Chemistry A2 course
amines as ligands
when NH3 and amines act as bases by accepting protons, they do so by donating a lone pair of electrons to the proton. they can also donate a lone pair of electrons to transition metal ions and act as ligands
when does the approximation of using delta H as a guide to feasibility become less justified?
when T is large because the magnitude of T delta S is bigger at low temps, delta G is approximately the same as delta H as TdeltaS is relatively small
butylamine with ethanoyl chloride
when amines react with acyl chlorides, an H atom on the amine is swapped for the acyl group to produce an N-substituted amide and HCl. the HCl reacts with another molecule of the amine to produce a salt to carry out this reaction, ethanoyl chloride is added to concentrated aqueous solution of the amine. a violent reaction occurs, which produces a solid, white mixture of the products
when to use amino
when compounds have a second functional group
where are carboxylic acids ionised
when dissolved in water the H+ ions in the solution mean that they show the characteristic reactions of acids with metals, bases and carbonates
why does rate constant change when temperature changes?
when k increases, so does rate constant this is because: as we increase the temperature the particles have more kinetic energy and they collide more often. this increases the rate
how does entropy effect the stability of a complex
when ligand exchange reaction occurs, dative bonds are broken and formed. the strength of the bonds being broken is often very similar to the strength of the new bonds being made. so the enthalpy change for a ligand exchange reaction is usually very small. for example, the reaction substituting ammonia with ethane-1,2-diamine in a nickel complex has a very small enthalpy change of reaction equation: this is a reversible reaction, but the equilibrium lies so far to the right that it is thought of as being irreversible - [Ni(NH2CH2CH2NH2)3]+2 is much more stable than [Ni(NH3)6]+2. this isn't accounted for by an enthalpy change. instead, it's to do with the entropy change of the reaction: when monodentate ligands are substituted with bi/multidentate ligands, the number of particles in solution increase - the more particles, the greater the entropy. reactions that result in an increase in entropy are more likely to occur. when EDTA4- (hexadentate ligand) replaces mono/bidentate ligands, the complex formed is a lot more stable [Cr(NH3)6]+3 + EDTA-4 --> [Cr(EDTA)]- + 6NH3 2 particles --> 7 particles the enthalpy change for this reaction is negligible and the entropy change is big and positive. this makes the free energy change (delta G = delta H - T x delta S) negative, so the reaction is feasible
when are cell emfs at the max?
when no current flows because no energy is lost due to the internal resistance of the cell as the current flows
when may isoelectric point differ
when the R groups differ. some amino acids are dicarboxylic or diamino like glutamic and aspartic acid which changes the isoelectric point significantly
When is gas chromatography used?
when the mobile phase is a gas
transition metals forming ions
when the transition metals form positive ions, outer s electrons are removed first, then the d electrons for example, Titanium can form Ti+2 and Ti+3 ions. give the electronic configuration for these 2 ions when titanium forms 2+ ions, it loses both its 4s electrons. Ti: 1s2 2s2 2p6 3s2 3p6 3d2 4s2 --> Ti+2 = 1s2 2s2 2p6 3s2 3p6 3d2 to form 3+ ions, it loses both its 4s electrons, and then a 3d electron as well. Ti+2 = 1s2 2s2 2p6 3s2 3p6 3d2 --> Ti+3 = 1s2 2s2 2p6 3s2 3p6 3d1
when is a mixture thermodynamically stable?
when there is no tendency for a reaction as delta G is positive
ions of the transition metals
when transition metals form their ions, e-s are lost initially from the 4s sub-shell and not the 3d sub-shell. this is because prior to holding any electrons, the 4s level is more stable than the 3d level. but once the 3d sub-shell is occupied by electrons, these 3d e-s, being closer to the nucleus, repel the 4s electrons to a higher energy level. the 4s e-s are repelled to an energy level higher than those occupying the 3d sub-sell. so, when transition metals form ions, they lose e-s from the 4s before the 3d level. this emphasises the fact that transition metals have similar chemical properties dictated by the behaviour of the 4s electron in their outer shell
how to obtain chromium hydroxide precipitate
when you mix an aqueous solution of chromium(III) ions with NaOH or aqNH3 you get chromium hydroxide precipitate - Cr(OH)3(H2O)3 (s)
how to predict whether a reaction is feasible by looking at ∆G = ∆H - T∆S
when ∆H is negative and ∆S is positive, ∆G will always be negative and the reaction is feasible when ∆H is positive and ∆S is negative, ∆G will always be positive and the reaction is not feasible
first order reactions
where [X] is the concentration of a particular reactant: - if the rate is proportional to [X], then the order of reaction with respect to D is 1. - so if [X] doubles, the rate will double and if [X] triples, the rate will triple
stationary phase
where the molecules can't move. this must be a solid or a liquid supported by a solid
oxidising ethanol by reflux
with acidified potassium dichromate (VI) turns into ethanoic acid colour change from orange to green
draw a labelled diagram showing how to set up cell to find the standard electrode potential of: V3+ (aq) + e- <--> V2+ (aq)
with the Fe3+ solution containing ions of V2+ and V3+ ions
can methanoic acid be oxidised?
yes by acidified potassium manganate --> carbonic acid (H2CO3) --> CO2 + H2O - no c-c to break
can catalysts appear in the rate equation
yes so if there is something in the rate equation but not chemical equation it is a catalyst
Cr3+ to chromate(VI) ions
you can oxidise Cr3+ to chromate(VI) ions with hydrogen peroxide in an alkaline solution
following rate of reaction using titration
you can take small samples of a reaction at regular time intervals and titrate them using a standard solution to slow reaction to take the sample we either do: 1. dilution with deionised water 2. cool it down 3. add a chemical to stop the reaction (quenching) the rate can be found from measuring the change in concentration of the product or reactant over time
hydrolysing esters with base
you have to reflux the ester with a dilute alkali, such as sodium hydroxide. you get a carboxylate ion and an alcohol. this reaction is irreversible - higher yield
what must the value of ∆G be for a feasible reaction?
zero or negative the more negative, the more feasible the reaction
Zinc/copper electrochemical cell observation at electrodes
zinc electrode will become thinner as more Zn2+ ions produced copper electrode will get thicker as Cu2+ ions receive electrons and turn into copper
what happens in zinc and copper electrochemical cells?
zinc loses electrons more easily than copper. so in LHS half-cell, zinc (from zinc electrode) is oxidised to form Zn2+ (aq) ions Zn(s) -> Zn2+ (aq) + 2e- this releases electrons into external circuit in other half-cell, the same no. of e-'s are taken from external circuit, reducing the Cu2+ ions to Cu atoms Cu2+ (aq) + 2e- -> Cu(s)
describe the application of NMR
• Provides structural information of elements eg. hydrogen, carbon, fluorine, silicon, phosphorus • optimise reaction dynamics, measure reaction kinetics, monitor reaction progress, control product purity • determine shapes and structures of large complex molecules, such as how proteins fold, twist and coil • evaluate proportions of solid/liquid components in fatty food such as margarines and low-fat spreads • pharmaceutical sciences and medicine (eg. diagnosis of tissue abnormalities, MRI scanning)
how to draw a titration curve
•Measure initial pH of the acid • Add alkali in small amounts noting the volume added • Stir mixture to equalise the pH • Measure and record the pH to 1 dp • When approaching endpoint add in smaller volumes of alkali •Add until alkali in excess
What is significant about the gradient of the line and the values P and Q shown in Fig. 18.1? Explain your reasoning.
∆G = ∆H - T∆S linked to y = mx + c (somewhere) gradient = -∆S P: ∆H / enthalpy change Q: temperature for reaction to be feasible/unfeasible
when ∆G = 0, the reaction is just feasible so temperature at which the reaction becomes feasible can be calculated by rearranging the equation:
∆H - T x ∆Ssystem = 0 so, T = ∆H / ∆Ssystem
why is it difficult to use enthalpy cycles to account for trends in the solubilities of ionic compounds
∆solH = a small difference between 2 large enthalpy changes - neither of which can be measured directly therefore small errors in estimating trends in the values can lead to large % errors in the prediction of ∆solH - both lattice and hydration enthalpies are affected in the same way by changes in the size of ions and their charges - this decreases the likelihood of any clear trends in ∆solH - the sign and magnitude of the ∆solH is not a reliable guide as to whether or not a solid will dissolve (e.g. entropy)
Carbon monoxide reacts with steam. CO(g) + H2O(g) U CO2(g) + H2(g) At 1100 K, Kc = 1.00 In an experiment, 1 mol of carbon monoxide was mixed with 1 mol of steam, 2 mol of carbon dioxide and 2 mol of hydrogen. Deduce, with reasons, the direction in which the reaction will shift to reach equilibrium.
(since Kc = 1) the concentrations of the products must be equal to the concentrations of the reactants at equilibrium the concentrations of CO2 and H2 / products need to decrease and those of CO and H2O / reactants need to increase so reaction shifts to the left
advantages and disadvantages of non-rechargeable cells
+ cheap - toxic waste material
advantages and disadvantages of rechargeable cells
+less waste than non-rechargeable cells +cheaper to run over lifetime +lower environmental impact -production of toxic waste that needs to be disposed at end of cells lifetime
explain why chemists are interested in determining rate equations and measuring rate constants
- Rate equations help to deduce reaction mechanisms; this information can inform methods of chemical synthesis. - Understanding catalytic activity (including enzymes) can lead to the development of more efficient manufacturing processes or guide drug design and development. - Rate measurements are important in controlling chemical changes in laboratory and industrial processes; chemical engineers use rate data when designing chemical plants
ways of modifying properties of polymers
- altering average length of chain - changing structure of monomer with different side group and different intermolecular forces - varying extent of cross-linking between chains - selecting a monomer that produces a biodegradable polymer - producing a co-polymer - adding fillers and pigments - making composites
what is the role of carbonic acid molecules and hydrogen carbonate ions in controlling the pH of blood
- blood needs to be kept at pH 7.4 - the pH is controlled using carbonic acid-hydrogencarbonate buffer system. Carbonic acid dissociates into h+ ions and HCO3- ions: H2CO3 (aq) <--> H+ (aq) + HCO3-(aq) if conc of H+ ions rises in blood, then HCO3- ions from the carbonic acid-hydrogencarbonate buffer system will react with the excess H+ ions, and the equilibrium shifts to the left, reducing H+ conc to its original value. this stops the pH of blood from dropping meanwhile, if the conc of H+ ions falls in blood, more H2CO3 molecules from the carbonic acid-hydrogencarbonate buffer system will dissociate, and the equilibrium will shift right, increasing H+ conc. this stops pH from rising the levels of H2CO3 are controlled by respiration. by breathing out CO2, H2CO3 is reduced, as it moves this equilibrium right the levels of HCO3- are controlled by the kidneys, with excess being excreted in the urine
non-rechargeable cells
- chemicals contained are used up over time and the emf(Ecell) decreases - once 1 or more reactant is used up, the emf is 0 volts, cell won't supply electricity - cells must be disposed after a single use
what do all the reagents to test for aldehydes have in common?
- contain transition metals - change oxidation state - undergo reduction -> = change in colour
describe and explain the different ways that a high res NMR spec can give info about a molecule
- delta gives chemical/ proton environment - quote data from sheet - no. of peaks gives no of different types of proton environments - ratio of peak areas gives no. of protons - splitting gives no of adjacent protons - n+1 rule - D2O can identify OH groups
why are there limitations to the use of bond energy data in estimating ΔH?
- due to significant differences between the values of ΔH estimated from bond enthalpies and experimental data : - variables in strength of one kind of bond in different molecules - one of the reactants/ products is not in gas state
compare methanol/ethanol to hydrogen fuel cells
- ethanol can be made renewably - less flammable than H2 - easier to store and transfer than H2
what does the relationship of -deltaH/T show?
- for a given quantity of energy, the increase in entropy is greater when the surroundings are cool than when they are hot - ^ adding energy to molecules in a cooler system has a proportionally greater effect on the number of ways distribution matter and eenrgy than adding the same quantity of energy to a system that is already very hot
why do arenes undergo substitution rather than addition
- for an arene to undergo addition, the delocalised electron ring would have to be broken - as the pi system is very stable, this would require much more energy than that needed to break the 1 double bond in an alkene - therefore they undergo substitution to retain their delocalised pi electrons
fuel cells
- fuel cells are different to other storage cells in that they have a continuous external supply of reactants (the fuel) into the cell - fuel cells use the chemical energy from the reaction of the fuel to create the voltage - do not need re-charging but do need a constant supply of the required chemicals - Hydrogen-oxygen fuel cell is the main type but others use methanol/ ethanol as fuels
examples of kinetic inertness
- fuel tank containing petrol and air - mixture of hydrogen and oxygen at room temperature - solution of hydrogen peroxide in absence of a catalyst
characteristics of transition metals
- hard metals with useful mechanical properties, high melting and boiling points - show variable oxidation numbers in their compounds - form coloured ions in solution - can act as catalysts both as the elements and as their compounds - form complex ions involving monodentate, bidentate and polydentate ligands
EDTA-4
- hexadentate ligand forming 6 co-ordinate bonds with the central metal ion in complexes - binds so firmly with metal ions that it holds them in solution and makes them chemically inactive - EDTA-4 with a Pb2+ ion is used to treat lead poisoning - it forms such a stable complex that they can be excreted through the kidneys
what are the rules for picking out which reactants from the chemical equation are involved in the RDS
- if a reactant appears in the rate equation, it must affect the rate. so this reactant, or something derived from it, must be in the RDS - if a reactant doesn't appear in the rate equation, then it isn't involved in the RDS (and neither is anything derived from it)
what are some rules for determining the mechanism of a reaction?
- if the reactant appears in the rate equation, it must appear in the RDS (or steps up to RDS) - + visa versa - the RDS doesn't have to be 1st step - substances that act as a catalyst will appear in the RDS but not the overall equation
rechargeable cells what needs to happen for them to work?
- in rechargeable cells the overall reactions are reversible - forward reaction occurs on discharge, giving out charge and supply electricity - reversed by applying an external voltage to change equilibria favouring the reverse reaction - this recharges the battery and the equation is written backwards reversible cells only work if product stays attached to the electrode and doesn't disperse
a reaction is only feasible if the total entropy change is positive. this means that:
- most exothermic reactions are feasible as the value of -delta H/T is more positive than entropy of a system which means that total entropy change is positive - an endothermic reaction can be feasible so long as the increase in the entropy of the system is greater than the decrease in entropy of surroundings - a reaction that does not tend to go at room temperature may become feasible as the temperature rises, because entropy of surroundings decreases in magnitude as T increases
disadvantages of fuel cells?
- need constant supply of fuel - expensive - H2 = flammable - limited lifetime - H2 is produced using fossil fuels
how does the nucleus environment change the absorption amounts of energy?
- nucleus is partly shielded from effects of external magnetic fields by its surrounding electrons - any other atoms around a nucleus also affect electron shielding - this means nuclei in a molecule feel different magnetic fields depending on environments so will absorb different amounts of energy at different frequencies - the differences in absorption of energy gives the NMR spectrum
for a reaction between MnO4- and C2O4-: 16H+ + 2MnO4- + 5C2O42- --> 2Mn2+ + 10CO2 + 8H2O why is heating very important at the start of the titration but less important as the titration proceeds 2CO2 + 2e0 <--> C2O42- E0 = +0.64V Mn3+ + e- <--> Mn2+ E0 = +1.49V MnO4- + 8H+ + 5e- <--> Mn2+ + 4H2O E0 = +1.51V
- reaction slow initially as MnO4- and C2O42- are both negative so repel each other - when sufficient Mn2+ ions form they autocatalyse the reaction - Mn2+ ions will reduce MnO4- ions (as E0 is more negative) forming Mn2+ ions: MnO4- + 8H+ + 4Mn2+ --> 5Mn3+ + 4H2O - Mn3+ ions then oxidise C2O42- ions (reforming Mn2+) as E0 is > C2O42- + 2Mn3+ --> 2Mn2+ + 2CO2
what are lithium cells used for? what is the advantage of them?
- rechargeable - phones and laptops - metals = low density and therefore cells are light - very reactive therefore E0 of Li half cell is high and each cell has a high emf
what are catalysts used for (transition metals)
- speed up reactions and products are obtained faster - allow processes to operate at lower temperatures with saving on energy and fuel - make possible processes that have high atom economies and produce less waste - can be highly selective so that only the desired product is formed without the obvious waste from side-reactions
what are the assumptions made when carrying out a clock reaction
- the conc of each reactant doesn't change significantly over the time period of your clock reaction - temperature stays constant - when the end point is seen, the reaction has not proceeded too far
hydroxynitrile can provide evidence for the reaction mechanism
- the groups surrounding the carbonyl carbon in a ketone/ aldehyde are planar so the CN- ion can attack from either side - when you react an aldehyde or asymmetric ketone with CN-, you get a racemic mixture - - the carbonyl group gets attacked equally from each side, producing equal amounts of the 2 products, which are optical isomers - as the product is present in a racemic mixture, the product optically inactive
when the conc of one of the components in the equilibrium is altered, what happens?
- there is an immediate change in Q - at this point Q does not equal k and the system is no longer at equilibrium - the equilibrium composition changes until the equilibrium is restored and Q = k (equilibrium constant) - k never changes
how do you stop the reaction going in the reverse direction to increase yield of ester?
- use excess acid/alcohol - use greater volume of conc H2SO4 than needed as a catalyst so it reacts with the water formed. but its effect is minimal as amount of catalyst is small - could distil ester/ water off as they form --> encourages reaction to go to completion
2 experiments were done to determine the kc of a reaction. why might the values of kc be different?
- value for kc should be constant at the same temp - could be due to equipment, % error, sample size - same diff in temp
standard enthalpy change of neutralisation for dilute solutions of strong acid with strong base is always close to:
-57.5kJmol-1
What is a limitation of using standard conditions to measure electrode potentials?
-Some reactions which aren't possible under standard conditions may occur under non-standard conditions and these are not accounted for -Changes in concentration, temperature and pressure can affect electrode potentials
Describe the limitations of E° data
-Tells us about thermodynamic feasibility but nothing about its rate of reaction. There may be something that inhibits the reaction kinetically eg: high act. energy which would make reaction mixture inert -Therefore in reality, these reactions may be very slow and no change is observed but the data does not indicate this
pKa
-logKa
pKw
-logKw Kw = 10^-pKw
pH definition
-log[H+]
storage cells are an application of standard electrode potentials, so some fundamental concepts of electrochemical cells still apply:
-the Ecell value for the electrochemical cell must be a positive voltage - the most positive E0 runs in the forwards direction, the most negative E0 is reversed supplying e- - when storage cells are operating, the cell reaction occurs as determined by E0 and the cell is said to be discharging
what does the lead-acid cells do normally (during discharge - normal running) what is the issue with the build up of lead sulfate?
-ve terminal = lead with a conc H2SO4 electrolyte. Pb(s) + SO42-(aq) --> PbSO4(s) + 2e- lead looses electrons positive terminal = lead coated with lead oxide PbO2(s) + SO42-(aq) + 4H+ +2e- --> PbSO4(s) +2H20 - lead oxide reacts with H+ and gains electrons insoluble therefore if the cells are discharged for a long period, the ppt becomes larger and thicker and the process can't be reversed when the cell are recharged
what are the 2 half equations when methanol is used instead of hydrogen in an acid fuel cell?
-ve terminal: CH3OH + H2O --> CO2 + 6H+ + 6e- +ve terminal: 3O2 + 6H+ + 6e- --> 3H2O
The pH range over which an indicator, HIn, changes colour is determined by...
...its strength as the acid. Typically the range is given roughly by pKa ± 1. The logarithmic form of the equilibrium law derived at the end shows why this is so. For an indicator it takes this form: pH = pKa + log([In-]/[HIn]).
evidence to support the delocalised structure rather than the Kekule 'triene' structure
1) C-C bond length - all the same length, in between C-C and C=C. if it were a triene, C-C would be longer than C=C (shown by x-ray diffraction) 2) addition reactions - Benzene does not readily undergo addition reactions (doesn't decolourised bromine water. if it were a triene, it would 3) enthalpy of hydrogenation - we would expect a triene to react with 3 H2 to form cyclohexane releasing 360kJ/mol of energy. it only releases 208kJ/mol showing that benzene is more stable than triene. this extra stability is due to the delocalisation of electrons and is known as the delocalisation stability 4: in IR benzene is missing C=C absorption
Suggest a simple practical demonstration of the difference between equimolar solutions of a strong acid and of a weak acid.
1) Dip an identical pair of electrodes into the 2 solutions, apply the same voltage across the electrodes and measure the current, which flows - the current is much larger with the strong acid. 2) Add identical strips of magnesium to the two solutions and observe the rate of reaction, which is faster with the solution of the strong acid.
Strategy for determining the value of an equilibrium constant involves: what is the problem with this method 2 solutions
1) Mix measured quantities of reactants and/or products. Then allow the mixture to reach equilibrium under steady conditions. 2) Analyse the mixture to find the equilibrium concentration of one of the chemicals at equilibrium. 3) Use the equation for the reaction and the information from steps 1 and 2 to work out the values for equilibrium concs of all the atoms/molecules/ions. Then substitute these values into the expression for Kc. Finding ways to measure equilibrium concentrations without upsetting the equilibrium - many methods of analysis use up the chemical being analysed - these are unsuitable due to Le Chatelier's principle. The two main solutions - a) find a method of 'freezing' (either by cooling or removing a catalyst...) the reaction and thus slowing down the rate so much that it is possible to measure one of the equilibrium concs by titration. b) Use an instrument that responds to a property of the mixture that varies with conc. Well-established methods for doing this include measuring the pH with a pH meter or measuring the intensity of a colour with a colorimeter.
what does NMR spec tell us
1) No. of different chemical enviro.s in the comp. 2) Relative no. of Hs in each enviro. 3) e- density around the Hs (indicated by chemical shifts). 4) How many neighbours each H atom has (from sub-peaks of high resolution):
hydrogen-oxygen fuel cells in acidic conditions
1) at anode the Pt catalyst splits the H2 into protons and electrons 2) the polymer electrolyte membrane (PEM) only allows H+ across and forces e- to travel around circuit to cathode 3) current created 4) at cathode, O2 combines with H+ from anode and e- from circuit to make H2O - waste product
2 types of NMR spectroscopy
1) carbon-13 NMR - gives you info about the no of C atoms in a molecule, and the environments they are in 2) high resolution proton NMR gives you info about no of H atoms in a molecule, and the environments they are in
convention for drawing electrochemical cells
1) half cells with more negative potential on left 2) oxidised forms go in centre (with the highest oxidation state) and reduced forms on outside 3) double vertical lines show salt bridge and single lines separate species in different physical states 4) if either half-cells use platinum, lead or other inert electrodes it goes on either side
Suggest two benefits that arise from the isomerization of the inactive enantiomer of ibuprofen.
1. All the ibuprofen is useful (rather than half) 2. No need for separation of isomers / enantiomers 3. No need for a more complex synthesis forming just one enantiomer 4. Sometimes one enantiomer has negative effects 5. Smaller dosage may be used
how does NMR work?
1. atoms with odd number of protons OR neutrons spin will behave as a magnetic dipole (magnetic field) -> NMR detects how field is affected by large field 2. nuclei spins randomly but when an external magnetic field is applied the magnetic field of this dipole aligned: -> parallel (same direction) to applied magnetic field -> opposed (op direction) to the applied magnetic field 3. nuclei that spin parallel with the external magnetic field have lower energy than ones opposite 4. NMR sends out radio waves 5. at specific frequency nuclei aligned with magnetic field absorb radio waves + move from lower (parallel) to higher energy (antiparallel) state = resonance condition 6. nuclei will jump back down to base state (lower state) which releases energy (proportional to absorption) 7. NMR measures amount of energy absorbed -> absorption depends on chemical environment -> amount that it differs is called the chemical shift, δ
what happens when a solid ionic lattice dissolves in water?
1. bonds between ions break - endothermic. enthalpy change is opposite of lattice enthalpy 2. bonds between ions and water are made - exothermic. enthalpy change here is called enthalpy change of hydration
in weak acids and bases what are the 2 types of enthalpy involved in neutralisation
1. enthalpy of dissociation 2. enthalpy when OH- and H+ ions react enthalpy of dissociation varies depending on the acid/ base. this means enthalpy of neutralisation also varies
how to measure equilibrium concentration
1. freezing the reaction to slow the rate and measure 1 equilibrium conc. by titration e.g. by cooling/ diluting the equilibrium mixture/ to remove a catalyst 2. measuring the pH with a pH meter as pH corresponds with concentration
how to set up an electrochemical cell
1. get strip of metals - these are the electrodes. clean surfaces using emery paper 2. clean grease from electrodes using propanone (wear gloves to prevent contamination) 3. place each electrode into a beaker filled with solution containing ions of that metal. e.g. Zn electrode in ZnSO4 (aq) - if using an oxidation agent containing oxygen you need to add acid too (e.g. MnO4-) 4. create salt bridge by soaking filter paper in salt solution e.g. KNO3 and drape between 2 beakers 5. connect electrodes to a voltmeter using crocodile clips and wires
using the following data: Ag+ + e- <--> Ag = electrode potential = +0.80 V Ag2+ + e- <--> Ag+ = electrode potential = 2.00V predict whether Ag+ ion will disproportionate in solution
1. identify which is oxidised and reverse oxidised equation 2. combine equations to obtain the feasible reaction: Ag + Ag2+ <--> 2Ag+ 3. compare this equation to the react stated in the question. we can see that Ag+ will not disproportionate in solution 4. confirm by calculating Ecell of the disproportionation reaction. all feasible reactions will have a positive Ecell value Ecell = 0.80 - 2.00 = -1.20
a clock reaction - an example of the initial rates method
1. in a clock reaction, you measure how the time taken for a set amount of product to form changes as you vary the conc of one of the reactants 2. as part of the clock reaction, there will be a sudden increase in the conc of a certain product as a limiting reactant is used up 3. there's usually an easily observable end point, such as colour change, to tell you when the desired amount of product has formed 4. the quicker the clock reaction finishes, the faster the initial rate of the reaction
what are the 2 ways to use hydrogen as fuel for cars
1. in fuel cells to power an electric motor 2. as fuel in an internal combustion engine
types of ligands
1. ligands with 1 lone pair - monodentate e.g. H2O:, :NH3, :Cl-, :OH- 2. ligand with 2 lone pairs - bidentate e.g. 1,2 - diaminoethane - forms 2 dative covalent bonds with metal ion 3. ligand with more than 2 - multidentate e.g. EDTA-4 has 6 lone pairs
steps to find out how many manganate (VII) ions are needed to react with a reducing agent
1. measure the quantity of reducing agent e.g. aqueous Fe2+ ions and put in conical flask 2. add dilute sulfuric acid in excess (acid ensures there are plenty of H+ ions to allow the oxidising agent to be reduced 3. do rough titration - gradually adding aqueous MnO4- to reducing agent 4. stop when turns purple
3 types of electrodes
1. metal electrodes - consist of metal surrounded by a solution of its ions e.g. Zn(s) I Zn2+ (aq) 2. gas electrodes - for a gas and a solution of its ions. here an inert metal (platinum) is the electrode to allow the flow of electrons e.g. Pt(s) I H2(g) I H+ (aq) 3. redox electrodes - for 2 different ions of the same element where the 2 types of ions are present in solution with an inert metal electrode e.g. Pt(s) I Fe2+ (aq), Fe3+(aq)
finding equilibrium constants by experiment in 3 steps
1. mix measured quantities of reactants and/or products. allow mixture to reach equilibrium under steady conditions 2. find equilibrium concentrations of one of the chemicals at equilibrium 3. use equation for reaction and info from steps 1 and 2 to work out values for equilibrium concs of atoms/ ions. substitute into Kc expression
steps to find equilibrium constants by experiment
1. mix measured quantities of reactants and/or products. then allow the mixture to reach equilibrium under steady conditions 2. analyse the mixture to find the equilibrium concentration of one of the chemicals at equilibrium using titration 3. use the equation for the reaction and info from step 1 and 2 to work out values for the equilibrium concentrations of all the atoms/molecules/ions. substitute into Kc
basic principles of chromatography
1. mobile phase moves through or over stationary phase 2. distance each substance moves up plate depends on its solubility in mobile phase and its retention by or adsorption to the stationary phase 3. more soluble components in mobile phase travel further up plate or faster through column. these differences in solubility and retention by stationary phase separate out different substances
find pH of the buffer solution made by mixing 25 cm3 of 1 moldm-3 ethanoic acid with 25cm3 of 0.4 moldm-3 sodium hydroxide ka = ethanoic acid = 1.74 x 10-5 b) calculate the new pH of the buffer if 0.2cm3 of 0.5moldm-3 sulfuric acid is added to the sample
1. n(HA) = 25 x 1/1000 = 0.025 2. n(OH-) = 25 x 0.4/1000 = 0.01 3. what is in excess - HA 4. HA + OH- --> A- + H2O before 0.025 0.01 after 0.015 0.01 5. left over [HA] 0.015/ (50/1000) = 0.3 6. [A-] = 0.01 / (50/1000) = 0.2 7. [H+] = ka[HA] / [A-] left over = 1.74 x 10-5 x 0.3 / 0.2 = 2.61 x 10-5 b) n(H+ added) = 2 x 0.2 / 1000 x 0.5 = 0.0002 A- + H+ --> HA before 0.01 0.0002 0.015 after 0.0098 0.0152 therefore [HA] = 0.0152 / 50.2/1000 = 0.0152 [A-] = 0.0098 / 50.2/1000 [H+] = 1.74 x 10-5 x [A-] / [HA] [H+] = 2.7 x 10-5 pH = 4.57
when may a positive electrode potential not be a feasible reaction
1. non standard conditions if we change the conc or temp this can cause the electrode potential to change e.g. a rust nail: Fe2+ + 2e- <--> Fe E0 = -0.44V O2 + 2H2O + 4e- <--> 4OH- E0 = +0.40V Ecell = 0.4--0.44 = + 0.684 if we increase conc of O2, equilibrium will shift right which means it is easier for O2 to gain e-s and the electrode potential for the half cell is more positive and the cell potential will be higher. if you increase the conc of Fe, equilibrium will shift left. less e-s will be used up. the electrode potential of Fe2+/Fe becomes less negative and cell potential will be lower. 2. Kinetics not favourable - rate of reaction is so slow it appears there is no reaction - if activation energy is too high it may stop the reaction happening all together unless we have a catalyst
ligands split the 3d subshell into 2 energy levels
1. normally the 3d orbitals of transition metal ions all have the same energy. but when ligands bond to ions, the 3d orbitals split into 2 different energy levels. 2. electrons tend to occupy the lower orbitals (ground state). to be excited, they need energy from visible light equal to the energy gap, delta E. 3. the larger the energy gap, the higher the frequency of light that is absorbed 4. the amount of energy (so frequency of light) needed to make electrons excited depends on the central metal ion, its oxidation number, the ligands and co-ordination number - these affect the size of the energy gap (delta E)
what are standard conditions for hydrogen electrodes?
1. solutions of ions you're interested in must have a concentration of 1.00 mol dm-3 2. temperature of 298K 3. pressure of 100kPa (only affects half-cells with gases)
how do you determine if a spectrum contains labile protons?
1. spectrum with normal sample and solvent 2. spectrum with deuterated water (D2O) - the deuterium atoms quickly replace the H on OH 3. 2nd spectrum will be missing any labile proton peaks so can compare to original peaks as the D2O does not show up as there is no spin on the D as it has 2 neutrons
errors in redox titrations
1. starch indicator for sodium thiosulfate needs to be added at right point or blue will be slow to disappear 2. starch solution freshly made or won't act normally 3. ppt of copper (I) iodide makes seeing colour of solution hard 4. iodine produced in reaction can evaporate from solution giving false titration reading so % of copper would be too low - instead keep solution cool 5. starch added too early and iodine starch complex solid forms
source of errors with titration of sodium thiosulfate and iodine
1. starch indicator for sodium thiosulfate titration needs to be added at right point when most of iodine has reacted, or the blue colour will be very slow to disappear 2. starch solution needs to be freshly made or won't behave as expected
compare the base strength of: 1. methyamine vs diethylamine 2. propylamine vs phenylamine 3. ammonia vs phenylmethyamine
1. stronger = diethylamine secondary vs primary greater electron density on nitrogen lone pair greater ability to accept H+ 2. stronger = propylamine lone pair on pheylamine N is partially delocalised into the benzene ring propylamine is has greater electron density on N lone pair therefore has greater ability to accept H+ 3. stronger = phenylmethyamine primary vs ammonia therefore electron density on N lone pair therefore greater ability to accept H+ pka values of their conjugate acids give a measure of the strength of ammonia. butyl amine (pka = 10.61) and phenylamine (pka = 4.62). a higher pka value corresponds to a stronger base
titration of iodine with sodium
1. take flask containing solution produced in stage 1 2. add sodium thiosulfate from burette 3. end point is pale yellow 4. add starch solution (to detect presence of iodine) - solution will go dark blue 5. add sodium thiosulfate until blue disappears - all of iodine has reacted 6. now calculate number of moles of iodine in solution
2 important things to remember about RDSs and mechanisms
1. the RDS doesn't have to be the first step in a mechanism 2. the reaction mechanism can't usually be predicted from just the chemical equation
steps of heterogenous catalysts
1. the reactant molecules are attracted to the surface of the solid catalyst and stick to it - adsorption 2. the surface of the catalyst activates the molecules so they react more easily. in the reaction between nitrogen monoxide and carbon dioxide, the bonds between the reactants' atoms are weakened making them easier to break and reform as the product 3. the product molecules leave the surface of the catalyst making way for fresh reactants to take their place - desorption
investigating the reaction of hydrogen peroxide with iodide ions by a clock reaction method:
1. use a pipette to add 10cm3 of hydrogen peroxide to a clean beaker 2. use a measuring cylinder to at 25cm3 of sulfuric acid 3. use burettes to add to a second clean beaker: - 5cm3 of potassium iodide solution - 20cm3 of distilled water - 2cm3 of sodium thiosulfate solution - a few drops of starch 4. pour contents of 1st beaker into 2nd beaker and start timing and swirl content thoroughly 5. record time take for blue colour of iodine-starch complex to appear 6. repeat steps using same quantities of H2O2 and H2SO4 but different volumes of KI solution and water - making sure total volume of KI and water is 25cm3 each time
3 reasons it is difficult to account for trends in solubilities of ionic compounds
1. ΔHsol is a small difference between 2 large enthalpy changes so even small errors in estimating trends in values of lattice energies and hydration enthalpies led to large % error 2. lattice energies and hydration enthalpies are affected in the same way by changes in charge density of ions 3. sign and magnitude (size) of enthalpy changes of solution is not a reliable guide as to whether a solid will dissolve. other factors need to be considered
A different monobasic weak acid has a dissociation constant of 1.8 × 10- 5 mol dm-3. Calculate the pH of a 1.00 mol dm-3 aqueous solution of this acid.
1.8 × 10−5 = [H +]^2 / 1 [H+ ] = √1.8 × 10−5 = 4.24 × 10−3 pH = −log (4.24 × 10−3 ) = 2.37/2.4
how to calculate overall yield of synthesis from various percentages: e.g. 34% for reaction 1 76% for reaction 2 89% for reaction 3
100 x 0.34 x 0.76 x 0.89
a commercial nitric acid solution, HNO3, has a conc of 15.9 mol dm-3. a 15cm3 sample was made up to 100cm3 by adding deionised water. calculate the pH of this solution
15.9 x (15/100) = 2.385 pH = -log(2.385) = -0.377 Note that if the [H +] ≥ 1M, the pH will become negative.
explain the effect of a change in temperature on the position of equilibrium
2 equations are used: ΔG = -RTInkc ΔG = ΔH-TΔSsys -RTlnkc = ΔH-TΔSsys RTlnKc = -ΔH + TΔSsys lnkc = (-ΔH/R) x 1/T + ΔSsys/R assume that ΔH and ΔSsys do not vary much with temp so this corresponds to: y = mx+c y = lnkc m = -ΔH/R x = 1/T c = ΔSsys/R
preparing aromatic amines
2 step process: nitration to make a nitro compound and then reduction. reduction of an aromatic nitro-compound is achieved by boiling under reflux with tin and conc. HCl the aromatic amine dissolves in excess conc. HCl, forming a salt. The free amines can be liberated from the solution by adding sodium hydroxide solution. it is then separated from the mixture by steam distillation
calculating pH of strong diprotic acid
2 x [H+] = Ans pH = -log(Ans)
what standard conditions which apply to Fe3+(aq)/Fe2+(aq) when measuring this potential?
298 K both Fe3+(aq) + Fe2+(aq) conc of 1 moldm-3
standard molar entropy standard conditions
298K, 1 atm
Electrode system Standard electrode potential E0/ V V2+(aq)|V(s) -1.18 V3+(aq), V2+(aq)|Pt -0.26 electrochemical cell using the 2 electrode systems above to measure Ecell with equation for the cell reaction
2V3+ + V --> 3V2+
A solution of glycolic acid of concentration 0.1 mol dm-3 has a pH of 2.4 What is the approximate pH of the resulting solution after it has been diluted by a factor of 100?
3.4 diluting a weak acid by 100 increases pH by 1 diluting a strong acid 10 increases pH by 1
For a given initial reactant pressure, the half-life for a first order gaseous reaction was 30 minutes. If the experiment were repeated at half the initial reactant pressure, the half-life would be
30 minutes
Domestic bleaches contain sodium chlorate(I), NaOCl. Write the ionic equation to show the disproportionation of the chlorate(I) ion. Use oxidation numbers to explain the meaning of the term disproportionation in this reaction.
3OCl- → 2Cl- + ClO3 - chlorine (in OCl- ) is (simultaneously) oxidised from +1 to +5 and reduced from +1 to -1
how to calculate enthalpy change of solution
= latt. + hyd. the enthalpy of solution is slightly endothermic, but there are other factors at work that means that sodium chloride still dissolves in water
what happens if there are more than 3 neighbouring inequivalent H atoms?
= multiplet
when 5 moles of A is mixed with 5 moles of B in a container of volume 1 dm^3 an equilibrium is established with 3 moles of C. what is the kc at room temp? A + 2B <--> 2C
A + 2B <--> 2C I 5 5 0 C -1.5 -3 3 E 3.5 2 3 c(A) = 5.5/12 = 7/24 c(B) = 1/6 c(C) = 1/4 kc = 7.71 dm3 mol-1
How does IR work?
A beam of IR is passed through the sample, a similar beam is passed through a reference cell, the freq. of radiation is varied, bonds vibrating with a similar freq. absorb the radiation, the amount absorbed by sample is compared with reference cell, results are collected, stored and plotted.
Fehling's solution
A blue solution of complexed copper (II) ions dissolved in sodium hydroxide which is used to distinguish between aldehydes and ketones. if heated with aldehyde, copper ions are reduced to brick-red ppt of coper (I) oxide blue solution --> brick-red ppt use fehlings over benedicts because benedicts is more stable so reacts less reliably
By considering both the thermodynamic stability and the kinetic inertness of a mixture of hydrogen and oxygen, explain why hydrogen does not react with oxygen unless ignited.
A mixture of hydrogen and oxygen is thermodynamically unstable because ∆Stotal is positive The mixture is kinetically inert /stable or reaction is (very) slow because the activation energy is (very) high
why is a reference electrode necessary
A reference electrode is needed to find the electrode potential of any other electrode as the potential of the reference electrode is known
combustion of alcohols
Alcohol + oxygen --> carbon dioxide + water
what is chromatography?
An analytical method used to separate the substances in a mixture
the born-haber cycle
An enthalpy cycle that allows you to calculate the lattice enthalpy change of formation for system.
equivalent protons
Are those in the same chemical environment with the same chemical shift. They do not couple with each other
expression for weak acids in aqueous solution with nothing else added and assumptions for this reaction: H2S(aq) + H2O(l) U H3O+(aq) + HS−(aq)
Assumption 1 only a small amount of the weak acid (HA) dissociates so we can assume that - [H2S]equilibrium = [H2S]initial Assumption 2 the dissociation of acid is greater than dissociation of water present in the solution so can assume all the H+ ions come from the acid [H+] = [HS−] [A-] ionisation is negliable temperature is at 298k
A haloalkane, RX, reacts with OH- ions to form an alcohol. RX + OH- --> ROH + X- The rate equation for the reaction is rate k[RX]. Which of these statements is incorrect? A Rate is directly proportional to [RX]. B RX is a primary halogenoalkane C The reaction mechanism is SN1 D A carbocation intermediate forms in the reaction
B
Take CH3COC4H9 (molecular ion has m/z=100) and explain how the position of the carbonyl group influences the fragmentation pattern.
Breaking the bond between the CH3 and the CO would produce peaks at 15 (CH3+) and 85 (C4H9CO+) breaking the bond between the C4H9 and the CO produces two peaks at 43 (CH3CO+) and 57 (C4H9+). [There would also be a peak at 72 due to loss of the CO].
suggest why dibutylamine solution is more alkaline than butylamine solution
Butyl / alkyl groups are electron donating / are electron pushing / are electron releasing Two alkyl groups in dibutylamine but only one in butylamine Lone pair of electrons on the nitrogen more readily available / higher electron density on the nitrogen or NH2 or amine group / N more delta negative / N or NH2 accepts a proton more readily
In liquid ammonia the following equilibrium is present. NH2 − + NH4 + U 2NH3 Identify the Brønsted-Lowry base(s). A NH2 − only B NH4 + only C NH2 − and NH3 D NH4 + and NH3
C
What is the difference between the structure of the nuclei of C-12 and C-13 atoms?
C-13 atoms have 1 extra neutron compared to C-12.
K2Cr2O7 as an oxidising agent - redox titrations
C2O72- + 4H+ + 6e- --> 2Cr3+ + 7H2O good: dilute H2SO4/ HCl bad: conc H2SO4 - oxidises some Fe2+ therefore decreases titre value conc HNO3 - oxidises some Fe2+ therefore decreases titre value HCl - is oxidised by MnO4- --> Cl2 so increases titre value and Cl2 is toxic ethanoic acid - weak and not enough H+ indicator = barium diphenylamine sulphate --> purple
benzene formula and structure
C6H6 planar molecule and 6 c-c bonds are same length - intermediate between single and double each C atom is bonded to 2 other C atoms and 1 H atom by single covalent sigma bonds leaving 1 unused electron on each C atom in the p orbital, perpendicular to the plane of the ring each p orbital overlap to form a pi-bond the overall result is a ring of electron cloud above and below the plane of the ring
1-butylamine, CH3CH2CH2CH2NH2, reacts with ethanoyl chloride to form...
CH3CH2CH2CH2NHCOCH3
what happens to an acidic buffer when you add a little H+ e.g. (CH3COOH + CH3COO-)
CH3COOH ⇌ H+ + CH3COO- The added H+ is removed by reaction with CH3COO- to form CH3COOH. The [CH3COO-] falls slightly and the [CH3COOH] rises slightly, but as [CH3COOH] & [CH3COO-] >> [H+], the ratio of [CH3COOH]/[ CH3COO-] remains roughly constant.
when happens to an acidic buffer when you add a little OH-
CH3COOH ⇌ H+ + CH3COO- The added OH- reacts with H+, and so some CH3COOH breaks down to replace that H+. The [CH3COO-] rises slightly and the [CH3COOH] falls slightly, but as [CH3COOH] & [CH3COO-] >> [H+], the ratio of [CH3COOH]/[CH3COO-] remains roughly constant.
What anhydrous salts can be used for drying?
Calcium chloride.(hydrocarbons) Calcium sulfate. Magnesium sulfate.
To what extent is the following statement true? 'Once a reaction reaches equilibrium, there is no further reaction.' (3)
Chemical equilibrium is dynamic [1]; on a molecular scale there is constant change with opposing reactions cancelling each other out. [1] However, the overall macroscopic effect is that nothing appears to be changing when the system is at equilibrium. [1].
what can chromatography be used for?
Chromatography can be used to separate mixtures and can give information to help identify substances.
Account for the fact that the mass spectrum of dichloroethene includes three peaks at m/z values of 96, 98 and 100 with intensities of the ratio 9:6:1.
Cl consist of 2 isotopes: 35 & 37, with abundances in ratio 3:1. In a mc ion of C2H2Cl2 with 2 Cl atoms, the possibilities are 35-35, 35-37 and 37-37. The adjacent peaks differ by two mass units and expected ratio is 9:6:1.
suggest why KCl is not a suitable salt for salt bridges in a Cu+ --> e- + Cu2+ cell
Cl- ions react with Cu2+
what salts should you use for a chromate half cell?
CrCl3, K2CrO7
Electrochemical cells are set by combining the two half cells shown. In each case: i) Give the conventional representation of the cell. ii) Calculate the cell emf. iii) Identify the anode. iv) Write a balanced equation for the reaction that will take place in the cell. v) Explain why the reaction takes place. Cu2+(aq) + 2 e- ⇌ Cu(s) E° = +0.34V Fe3+(aq) + e- ⇌ Fe2+(aq) E° = +0.77 V
Cu(s) | Cu2+(aq) || Fe3+(aq), Fe2+(aq) | Pt(s) emf = +0.43 V anode = Cu2+/Cu 2Fe3+ + Cu → 2Fe2+ + Cu2+ E°(Fe3+/Fe2+) > E°(Cu2+/Cu) and therefore Fe3+ gains electrons from Cu
phenol with nitric acid
Dilute nitric acid reacts rapidly at room temperature to form a brown mixture. main products are 2-nitrophenol and 4-nitrophenol
use of drying agents
Drying agents can be used to remove traces of water from an organic product Drying agents are usually anhydrous inorganic salts that readily hydrate in the presence of water - Anhydrous calcium chloride is commonly used to dry hydrocarbons - Anhydrous calcium or magnesium sulfate are used more general purpose drying agents A spatula of drying agent is added into the organic product and swirled - If the organic product has a low boiling point, a lid / stopped can be added to reduce the potential evaporation of any product If the drying agent clumps together, then there is still water in the organic liquid More drying agent is added until some remains dispersed in the organic liquid as a fine powder The dry organic liquid can then be decanted or filtered - If the organic liquid is dry then it should appear clear
problems with IR spec
Due to many different vibrations taking place in a mc, total characterisation of a substance based only on its IR spec is ~ impossible unless one has computerised data handling facilities for comparison with spec of known comps. This is useful in conjunction with other techniques.
how to calculate activation energy from a graph
Ea = − R x gradient/ 1000
arrhenius equation
Ea is in Joules mol-1
Explain why the dissolving of silver fluoride in water is always a spontaneous process.
Entropy change is positive/entropy increases and enthalpy change negative/exothermic So ∆G is (always) negative
How would you find the value for Kp for the equilibrium between N2(g), H2(g) and NH3(g) given that at equilibrium the mixture contains 2.15 mol pf N2, 6.75 mol of H2 and 1.41 mol of NH3 at a total pressure of 10.0 atm.
First work out the mole fractions of the gases (2.15/10.3 for N2...). Multiply the total pressure by the mole fractions to get the partial pressures (0.208 x 10 = 2.08 atm for N2). Check that the sum of the partial pressures equals the total pressure. Finally substitute in the expression for Kp (p(NH3)^2/p(N2) x p(H2)^2) and give units (atm^-2).
the pH of acidic buffer solution equation
H+ = ka x [HA]/ [A-]
2H+ + CO32- --> H+ + HCO3- H+ + NH3
H2O + CO2 H2O + CO2 NH4+
which substance is created when water ionises
H30+ and OH-
at a certain temperature, a buffer solution contains 0.40 mol dm-3 methanoic acid, HCOOH, and 0.60moldm-3 sodium methanoate, HCOO-Na+. at this temperature, Ka for methanoic acid = 1.8 x 10-4 mol dm-3. what is the pH of this buffer
HCCOH (aq) <--> H+ (aq) + HCOOH- (aq) ka = [H+][HCOO-] / [HCCOH] [H+] = ka x [HCCOH] / [HCOO-] [H+] = 1.8 x 10-4 x 0.4/0.6 = 1.2 x 10-4 mol dm-3 pH = -log[H+] = -log(1.2 x 10-4) = 3.92
Account for the fact that the reaction between conc H2SO4 and NaCl can give a good yield of HCl gas, despite the fact that conc H2SO4 and HCl are strong acids.
HCl is a gas, so it escapes from the reaction mixture as it forms. This means that the backward reaction does not start so the forward reaction continues essentially to completion.
strong and weak acids dissolved in water
HCl(aq) -> H+(aq) + Cl-(aq) CH3COOH(aq) <-> CH3COO-(aq) + H+(aq) the weak acid is only slightly ionised
in indicators, what is the main species present at low pH? and high pH?
HIn is mean species present as the H+ ion present in an acidic solution will push position of equilibrium towards reactants and therefore colour is the colour of HIn high pH = the OH- ions react and remove the H+ ions to form water causing the equilibrium to shift to products therefore colour is colour of In- HIn <--> H+ + In-
Calculate the concentration, in mol dm-3 , of a solution of chloric(l) acid, HOCl, which has a pH of 4.23. Chloric(l) acid is a weak acid with Ka = 3.72 × 10-8 mol dm-3.
HOCl: pH = 4.23 ∴ [H+ ] = 5.89 × 10-5 mol dm-3 (1) Ka = [H+ ] [OCl- ] [H+ ] = [OCl- ] (1) [HOCl] = [H+ ] 2 / Ka = 0.0932 mol dm-3
what is used when gas chromatography can't
HPLC is used for example when the sample is heat sensitive or has a high boiling point
describe a standard hydrogen electrode
Hydrogen gas 1.0 mol dm-3 H+ 298K 100kPa Pt electrode
what is the ionic equation for a neutralisation reaction?
H⁺(aq) + OH⁻(aq) → H₂O(l)
what is the reaction mechanism for: 2ICl + H2 --> 2HCl + I2 rate = k [ICl][H2]
ICl + H2 --> HCl + HI ICl + HI --> HCl + I2
Explain 'The stronger the base, the weaker its conjugate acid.
If a base is strong, it tend to accept protons and turn into its conjugate acid. The conjugate acid has a correspondingly limited tendency to give up the protons and ionise.
Explain 'the stronger the acid, the weaker its conjugate base.
If an acid is strong it means that it tends to ionise in water to form hydrated hydrogen ions and the conjugate base In other words, the conjugate base has a weak hold on protons in competition with water molecules.
C-13 chemical shifts: shifted downfield
If the C is attached to electro-ive atoms/e- withdrawing groups it's said to be deshielded from the external magnetic field and has a greater chemical shift value.
When do substances dissolve?
If the energy released is roughly the same or greater than the energy taken in; Soluble substances tend to have exothermic enthalpies of solution.
What is the eluent in column chromatography?
In column chromatography, the liquid flowing through the column is the eluent. It washes the components of the mix through the column - this is elution.
homogeneous catalyst
In the same state as the reactants It interacts with a reactant to form an intermediate substance, which then decomposes or reacts with another reactant in one or more steps to regenerate the original catalyst and form product.
Other than providing a suitable peak, what other requirements must there be for a standard added to a solution in an NMR test?
Inert so doesn't react with samples. Low BP so easily removed from a sample after the NMR test has been run. Non-toxic. produces a single peak for both proton and carbon-13 NMR
When a metal is placed in a solution of its ions, the electrical potential set up between the metal and the solution cannot be measured without using a reference electrode. Explain why this is so
Introducing another metal wire would set up its own p.d. / can only measure a potential difference / voltmeter requires two connections
The value of Kw varies with temperature. At 273K its value is 1.10 x 10^-15 mol2dm-6, while at 303K it is 1.5 x 10^-14 mol2dm-6. Is the ionisation of water an exothermic or an endothermic process? What happens to the hydrogen ion concentration in pure water, and hence the pH, as the temperature rises? Does pure water stop being neutral if its temperature rises?
Ionisation is exothermic. Extent of ionisation increases as the temperature rises and the H+ conc rises as the temp rises. The number value of the pH falls. The water remains neutral because [H+(aq)] and [OH-(aq)] are equal.
What is a molecular ion radical (M+)?
It's a radical ion having a +ive charge and an unpaired e- formed by e- loss from the mc. The peak of highest m:z corresponds to this mc ion radical.
what does having a large Ka or a large pKa show
Ka = acid is strong pKa = acid is weak
To what extent is the following statement true? 'Adding more of one of the reactants to an equilibrium mix increases the yield of products because the value of the equilibrium constant increases.' (3)
Kc does not vary with concentration. [1] However, it is true that adding more of a reactant to a system at equilibrium means that the mixture is no longer at equilibrium [1]; some of the added chemical reacts and more products form until the system is again at equilibrium. [1]
in cell Li oxidised to Li+. explain why H2O is not used as a solvent
Li reacts with H2O to form LiOH
what is a difficulty with lithium cells? how is this overcome?
Li readily combines with O2 in the area to form a layer of non-conductive Li2O - Li also reacts with water developing electrodes with Li atoms and ions inserted into the crystal lattices of other materials
what is plane polarised light
Light that only vibrates in one direction in the same plane from a polaroid
excess zinc powder is added to an acidifed solution of the compound NH4VO3. using the data table, explain the sequence of reactions that takes place in your answer, include your observation, and the relevant ionic equations with their calculated Ecell values. state symbols are not required V+2 + 2e- <--> V(s) E0 = -1.18 V+3 + e- <--> V2+ E0 = -0.26 VO+2 + 2H+ + e- <--> V+3 + H2O E0 = +0.34 VO3- + 4H+ + e- <--> VO+2 + 2H2O E0 = +1 Zn+2 + 2e- <--> Zn E0 = -0.76
M1 and M2 -colours Yellow --> blue --> green --> violet oxidation states 4 colours linked to correct species / oxidation states / reactions (1) M3 - statement Statement that sequence is from +5 to +4 to +3 to +2 or (step-wise) reduction / zinc is a reducing agent (1) Zn + 2VΟ3 − + 8Η+ → Zn2+ + 2VΟ2+ + 4Η2Ο and E o = (+)1.76 (V) Ζn + 2VΟ2+ + 4Η+ → Zn2+ + 2V3+ + 2Η2Ο and E o = (+)1.1(0) (V) Zn + 2V3+ → Ζn2+ + 2V2+ and E o = (+)0.5(0) (V) stops at V2+ No (further) reduction (feasible) to V metal / V(0) or Zn + V2+ → Ζn2+ + V not feasible or E o = −0.42 (V)
Sulfurous acid, H2SO3, is also a diprotic acid. The values of Ka1 and Ka2 can be determined from the results of an acid-base titration. Diprotic acids require two OH- ions per molecule for complete neutralization. Sulfurous acid, H2SO3, is a stronger acid than H2S and a 0.100 mol dm−3 solution has a pH of 1.5. (i) On the grid below, sketch the likely shape of the titration curve for sulfurous acid, H2SO3,during the neutralization process.
M1: General shape of an acid-base curve with the pH increasing and either one or two steep / vertical sections shown NOTE Penalise a pH range for a single vertical with a range of eight or more pH units (as this is the typical range for a strong monobasic acid with a strong base titration curve) M2: Vertical / steep section at 25 cm3 M3: Vertical / steep section at 50 cm3 M4: Either equivalence point labelled anywhere on vertical section or x-axis M5: Initial pH = 1.5 and a recognisable 'plateau' in the pH range of 12 to 13
KMnO4 as an oxidising agent - redox titrations
MnO4- + 8H+ + 5e- --> 4H2O + Mn2+ good: dilute H2SO4 - needs a lot otherwise it produces MnO2 = brown ppt = obscures endpoint and a > volume of MnO4- used bad: conc H2SO4 - oxidises some Fe2+ therefore decreases titre value conc HNO3 - oxidises some Fe2+ therefore decreases titre value HCl - is oxidised by MnO4- --> Cl2 so increases titre value and Cl2 is toxic ethanoic acid - weak and not enough H+ (and forms MnO2) it is self indicating as it goes from colourless to purple and has high emf so is a very powerful oxidising agent
what happens to a basic buffer when you add a little H+ e.g. NH3, NH4+
NH3 + H2O ⇌ NH4+ + OH- The added H+ is removed by reaction with OH-, so some NH3 reacts to replace the OH- . The [NH3] falls slightly and the [NH4+] rises slightly, but as [NH3] & [NH4+] >> [OH-], the ratio of [NH3]/[NH4+] remains roughly constant.
what happens to a basic buffer when you add a little OH-
NH3 + H2O ⇌ NH4+ + OH- The added OH- is removed by reaction with NH4+ to form NH3. The [NH3] rises slightly and the [NH4+] falls slightly, but as [NH3] & [NH4+] >> [OH-], the ratio of [NH3]/[NH4+] remains roughly constant.
explain the pH of NH4Cl
NH4Cl <--> NH4+ + Cl- NH4Cl full dissociates NH4+ ions interact with OH- ions from the water, removing them from solution, therefore equilibrium shifts right therefore increasing [H+] , the acidity of solution increases so pH decreases therefore NH4Cl(aq) = acidic by hydrolysis
The student made the following statement: 'The pH of pure water is always 7.0' Is the student correct? Use the following information to justify your answer. H2O(l) <--> H+(aq) + OH- (aq) Kw = 1.0 x 10-14 mol2 dm-6 at 298 K delta H is positive for the forward reaction in the equilibrium
No, as T increases eqm moves to RHS / Kw increases So [H+] ions increases / more H+ ions [H+] > 1 x 10−7 pH decreases
what is nuclear shielding
Not all C-13/H-1 atoms in a molecule resonate at exactly the same frequency due to differences in magnetic field strength; this is due to nuclear shield - where e-s modify the magnetic field experienced by the nucleus. Nuclei with more e-s around them are better shielded from the external magnetic field and so they resonate at lower freq.s. Each peak in an NMR spectrum corresponds to 1+ atoms in a particular enviro.
suggest why e.m.f. of a hydrogen-oxygen fuel cell, operating in acidic conditions,exactly same as that of an alkaline fuel cell?
Overall reaction same(2H2 + O2 → 2H2O)
where does oxidation and reduction occur in an electrochemical cell
Oxidation at anode (-ve electrode), reduction at cathode (+ve electrode)
Tests for carboxylic acids
PCl5 - Steamy fumes Named metal carbonate (solution) - Effervescence
describe the features of a proton NMR spectrum
Peak: represent the number of hydrogen environments Peak integral: ratio of area under each peak which equal to number of hydrogen atoms in the environment -> divide by smallest, multiply by common denominator Peak at 0: protons in reference of tetramethylsilane Spin coupling pattern: number of adjacent protons Chemical shift: different external magnetic fields to bring them to resonance at particular radio frequency -> allows us to see what environment hydrogen is in
Why do peaks appear? What do they tell us?
Peaks appear due to characteristic fragments (e.g. C2/H5+ at 29) and difference between peaks also indicates loss of certain units - e.g. 18 for H2O or 28 for C=O. Simple HCs have peaks where C-C bonds break - notably 15 (CH3+), 29 (C2H5+), 43 (C3H7+), 57 (C4H9+)...abundance dep. on stability of carbocat formed: 3˚>2˚>1˚.
carboxylic acid to acyl chloride
Phosphorus (V) chloride reacts vigorously at room temperature. the reaction replaces the -OH group with a Cl atom form acyl chloride + POCl3 + HCl HCl forms steamy fumes in moist air
write a cell diagram for this cell using the conventional representation of: 2CO2(g) + 2e- <--> C2O42-(aq) 8H+ + MnO4- + 5e- <--> Mn2+ + 4H2O
Pt I C2O42-(aq) I 2CO2(g) II MnO4-, 8H+, Mn2+, 4H2O I Pt
write the conventional representation for an alkaline hydrogen-oxygen fuel cell
Pt|H2(g)|OH-(aq),H2O(l)||O2(g)|H2O(l),OH-(aq)|Pt
N2O4 (g) <--> 2NO2 (g) what would happen if the conc of N2O4 increases
Q does not equal K and Q < K as the value for the denominator in Q has increased, and the numerator is unchanged - the equilibrium composition changes so that the conc of NO2 increases and the conc of N2O4 decreases (numerator increases, denominator decreases) so that the equilibrium can be restored and Q can = k overall, the equilibrium shifts to the right so Q = k
acylation of benzene
RCOCl reacts with aluminium chloride to form an acylium ion which acts as an electrophile in the 2-step electrophilic substitution reaction heated under reflux in a non-aqueous solvent e.g. dry ether
The rate of the reaction was measured at several different concentrations of A, in the presence of a large excess of compound B and a constant amount of catalyst X, to find the order of reaction with respect to A. The results are shown on the graph below. Suggest an explanation, other than human error, for the two anomalous results circled on the graph
Rate higher than expected for the anomalous points on the graph Reaction is exothermic So there are more particles/collisions with energy > Ea
In industry, catalysts are used even though they are often expensive. State and explain one benefit to the environment result from the use of catalysts in industrial processes.
Reaction occur at lower temp/lower Ea/less fuel needed/fewer emissions/fewer raw materials needed/less natural resources used.
Hydrogen is manufactured by steam reforming of methane from natural gas. CH4(g) + H2O(g) U CO(g) + 3H2(g) ∆H = + 210 kJ mol−1 Reaction I occurs at a temperature of 1000 K and a pressure of 30 atm over a nickel catalyst Increasing the pressure on this heterogeneously-catalysed reaction I has very little effect on the rate of the reaction. Suggest why this is so.
Reaction takes place on surface of the catalyst Active sites/(catalyst) surface is saturated with reactants at the pressure of the reaction
what is the advantage and disadvantage of recrystallisation being repeated
Recrystallisation can be repeated more than once to ensure a very pure product, but each time the yield of product will decrease. Slow cooling results in bigger well defined crystals which are easier to filter and dry.
recrystallisation
Recrystallisation is used to purify impure solids The principle is that a hot solvent is used to dissolve both the organic solid and the impurities and then as the solution cools the solid crystallises out and leaves behind the impurities in the solution The key is using the minimum amount of solvent to dissolve the solid and avoid loss of the product If any solid impurities remain in the solution, a hot filtration can be carried out Once the solution has cooled down to room temperature and crystallised then the product crystals can be recovered by filtration This is faster using Buchner apparatus in which filtration occurs under reduced pressure
routes to make acidic buffer solutions
Route 1 Mixture of weak acid and one of its salts e.g. ethanoic acid + sodium ethanoate Route 2 Mixture of an excess of weak acid and a strong base e.g. excess ethanoic acid + sodium hydroxide
routes to make basic buffer solution
Route 1 Mixture of weak base and one of its salts e.g. ammonia + ammonium chloride Route 2 Mixture of an excess of weak base and a strong acid e.g. excess ammonia + hydrochloric acid
explain the function of a salt bridge?
SALT BRIDGE BALANCES CHARGE COMPLETE THE CIRCUIT PREVENTS/ALLOWS IONS TO FLOW BETWEEN SOLTIONS BUT PREVENTS THEM MIXING
difference between SN1 and SN2 mechanism
SN1 - only involves 1 molecule/ ion in RDS SN2 - involves 2 molecules/ions involved in RDS primary - only SN2 as lots of space around carbon to attack - not SN1 as carbocation formed = unstable tertiary - only SN1 - very little space around carbon to attack. also tertiary is stabilised by positive inductive effect SN2 - single, rate determining step - transition state forms
simple distillation
Simple distillation is a common practical completed in organic chemistry It is used as there are times that a reaction does not go to completion or there are other chemicals produced as well as the desired product Simple distillation allows you to separate compounds by their boiling point Chemicals with the lowest boiling point will distil first One of the most common simple distillation practical is the oxidation of primary and secondary alcohol to aldehydes and ketones
what is the iodine clock reaction?
Small amount of sodium thiosulpate solution and starch are added to an excess of hydrogen peroxide and iodide ions in acid solution H2O2 + 2I- + 2H+ --> 2H2O + I2 starch is added after the solution -> pale yellow (to avoid insoluble substances building up) the sodium thiosulfate added reacts instantly with the I2 that forms: 2S2O3- + I2 --> 2I- + S4O62- when the thiosulfate is used up, the iodine from reaction 1 will stay in solution therefore colourless -> blue black
how many molecules do the Sn1 and Sn2 nucleophilic substitution reactions involve in the rate determining step
Sn1 reactions only involve 1 molecule or ion in the RDS Sn2 reactions involve 2 molecules, 1 molecule and 1 ion, or 2 ions in the RDS primary halogenoalkanes only react by Sn2 secondary react by both tertiary only react by Sn1
in the iodine clock reaction what is the purpose of the sodium thiosulfate solution.
Sodium thiosulfate rapidly reacts with / reduces the iodine as it is formed So prevents the starch-iodine colour appearing until a fixed amount of reaction has occurred
process of column chromatography
Solution of sample added to column top; then solvent added slowly and continuously to run through the column. The substances in sample separate and emerge at different times from the column.
how to carry out steam distillation
Steam distillation is used to separate an insoluble liquid from an aqueous solution Steam is bubbled through a reaction mixture containing the aqueous solution and the insoluble liquid that forms a separate layer As the steam bubbles through the reaction mixture it mixes the layers so they form part of the evaporating liquid
optical isomers
Stereoisomers that are non-superimposable mirror images of each other
strength vs concentration
Strength is the extent of ionisation. Concentration is the amount in moles of acid in a cubic decimetre. It takes just as much NaOH to neutralise 25cm3 of a 0.1 moldm-3 solution of a weak acid as it does to neutralise 25 cm3 of a 1.0 moldm-3 solution of a strong acid.
uses of TLC
TLC can be used quickly to check that a chemical reaction is going as expected and making the required product. After attempts to purify a chemical, TLC can show whether all impurities have been removed.
state the advantages TLC has over paper chromatography
TLC runs faster the spots are usually spread out less the pates are more robust smaller amounts of mixtures can be separated
The kinetics of the fast reaction below were investigated in a series of experiments. A + B C + D ǻH is negative catalyst X (a) Outline a titrimetric method that could be used to measure the change in concentration of compound A with time. Compound A is an alkali, whereas compounds B, C and D are neutral
Take samples (of reaction mixture) at various times OR Using of different mixtures (e.g. in separate conical flasks) (1) THEN: EITHER Quench (with ice) / remove the catalyst (1) Titrate with acid of known concentration/standard (using a suitable indicator) (1) OR Quench with acid (1) Titrate with alkali of known concentration (using a suitable indicator)
Suggest why the addition of anhydrous magnesium chloride to water results in an increase in temperature and a decrease in volume.
Temperature increases because the reaction/process/dissolving/hydration of ions is exothermic. Volume decreases as shorter bonds between ion and water molecules
What does mass spec give you?
The Mr of a comp & can suggest likely structure for a compound from fragmentation peaks.
What does the m:z value of the M+ peak tell us?
The Mr of the mc, assuming that just one e- has been knocked off the mc.
Boiling point determination
The boiling point of a liquid is indicative of its purity and identity Boiling point is determined by distillation The sample is gently heated until it boils and this temperature is recorded This boiling point can then be compared against literature / database values If the sample contains impurities: The boiling point may appear higher than the literature / database values The sample may boil over a range of temperatures instead of at a single temperature
One example of an 'iodine clock' reaction that involves the iodate(V) ions and iodide ions in acidic solution is IO3 − (aq) + 5I − (aq) + 6H+ (aq) → 3I2(aq) + 3H2O(l) (a) State why the order of reaction with respect to iodide ions cannot be five, even though 5mol of iodide ions are shown in the equation. A series of experiments was carried out by a student to determine the order of reaction wrt iodate(V) ions. The total volume of the reaction mixture was kept constant by the addition of suitable volumes of deionised water state why the total volume should be kept the same
The chance of five or more ions colliding in the rate determining step is negligible b) So that the volume of iodate(V) ions is proportional to the concentration
what is reaction rate?
The change in the amount of reactants or products per unit time
standard enthalpy of neutralisation
The enthalpy change when an acid and an alkali react together under standard conditions to form 1 mole of water
enthalpy of atomisation
The enthalpy change when one mole of gaseous atoms is formed from an element in its standard state
iron (II) complex in haemoglobin containing a haem group = the multidentate ligand
The haem group is made up of a ring containing 4 nitrogen atoms. this means it's able to form 4 dative covalent bonds to the iron(II) ion. there are 2 other ligands bonded to the iron(II) ion - a protein called globin and either oxygen or water
advantage of steam distillation over normal distillation
The insoluble liquid distils at a temperature below its usual boiling point It reduces the chances of thermal decomposition of the insoluble liquid
Why is the mass spectrum of a molecule more complex?
The mass spec shows the presence of the mc ion radical (M+) peak and many peaks of lower m:z due to fragmentation of some of the mc ion radicals.
Melting point determination
The mp of a solid is indicative of its purity and identity A mp can be matched to a known substance as a means of identification or confirmation of a desired product The proximity of a mp to the actual data book value can express purity Impurities tend to lower the mp of a solid The mp range also reveals the degree of purity Pure substances have sharp well defined mp - Impure substances have a broad mp range The skills needed in performing a mp test are largely dependent on the specific melting point apparatus you are using However, there are some common key skills: Correctly preparing the melting point tubes Heating the tubes very slowly Repeating to get a range of measurements (3) The sample solid must be totally dry and finely powdered - this can be achieved by crushing it with the back of a spatula onto some filter paper or the back of a white tile (this absorbs any moisture) Use the first tube to find the approximate mp and then repeat using a much slower heating rate exam tip: Always quote a melting point as a range + or - and reference a data book value if you have one.
What does IR spec show you?
The presence of particular functional groups by detecting their characteristic vibration frequencies.
Suggest a reason why doctors and radiographers refer to MRI scanning rather than to NMR imaging, even though the technologies used in medicine and chemical research are essentially the same.
The public is generally nervous of 'nuclear technologies' because of the association in their minds of nuclear power with nuclear weapons or the bad press associated with the disposal of waste from nuclear power stations.
what happens to an acidic or basic buffer when you add water
The ratio of [CH3COOH] to [CH3COO-] remains constant and so the pH remains constant. same for a base
overall order of the reaction definition
The sum of the orders for all the substances that appear in the rate equation
Key points of 13C NMR
There is one signal for each set of equivalent C atoms, there is no coupling and the size of the signal is not relative to the number of equivalent C atoms (unlike H-1 NMR). Like H-1 NMR, the chemical shift is measured relative to TMS and although deuterated solvents are usually used, there will be a signal for any C atoms in the solvent.
Account for the fact that the mass spectrum of dichloroethene includes two peaks at m/z values 61 and 63 with intensities in the ratio of 3:1.
These 2 peaks are fragments formed by the loss of one chlorine atom leaving a single chlorine atom in the fragment, so the ratio of abundances corresponds to the ratio of the abundance of the isotopes
What is the significance of peaks at m/z = M and (M+2) in certain compounds.
These usually show presence of other isotopes, often of Cl/Br. E.g. C2H5Cl will have peaks at: m:z = 64 & 66 in a ratio 3:1.
why do strong acid - strong base reactions have similar enthalpies?
They all undergo the same reaction; H⁺(aq) + OH⁻(aq) → H₂O(l); due to full ionisation of the reactants
Why is there often a very small peak at m:z = (Mr + 1)?
This is due to presence of the 13C, which has a natural abundance of only 1.1%.
Why is the air inside the container of the TLC solvent saturated with vapour of the solvent before adding the TLC plate?
This keeps S phase from drying out before process is finished. The solvent evaporating from paper saturates chamber's air so it doesn't wick solvent off the S phase as quickly. If S phase dries out early, comps in sample won't separate properly -> incorrect results.
non-superimposable
This means that the molecules cannot be placed on top of one another and give the same molecule.
simple distillation process
To produce an aldehyde from a primary alcohol, a reaction mixture containing the primary alcohol and acidified potassium dichromate solution is placed into a pear-shaped or round bottomed flask Anti-bumping granules are added to promote smooth boiling Quickfit apparatus is then set up, including a still head and condenser connected to the side The joints of the Quickfit apparatus are often have a thin layer of silicon grease smeared over them to give a better seal as well as to make it easier to disassemble the equipment afterwards A Quickfit thermometer can be used, with the thermometer bulb sitting where the vapours will pass into the condenser A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser The reaction mixture is heated until it boils using a heating mantle Electric heating mantles are used for this because the temperature can be controlled, and because you are using chemicals which are flammable The distillate which forms in the condenser drips directly into a receiving vessel The distillate which should be collected, is that which is given off at +/- 2 oC of the boiling point of the desired product Some distillate may be given off below this temperature - this needs to be discarded and a clean vessel used to collect the desired product Stop collecting the distillate if the temperature rises above +/- 2oC of the boiling point of the desired product The aldehyde product has a lower boiling point than the alcohol (since it has lost the H-bonding) so it can be distilled off as soon as it forms
A further experiment was carried out to confirm the order of the reaction with respect to iodide ions. 2 mol dm-3 of (NH4 ) 2 S2 O8 was mixed with 0.024moldm-3 of KI to form a solution. The conc of I2 was measured at various times until the reaction was complete. (i) Outline a method, not involving sampling the mixture, which would be suitable for measuring the iodine concentrations in this experiment - stating how you would use your measurements to obtain iodine concentrations
Use a colorimeter (as iodine is the only coloured solution) Measure absorbance at various times Use a calibration curve to convert absorbance into concentration.
explain how vanadium (V) oxide acts as a catalyst in one step of the contact process equation for this step: 2SO2(g) + O2(g) <--> 2SO3 (g)
V changes its oxidation state from +5 to +4 as it oxidises the sulfur dioxide - Vanadium is reduced in the reaction with SO2 Then returns to +5 oxidation state by reacting with oxygen - 2 V2O4 + O2 --> 2 V2O5
following rate of reaction using gas volume
Vmax - Vt - on the y axis so it proportional to concentration if a gas is given off, collect it in a gas syringe and record how much you've got at regular time interval to find concentration of a reactant at each time point: - use the ideal gas equation to calculate moles of gas - and then use molar ratio to calculate conc
Explain why it takes the same amount of NaOH to neutralise the same volume and concentration of weak or strong acids.
Weak acids in solution are only partially ionised. However, on adding the alkali the hydrogen ions are neutralised and turned to water. As a result the equilibrium shifts to the right to form more H+ ions hence the same quantity of alkali is needed for both.
ways to improve preparing a solution of an acid
Weigh by difference or rinse weighing bottle and add to beaker Rinse beaker and add washings to graduated flask Invert flask several times to ensure uniform solution Use a funnel to transfer to the flask and rinse the funnel Use a stirrer to prepare the solution and rinse the stirrer
When added to a solution, an indicator gains or loses protons depending on the pH of the solution...
When added to a solution, an indicator gains or loses protons depending on the pH of the solution, a weak acid indicator as HIn: HIn(aq) ⇔ H+(aq) + In-(aq). Only a little of the In is added so it cannot affect the pH of the mix. The position of the equilibrium for the ionisation of the indicator is determined by the changing pH of the solution in the titration flask.
solvent extraction
When organic liquids are being prepared, water can often be obtained along with the organic product The water will usually form an aq layer with the product in the organic layer 1. add the impure compound to a separating funnel and add some water - shake well 2. add an organic solvent in which the product is more soluble than it is in water. shake the separating funnel well. the product will dissolve into the organic solvent, leaving the impurities dissolved in the water 3. you could add salt (e.g. NaCl) to mixture. this will cause the organic product to move into the organic layer, as it will be less soluble in the very polar salt and water layer 4. open the tap and run each layer off into a separate container It can sometimes be hard to identify which layer is the organic layer - this can be achieved by simply adding water and seeing which layer increases in volume Other organic reactions may need to be neutralised before being purified - by adding sodium carbonate solution to the reaction vessel or separating funnel This can also be used to remove other impurities by washing The contents of the reaction vessel are transferred to a separating funnel and a stopper added The separating funnel is inverted and the stopcock opened to release the pressure - this is repeated 15-20 times If neutralisation has occurred then the stopcock is opened slowly to avoid losing any product The 2 layers are allowed to separate
mobile phases
Where the molecules can move. gas or liquid pass through the stationary phase taking with it the components of the mixture
suggest the rate equation for: A + 2B + C --> D + E 1. A + B --> X + D 2. X + C --> Y slow 3. X + B --> E
X is an intermediate rather than a reactant/ catalyst. it is dependent on [A] and [B] so: rate = k [A][B][C]
Is a mass spectrum unique to a particular compound?
Yes. Each compound fragments in its own particular way producing what amounts to a fingerprint for that comp. Comparing its mass spec with a database of all known spectra can identify it.
why will the value of cell potentials decrease as the cell is used (use examples of zinc and copper electrodes)
Zn I Zn2+ II Cu2+ I Cu E0 cell = +1.1V - electrons flow from the negative zinc electrode --> positive Cu electrode - therefore Zn --> Zn2+ 2e- and Cu 2+ + 2e- --> Cu - so as current is drawn from the cell, the conc (Zn2+) increases and conc (Cu2+) decreases - electrode potentials are measured when a half cell is in equilibrium e.g., Zn2+ + 2e- <--> Zn E0 = -0.76V - an increase conc (Zn2+) will shift equilibrium to RHS making the E0 less negative as it becomes a worse reducing agent - the E0 of the copper (II) ion/copper half cell will become more negative as the conc (Cu2+) decreases - since the E0cell is determined by the difference between the 2 electrode potentials, the E0 ell value will decrease as the cell is used
what is an example of a linear complex used for aldehydes identification
[Ag(NH3)2]+ in tollens reagent
calculate the conc of methanoic acid with pH 4.02 and ka = 1.35 x 10-5
[H+] = 10-4.02 = 9.55 x 10-5 [HA] = [H+]^2 / ka = (9.55x10-5)^2 / 1.35 x 10-5 = 6.76 x 10-4 mol dm-3
definition of neutral
[H+] = [OH-]
explain why the pH of a 1 x 10^-8 moldm-3 solution of nitric acid is not 8
[H+] is √1x10-14 = 1 x 10^-7 so total [H+] is > 1x10^-7 or: -pH can't be more than 7 for an acid conc of H+ from H2O can't be ignored
ligands forming bonds using lone pairs
a
bases as proton acceptors - Bronsted-Lowry theory
a base is a molecule which can accept a hydrogen ion from an acid. a base has a lone pair of electrons which form dative covalent bond with a proton
crystalline derivative
a chemical which can be purified by recrystallisation and then identified by measuring its melting temperature
What is an ammine?
a complex in which 1 or more ammonia molecules are co-ordinately bonded to a metal ion
what is standard lattice energy a measure of
a measure of ionic bond strength. the more negative the lattice energy, the stronger the bonding
definition of a complex ion
a metal ion surrounded by dative covalently (co-ordinately) bonded ligands
racemic mixture
a mixture containing a 50/50 mixture of 2 enantiomers of a chiral compound
2 types of chromatography
a mobile phase: molecules can move - either liquid or gas a stationary phase: molecules can't move - solid or liquid on a solid support
what does the energy gap correspond to?
a particular frequency of radio waves
spontaneous reaction
a reaction which occurs without being driven by any external agency
definition of a mechanism
a series of steps through which a reaction progresses, showing bonds breaking and forming and often forming intermediate compounds
SN2 mechanism
a single enantiomer reactant produces a single enantiomer product step 1: the nucleophile always attacks the opposite side to the leaving group so 1 product is produced. the product will rotate plane-polarised light differently to the reaction, the extent and direction of rotation occurs can be measured experimentally
what is ethyl ethanoate used as
a solvent in chromatography
what are different absorptions measured relative to
a standard substance (used to calibrate) - tetramethylsilane (TMS) it produces a single absorption peak in both types of NMR as all its carbon and hydrogen nuclei are in the same environment it is chosen as standard as the absorption peak is at a lower frequency than most - approx 0 ppm all other peaks in other substances are measured as chemical shifts relative to this
what are lead-acid cells?
a storage cell that can be used to provide power for vehicles etc. when 6 are joined together
acyl chlorides to carboxylic acid
a vigorous reaction with cold water, producing carboxylic acid
a) How many peaks in carbon-13 NMR, would you expect in a) propane, b) 2-bromobutane, c) 2-methyl butan-2-ol. b) Methyl ethanoate has peaks at ~170, ~50 and ~20 ∂/ppm. Assign these peaks to the appropriate carbons. c) Peak spectra for butanone
a) A) 2 peaks, b+c) 4 peaks b) 170 = -C=O, 50 = -OCH3, 20 = CH3C=O c) ^
when you are finding out how much iron is in a sample, what happens to the Fe ions? what is the oxidising agent? what happens if the iron is not in the Fe2+ state
a) Fe2+ --> Fe3+ + e- oxidised by an oxidising agent b) kMnO4 (acidified) or K2Cr2O7 (goes purple) MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O or Cr2O72- + 14H+ + 6e- --> 2Cr3+ + 7H2O if Fe: react with H2SO4 to oxidise to Fe2+ if Fe3+: react with Zn to reduce to Fe2+ -> remaining zinc -> removed to stop reaction
'Bromine clock' involves a reaction between bromide ions and bromate(V) ions in acid solution: 5Br - (aq) + BrO3 - (aq) + 6H+(aq) l 3Br2(aq) + 3H2O(l) To monitor this reaction, phenol and methyl orange are added to mixture. - A small vol of phenol is present which reacts immediately with Br2 as it is produced, thus removing it from solution. - Once Br2 produced has reacted with all of the phenol , then any further Br2 produced will bleach the methyl orange solution providing a means to monitor the reaction rate. a) it is assumed that initial rate of reaction is proportional to 1/time taken for methyl orange to be bleached. Why is it essential to have a small vol of phenol compared to reactants for this assumption to be valid? b) A series of experiments was carried out where only the conc of bromide ions present was varied and the solution contained a large excess of BrO3 − and H+ ions. Total vol of the mixture was kept constant. Why was it important that the solution contained a large excess of BrO3 − and H+ ions?
a) So that the phenol is used up / methyl orange is bleached before the rate changes (significantly) b) So that only the concentration of bromide ions varies significantly during the course of the reaction
Stationary phase plus benefits of TLC
a) Stat phase is solid silica/alumina supported on a glass/plastic plate. It is quick, cheap, only needs small sample.
The pH of 0.100 mol dm-3 sulphuric acid is 0.98. (i) Calculate the concentration of hydrogen ions, H+ , in this solution. Write equations to show the two successive ionisations of sulphuric acid, H2SO4, in water Suggest why the concentration of hydrogen ions is not 0.20 mol dm-3 in 0.100 mol dm-3 sulphuric acid
a) [H+ ] = 0.10 / 0.1047 / 0.105 b) H2SO4 <→ H+ + HSO4 - HSO4 <-> H+ + SO4 2- must be reversible c) second ionisation suppressed by the first ionisation
a) What type of bond links the water molecule to a proton in an oxonium ion? b) predict the shape of the oxonium ion
a) dative covalent bond b) trigonal pyramidal
a) why are polystyrene cups used in enthalpy changes in solution? b) what does this mean? c) if the solutions are dilute... d) what thermometer needs to be used
a) good insulator - negligible specific heat capacity b) if the reaction is exothermic, the energy released can't enter surroundings so it heats up the solution - opposite with endothermic c) it is accurate to calculate the enthalpy changes by assuming the solutions have the same density and specific heat capacity as water d) graduated 1/10 degrees Celsius as changes are often small
explain why some a) solids, b) liquids, c) gases have higher entropy than (a,b,c) standard
a) heavier and larger atoms can vibrate > freely and share out their energy in more ways therefore have higher entropy b) molecules with more atoms have a higher entropy as they can vibrate, rotate and arrange in more ways c) same as b
state and explain if the value of the potential for this half equation would become > or < than 0V for standard hydrogen electrode: a) use a piece of platinum with a greater surface area b) use H2SO4(aq) with conc 0.5mol dm-3 c) use HCl(aq) with conc of 2 moldm-3
a) no effect, allows for faster rate of transfer of electrons but has no effect on potential b) no effect as conc of H+ = 1 mol dm-3 c) positive, equilibrium shifts right to lower conc of H+
what does it mean if delta Stotal is a) positive b) zero c) negative
a) positive: the reaction is feasible at that temp (will occur spontaneously) b) 0: reaction is at equilibrium at that temp c) negative: reaction is not spontaneous/ feasible at that temp
what does it mean if the kc value is: a) large (kc = 100) b) small (kc = 0.01) c) closer to 1 d) very large (kc > 1x10^10) e) very small (kc < 1 x 10^-10)
a) reaction reaches an equilibrium in which the products are in a higher conc (equilibrium shifts RHS) b) reaction reaches an equilibrium in which the reactants are in a higher conc (equilibrium shifts LHS) c) roughly equal amounts of reactants and products d) reaction goes to completion - no equilibrium e) reaction does not go
write a possible mechanism for: a) A + 2B --> C + D rate = k[A][B] b) A + 2B --> D + E rate = k[A][B]^2
a) step 1: A + B --> P (RDS) step 2: P + B --> C + D b) 2B --> X A + X --> D + E (RDS)
Two half cells are joined together. Each half cell contains copper nitrate solution. The standard potential for Cu2+/Cu is +0.34 volts. Cu(s)⏐ Cu2+ (aq) ⏐⏐ Cu2+(aq)⏐Cu(s) In the left hand half cell the copper nitrate has a concentration of 1.00 mol dm-3 (with a potential of +0.34 V) and in the right hand half cell the concentration is 0.500 mol dm-3 . a) Will the potential of the right hand half cell be the same as +0.34 V, or more or less than this value. Explain your answer. b) Which electrode will be the anode? c) Which way will electrons flow in the cell? Explain your answer.
a) the equilibrium Cu2+(aq) + 2 e- ⇌ Cu(s) shifts left if concentration of Cu2+ is 0.500 mol dm-3 and so E° becomes less than +0.34 V b) right hand electrode is anode c) from right hand side to left hand side
a) when might a substance be insoluble b) what is an exception to this? what explains why some ionic solids dissolve in water and others don't
a) the lattice energy (endothermic) is much larger than the hydration enthalpy (exothermic) therefore, its not energetically favourable to dissociate the ions so: ΔHsol = endo b) NaCl, whose ΔHsol = endothermic is soluble - entropy of the system increases when a solute dissolves, as it becomes > disordered as the ions spread out and number of moles increases - when the ions are hydrated, water molecules become more ordered as they arrange themselves around the +ve and -ve ions therefore entropy of H20 decreases so water becomes > ordered with highly charged, small ions
hydrolysing esters with acid
acid hydrolysis splits the ester into a carboxylic acid and alcohol you reflux the ester with a dilute acid e.g. HCl
diprotic acid
acid that releases 2 H+ ions per molecule e.g. H2SO4 and H2C2O4
monoprotic acid
acid that releases one H+ ion per molecule e.g. HCl, HNO3, CH3COOH
conjugate acid-base pair
acid turns into its conjugate base when it loses a proton a base turns into its conjugate acid when it gains a proton in the example, there is a competition for protons between ammonia molecules and water molecules. on the RHS of the equation, the H+ ions are held by lone pairs in the ammonia molecules. on the LHS, they are held by lone pairs on water molecules the position of equilibrium shows which of the 2 bases has the stronger hold on the protons
how is chromium hydroxide amphoteric?
acts as both acid and base if you add excess NaOH to a chromium hydroxide ppt, the H2O ligands deprotonate, and a solution containing [Cr(OH)6]-3 (aq) forms. if you add acid to the chromium hydroxide ppt, the OH- ligands protonate and a solution containing [Cr(H2O)6]+3 (aq) forms the reaction above are not ligand exchanges. instead, they are acid-base reactions - the ligands are chemically modified by the acid or the alkali (by the addition/ removal of H+) but, if you add excess ammonia to the chromium hydroxide ppt, a ligand exchange occurs
Grignard reagent to carboxylic acid
add Co2 it occurs in 2 steps: 1. in dry ether, we bubble carbon dioxide in Grignard reagent 2. add dilute acid to solution e.g. bromopropane --Mg and dry ether --> CH3CH2CH2MgBr --1) CO2, dry ether 2) dilute HCl --> CH3CH2CH2COOH + MgBrCl
when finding the kc of the equilibrium between ethanoic acid, ethanol and ethyl ethanoate and water, what should you do to the tubes containing equilibrium mixture
add a stopper so that losses of evaporation are prevented no water vapour gets into the flask no ethanol is oxidised - keep at constant temp for 48 hours measure equilibrium after a week and kc should be the same repeatedly if it has reached equilibruim
acyl chlorides to ester
add alcohol a vigorous reaction at room temperature, producing an ester (irreversible and faster way to produce an ester than esterification)
acyl chloride to N-substituted amide
add amine a violent reaction at room temperature, producing an N-substituted amide
acyl chlorides to amide
add conc ammonia a violent reaction at room temperature producing an amide
chlorination of 2-methyl propan-2-ol how do you form the crude product?
add concentrated HCl and 2-methylpropan-2-ol to conical flask with bung and swirl open bung to release pressure and repeat regularly for 20 mins add anhydrous CaCl2 and shake 2 layers will form upper = organic product lower = aqueous layer
chlorination of 2-methyl propan-2-ol how do you remove the unreacted HCl and water
add solution of NaHCo3 and swirl. invert and open tap to release pressure a few times remove stopper and run off aqueous layer and run organic layer into clear conical flask to remove water add anhydrous Na2SO4 as drying agent
Le Chatelier's Principle and buffer solutions
adding a little strong acid temporarily increases the conc of H+ so at that instant the system isn't at equilibrium and shifts to left side to counteract. adding a little strong alkali temporarily decreases the conc of H+ so reaction shifts right
compare boiling point of aldehydes and alcohols
alcohol is higher more london forces as 1 more H same permanent dipole dipole forces alcohol -> H bonding
what is the difference between a base and an alkali
alkali is a solution of the base which is completely soluble in water
how is the reaction of alkenes vs benzene with bromine different?
alkene: can occur with just bromine water (no catalyst) - the pi electrons induce a dipole in the bromine molecule and the delta+ bromine atom is sufficiently strong electrophile for the reaction benze: more stable so the induced dipoles and the delta + bromine atoms = not sufficient electrophilic to react with benzene so for the reaction to occur, the 1st step has to be to produce a stronger electrophile using a catalyst to enable the electrophilic substitution to occur. --> involves the replacement of a H atom following attack by an electrophile
explain the solubility of amines
all amines can form hydrogen bond with h2o therefore soluble -> even tertiary ones, using just the lone pair amines with short hydrocarbon chains = very soluble
amino acid structures
all amino acids that occur in proteins, except glycine, have a central carbon atom attached to 4 different groups so they have a chiral centre. as a result of their chirality, the separate (+) and (-) isomers of all naturally occuring amino acids can rotate the plane of plane-polarised light
why is heating under reflux used
allows heating for a long period of time prevents flask from boiling dry prevents volatile reactants/ products escaping - main point ensures even heating
deduce 1 essential property of non-reactive porous separator?
allows ions to pass
can aldehydes and ketones form hydrogen bonds with water? what does this mean for dissolving
although no polar -OH group, O atom has lone pair of electrons on C=O bond so form hydrogen bonds with H in water. so small carbonyls will dissolve in water but long carbon chains cannot form hydrogen bonds as they disrupt the formation so won't dissolve
polyamides
amides bonded by an amide bond. same as the amide bond in proteins. the first synthetic and commercially important polyamides were various forms of nylon. these were not produced from amino acids but between diamines and dicarboxylic acids. the most common is nylon-6,6. polypeptides are naturally occurring polyamides
the preparation of amides
amides form rapidly at room temperature when acyl chlorides react with ammonia or amines. E.g. when ethanoyl chloride is added to conc ammonia (aq), a vigorous reaction takes place producing fumes of hydrogen chloride and ammonium chloride and residue of ethanamide
amine reaction with water
amines act as bronsted-lowry bases by removing protons from water to form alkaline solution containing OH- ions reversible reaction so alkyl amines are also weak bases.
ammonia reaction with water
ammonia acts as a brosted-lowry base and a small proportion of the dissolved molecules react with water to form a weakly alkaline solution containing OH- ions
ammonia + water
ammonia doesn't produce OH- ions directly. it reacts with water first and accepts a proton to produce ammonium ions and OH- ions
ammonia + acid
ammonia reacts with acid to make ammonium salts but NO water produced 2NH3 + H2SO4 -> (NH4)2SO4
Another buffer solution was formed by mixing 20.0 cm3 of sodium hydroxide solution of concentration 0.100 mol dm-3 with 25.0 cm3 of ethanoic acid of concentration 0.150 mol dm-3. CH3COOH + NaOH r CH3COONa + H2O Calculate the pH of this buffer solution. [Ka for ethanoic acid = 1.74 × 10-5 mol dm-3]
amount of NaOH = amount of salt formed =0.100 x 20.0/1000 = 0.00200 initial amount of acid = 0.150 x 25.0/1000 = 0.00375 amount of acid left = 0.00375 − 0.00200 = 0.00175 total volume = 20.0 + 25.0 = 45.0 (cm3 ) [salt] = 0.00200 x 1000/45.0 = 0.0444 (mol dm―3) [acid] = 0.00175 x 1000/45.0 = 0.0389(mol dm―3) Ka = [H+][salt]/[HA] so [H+] = Ka[acid]/[salt] [H+] = 1.74 x 10―5 x 0.0389/0.0444 = 1.52446 x 10―5 (mol dm―3) pH = −log[H+] = −log(1.52446 x 10―5 ) = 4.817
What does the Bronsted-Lowry theory state?
an acid is a hydrogen-ion donor and a base is a hydrogen-ion acceptor
What is an N-substituted amide?
an alkyl group is substituted for one of the hydrogen atoms of the -NH2 group
what is a quaternary ammonium salt
an ammonium salt where all 4 hydrogens are replaced by alkyl or aryl groups
What is NMR (nuclear magnetic resonance) spectroscopy?
an analytical technique that you can use to work out the structure of an organic molecule
phenol with bromine water
an aq solution of phenol reacts with bromine water to produce an immediate white ppt of 2,4,6-tribromophenol as the orange bromine colour fades. at room temperature 2,4,6tribromophenol is insoluble in water and smells of antiseptic
what is a ligand
an atom, ion or molecule that donates a pair of electrons to a central metal atom or ion. a ligand must have at least one lone pair of electrons, otherwise it won't have anything to form a dative covalent bond with
what is Kp
an equilibrium constant that you can calculate dealing with equilibria involving gases instead of concs. put partial pressures in expression
what happens when an indicator is added to a solution
an indicator gains or loses protons depending on the pH of the solution. indicator changes colour when conc of HIn (weak acid) = conc of In-
what is a zwitterion
an ion with both a positive and a negative charge.
complexes with six-fold coordination
an octahedral shape bond angles are all 90 degrees
during step 1 Sn2, the 3 groups that remain attached to the central C atom are turned inside out and the whole molecule is inverted - what does this mean
an optically active halogenoalkane gives rise to an optically active alcohol if substitution takes place by the Sn2 mechanism
how to make a reaction rate graph to go to equilibrium and what does the gradient of the graph measure
analyse results of the experiment to plot a concentration-time graph the gradient of the graph gives a measure of the rate of reaction at that time - change in Y/ change in X
aramids
aramids e.g. kevlar is strong, rigid and lightweight with extensive hydrogen bonds kevlar is a polyamide with aromatic hydrocarbon sections
why do most exothermic reactions tend to go?
around room temp, the value of -deltaH /T is much larger and more positive than delta S sys so deltaS total is more positive
Why do most exothermic reactions tend to go?
around room temp, the value of -deltaH/ T is much larger and positive than delta Ssys so delta Stotal is more positive
why can water be left out of the acid dissociation expression for water
as H2O is present in excess
effect of temperature on pH of water
as temperature increases, the equilibrium moves right to oppose the increase, therefore [H+] and [OH-] increase therefore Kw increases and pH decreases as H+ ion conc increases
why does a potential difference in build up in half-cells
as the substances in the half-cells are oxidised or reduced, the potential difference builds up, due to the difference in charge between the electrode and the ions in solution
when doing buffer calculations what can you assume?
as the weak acid = slightly ionised + salt = fully ionised assume: - all molecule, HA come from added acid - all - ions, A- come from added salt
how to work out a theoretical lattice energy
assume ions are spherical and have their charge evenly distributed around them - a purely ionic lattice work out how strongly the ions are attracted to each other based on their charge, distance etc. that gives you a value for energy change when ions form the lattice
when can amino acids form zwitterions
at a particular pH. if the pH is too high, the OH- ions remove H+ ions from the zwitterions, forming negative ions. if the pH is too low, the H+ ions react with zwitterions, producing positive ions. amino acids can therefore exist in 3 forms depending on pH: cation, zwitterion and anion form. notice from image that the net charge on an amino acid molecule varies with pH. the net charge is +ve in acid solution and -ve in alkaline solution. at isoelectric point, the + and - charges balance and net is 0
why is H2O removed from the equation for the ionic product of water?
because [H2O(l)] is much bigger than the concs of the ions, we assume its value is constant and make a new constant Kw
compare the pH of a strong acid after dilution 10, 100 and 1000 times
because pH is a logarithmic scale, diluting a strong acid 10 times will increase its pH by one unit, and diluting it 100 times would increase its pH by 2 units
why do nuclei in different parts of the molecule experience slightly different magnetic field?
because they are shielded to a lesser/ greater extent from the field applied by the tiny magnetic fields associated with the electrons of neighbouring bonds and atoms
CH3COOH + H2O -> CH3COO- + H3O+ why does only the proton from the carboxylic acid group and not the methyl group get donated to a water molecule?
because when you loose the H from the OH, the CH3COO- ion is stabilised by delocalisation of the charge would be unable to stabilise by delocalisation if the H was lost from the carbon
example of Friedel-crafts reaction to form methylbenzene
benzene refluxed with chloromethane and aluminium chloride - substitution
forming a phenylketone
benzene refluxed with ethanoyl chloride and aluminium chloride catalyst
hydrogenation of benzene
benzene to cyclohexane reagent: hydrogen conditions: nickel catalyst at 200 degrees at 30 atm important industrially in the manufacture of cyclohexane which is used to make nylon
explain the similarities of the reactions of benzene and alkenes?
both have electron dense regions (delocalised ring and single localised pi bond) therefore they both react with electrophiles in many of their reactions
why are enthalpy changes of hydration and lattices energies exothermic processes?
both involve electrostatic attractions between opposite charges
hydrolysis of proteins
breaking peptide bonds by refluxing with concentrated HCl, the product contains the cation forms of the a-amino acids. these are converted to the a-amino acids on dilution with water. after hydrolysis, the mixture of amino acids produced can be separated and identified by chromatography
chlorination of benzene
bubble chlorine through benzene at room temperature in presence of a catalyst (FeCl3 or AlCl3) catalyst equation: 2Fe + 3Cl2 -> 2FeCl3 - made in situ both the catalysts are covalent molecules that are electron deficient enabling them to act as a Lewis acid (electron pair acceptor)
water + butanoic anhydride
butanoic acid
State why the electrode potential for the standard hydrogen electrode is equal to 0.00V.
by definition
how is the initial rate of the iodination of propanone found
by following a continuous monitoring titrimetric method CH3COCH3(aq) + I2(aq) --(H+)--> Ch3COCH2I(aq) + H+(aq) + I-(aq) you can monitor the reaction by taking samples at regular intervals you 1st need to stop the reaction in each sample by adding sodium hydrogen carbonate to neutralise the acid then titrate each sample against sodium thiosulfate and starch to work out the conc of iodine you'll need to carry out the experiment several times and in each change the conc of just 1 reactant
how do antibumping granules produce smooth boiling
by forming smaller bubbles and preventing larger bubbles by providing a surface for bubbles to form on
To what extent is the following statement true? 'Adding a catalyst to make a reaction go faster can increase the amount of product at eq.' (3)
c) Adding a catalyst does not affect the value of Kc. [1] At a constant temperature, the yield of product at equilibrium is not changed by adding a catalyst [1]; however, many processes are so slow that they never reach equilibrium in the absence of a catalyst, so in that sense an added catalyst increases the yield by enabling the reaction mixture to reach equilibrium. [1]
what does amphoteric mean
can act as an acid or a base
nylon-6,6
can be produced more readily in the lab using hexanedioyl dichloride in place of the less reactive hexanedioic acid. a solution of hexanedioyl dichloride in cyclohexane is floated on an aq solution of 1,6-diaminohexane
solubility of carboxylic acids
can form hydrogen bonds with each other so have high boiling points. they can also form hydrogen bonds with water as the chain length increases the solubility decreases as it cannot form hydrogen bonds and cannot dissolve
what are Grignard reagents used to help
carbon-carbon bond formation
when reacting mild oxidising agents Fehlings, Benedicts and tollens act as in the alkaline conditions when oxidising the aldehyde to a carboxylic acid - the carboxylic acid is present as what in solution
carboxylate ions (they are formed when a carboxylic acid molecule loses a proton
carboxylic acids and bases to form salts
carboxylic acids are neutralised by bases (aq) to form salt and water e.g. ethanoic acid + NaOH -> sodium ethanoate + H2O
structure of amide and what are they derived from
carboxylic acids with the -NH2 group replacing the -OH group
how a catalytic converter works
catalytic converters are used in cars to reduce emissions of nitrogen monoxide and carbon monoxide produced by internal combustion engines. they use a platinum or rhodium catalyst to convert these gases into nitrogen and carbon dioxide.
the role of aluminium chloride on the Friedel-Crafts reaction
catalytic role in creating the electrophiles which attack benzene. so, when chloromethane is mixed with aluminium chloride, molecules remove Cl- ions from polar CH3-Cl molecules, allowing reactive carbocations, CH3+ ions, to act as electrophiles these reactive CH3+ electrophiles then attack the delocalised pi system of benzene molecules to form and intermediate cation, which breaks down producing methylbenzene and H+ ions Finally, AlCl3 catalyst is regenerated as H+ ions released in electrophilic substitution react with AlCl4- ions: H+ + AlCl4- -> HCl + AlCl3
what happens to the cell potential value if we change equilibrium position
cell potential changes too this is why we use standard conditions to measure electrode potentials so we can compare values
why are weak acid - weak base titrations not performed?
change in pH is not marked over a small volume making it hard to determine the end point of the titration
Rate of reaction and equation
change in the concentration of reactants or products per unit time. or something proportional to concentration
titration: weak acid - weak base and strong acid - strong base
change of pH around the equivalence point is gradual and not very marked meaning it is hard to fix the end-point precisely. if the dissociation constants for the weak acid and weak base are approx equal, the salt formed at equivalence point is neutral and pH=7
why are transition elements useful as oxidising and reducing agents
changing oxidation number so they can readily give out or receive electrons
what effects enthalpy of hydration?
charge on ions - ions with a higher charge attract water molecules greater so the electrostatic attraction is stronger. - more energy is released when the bond is made so they have a more exothermic enthalpy of hydration size of ions - smaller ions have higher charge densities so they attract water molecules more - more energy is released when the bond is made so they have a more exothermic enthalpy of hydration
how are optical isomers found in nature?
chiral molecules are found as pure enantiomers in nature
what is an optical isomer a result of
chirality in molecules with a single chiral centre in a molecule with asymmetric carbon atoms
what does chromatography involve?
chromatography involves a mixture being dissolved in a mobile phase (which could be liquid/gas) that is passed through a stationary phase (usually a solid) the phases are chosen so that components in the mixture have differing interactions in each phase the balance of these 2 factors determines the rate of movement of a component which is recorded as either an Rf value or retention time and used in a component's identification
oxidation numbers of chromium
chromium exists in compounds in many oxidation numbers. they +3 state is most stable, followed by +6 and then +2 chromium forms 2 ions with oxygen in the +6 oxidation number - chromate(VI) ions, CrO4-2, and dichromate(VI) ions, Cr2O7-2 - these ions are good oxidising agents as they easily reduce to Cr3+ ions when Cr3+ ions are surrounded by 6 water ligands, they are violet. but water ligands are usually substituted with impurities in the water e.g. Cl-. this makes the solution look green.
square planar and octahedral complex ions showing cis/trans isomerism
cis isomers have the same groups on the same size trans have the same groups opposite each other
conditions for dynamic equilibria
closed system and reversible reaction
tollen's reagent
colourless solution of silver nitrate dissolved in aqueous ammonia. when heated with an aldehyde, a silver mirror forms. don't heat test tube over direct flame as most organic compounds are flammable so use water bath - tests positive for aldehydes
how to work out the voltage produced by energy storage cells (batteries) using the electrode potentials of the substances in the cell
combine 2 half-equations in the feasible direction (electrode potential is positive) this involves switching reaction with less positive electrode potential around e- and OH- are cancelled out to calculate cell voltage use same formula for cell potential Ereduction - E oxidation
combustion of benzene
combusts with a sooty flame due to higher carbon to hydrogen ratio - unlike an alkane
what is a chelate
complex ions involving multidentate ligands
what are amino acids
compounds that contain an amino group -NH2 and carboxylic acid group -COOH. amino acids are the monomers that make up proteins they are amphoteric
what factors do not have an affect of values of Kc or Kp
concentration and pressure - they change the amount of products and reactants present at equilibrium but ratio stays the same
When does Q = K?
concs need to be in the same ratio (at equilibrium)
when may predictions using Electrode potential be wrong
conditions are not standard e.g. change in conc. or temp. as the equilibrium will shift e.g. for Zn/Cu cell increasing conc of Zn2+, equilibrium will shift left, reducing ease of electron loss of Zn. the electrode potential of Zn/Zn2+ becomes less negative and the whole cell potential will be lower vice versa for increasing conc of Cu2+ or: when the reaction kinetics are not favourable. the rate of reaction may be so slow that the reaction might not appear to happen or if the reaction has a high activation energy
2,4-DNP test
conditions: room temperature orange solution to orange ppt positive test for aldehydes and ketones
ionic crystals
consist of giant lattices containing positive and negative ions packed together in a regular pattern e.g. Na+ and Cl- ions. there are strong net electrostatic attractions between ions in all directions throughout the lattice - ionic bonding
where are transition metals in the periodic table?
d block
definition of transition metals
d-block elements that can form one or more stable ions with incompletely filled d-orbitals
butylamine with copper (II) sulfate aq
deep blue solution forms. 4 butylamine molecules replace 4 water ligands.
what is entropy?
definition: a measure of the disorder of a system and that the natural direction of change is increasing total entropy (positive entropy change 1. a measure of disorder of a system - tells you the no of ways that particles can be arranged and the no of ways that the energy can be shared out between particles 2. the more disordered the particles, the higher (more positive) the entropy number of atoms physical state number of electrons - more ways of distributing quanta of energy
alcohols to alkenes
dehydration in elimination reaction concentrated phosphoric acid heat mixture to 180 degrees
why might a substance be insoluble? what is an exception to this? what explains why some ionic solids dissolve in water and others don't
delta Hsol = delta Hlatt + delta Hhydr - the lattice enthalpy (endothermic) is much larger than the hydration enthalpy (exothermic) - therefore its not energetically favourable to dissociate the ions so delta Hsol = endo b) - NaCl, whose delta Hsol = endothermic is soluble - entropy explains why some substances dissolve if delta Hsol = endothermic - entropy of a system increases when a solute dissolves as it becomes more disordered as the ions spread out and number of moles increases - when the ions are hydrated, water molecules become more ordered as they arrange themselves around the positive and negative ions. therefore, entropy of water decreases so water becomes more ordered with highly charged and small ions. - this can outweigh the disorder from breaking down the lattice - so delta Ssys = negative therefore delta Ssurr needs to be positive therefore delta H needs to be negative (exothermic) so that delta Stot = positive - so in the case of small and highly charged ions, the delta Hsol needs to be exothermic but with large and small charged ions, the balance is such that the delta Hsol can be endothermic.
2SO2(g) + O2(g) ⇌ 2SO3(g) delta H = -196kjmol-1 explain, in terms of entropy, why the equilibrium shifts to the right when temp reduces why would you not operate at high pressures for this reaction
deltaS surr increases as T decreases because delta S surr = -deltaH/T so deltaS total increases as temp decreases so reaction becomes more favourable maintaining high pressure is expensive equilibrium conversion to SO3 must be very large as K is so large overall yield can be increased more cheapy by recycling unreacted SO2 + O2
factors that affect retention time
depends on time taken of the component spends moving along with the carrier gas time taken being stuck to the viscous liquid
dichromate(VI) ions reduced to Cr3+ Cr3+ reduced to Cr2+ in further reduction
dichromate(VI) ions can be reduced using a reducing agent such as zinc and dilute acid zinc will reduce Cr3+ to Cr2+. you will need to use an inert atmosphere - Cr2+ is so unstable that it oxidises straight back to Cr3+ in air
polyesters
diols and dicarboxylic acids can react together to form long ester, chains - a polyesters a condensation polymerisation
explain the intermolecular forces of amines and the properties
dipoles: due to lone pair on N H bonding (N-H) on primary and secondary but not tertiary as no N-H therefore can't hydrogen bond with themselves primary amines have highest bp as 2 hydrogens attached to N
chlorination of 2-methyl propan-2-ol how do you purify the product
distillation - separating the products on boiling point collect liquid boiling between 50-52 degrees
when diluting solutions, why does the water have to be boiled?
distilled H2O, contains dissolved Co2 which affects pH boiling removes gases as solubility decreases as temperature increases
tertiary amine reacting to form a quaternary ammonium salt
do this with excess halogenoalkane
Kc
don't include solids or pure liquids in Kc expression in heterogeneous equilibria as their concs stay constant throughout reaction if you have a very dilute solution and water is not a reactant, the conc of water is constant as it is in so much excess
explain arenes and water/ other solvents
don't mix with water, do mix with non-polar solvents
equation for the entropy change of a system
during a reaction there's an entropy change delta S, between reactants and products - the entropy change of the system entropy of products - reactants
equivalence point
during a titration is reached when the moles alkali equals the moles of acid according to the balanced equation
ionic oxide reacting with water
e.g. calcium oxide completely reacts with water to form calcium hydroxide.
complexes with four-fold coordination resulting in square planar shape.
e.g. cis-platin bond angles are 90 degrees cis-platin is a complex of platinum(II) with 2 chloride ions and 2 ammonia molecules and is an anti-cancer drug. the 2 chloride ions are next to each other, but if they were opposite each other you would have trans-platin, which is toxic
what can you do to the Arrhenius equation straight line graph if all concentrations are kept the same?
e.g. iodine clock reaction rate = s^-1 (1/t) therefore rate is proportional to 1/time so 1/t is used instead of k so plot graph of 1/T (k^-1) in x axis and (1/t) on y axis. gradient still = -Ea / R
how can components be identified in gas chromatography?
each separate substance will have a unique retention time so the retention time can be used to identify the components of the mixture you have to run a known sample under the same conditions for comparison
what is a benefit of using solid heterogeneous catalyst when making ibuprofen?
easier to recover/ separate through filtration Facilitates the use of flow (rather than batch) systems
suggest why recharging of lithium cell may lead to release of CO2 into atmosphere?
electricity for recharging cell may come from burning fossil fuels
storage cells definition
electrochemical cell that is based on reversible chemical changes so that it can be recharged by an external electricity supply
how to work out how polar a covalent bond is
electronegativity is the ability of an atom to attract the bonding electrons in a covalent bond. Pauling scale is used to measure the electronegativity of an atom. the greater the difference in electronegativity, the greater the shift in electron density, the more polar the bond. bonds are polar if the difference is more than 0.4
bromination of benzene
electrophilic substitution first step involves use of a catalyst to produce a stronger electrophile. when benzene is warmed with bromine in the presence of iron filings, the reaction is: 2Fe(s) + 3Br2(l) --> 2FeBr3(s) which polarises the bromine molecules to form Br+ ion which acts an electrophile. What does FeBr3 act as - a HALOGEN CARRIER
what does the possibility of a reaction taking place depend on?
entropy the enthalpy temperature
conditions affecting the value of the electrode potential
equilibrium position in a half-cell is affected by changes in temperature, pressure and concentration changing the equilibrium position changes the cell potential so standard conditions are used
oxidation of ethanol by distillation
ethanol is distilled with acidified potassium dichromate (VI) into ethanal changing from green to orange
The amino acid alanine, H2NCH(CH3 )COOH, exists as a solid at room temperature. The most important reason for this is that it
exists as a zwitterion
explain deltaSsurr for exo and endothermic reactions
exo: energy is given out to surrounding. more energy = more number of ways of arranging energy so more energy given out therefore deltaSsurr increases and is more positive endo: energy is taken in from surroundings - number of ways of arranging energy in surroundings therefore decreases - deltaSsurr is negative (decreases)
Is a neutralisation reaction exothermic or endothermic?
exothermic
are feasible reactions normally exo or endothermic?
exothermic because they have a negative standard enthalpy change of reaction however, some are endothermic which shows the limitation of using the enthalpy change to decide the likely direction of change e.g. ethanoic acid with ammonium carbonate as the mixture fizzes and gets colder so delta H is positve
how to obtain the best balance of high rate, high yield of ethanol and cost from ethene and steam - exothermic reaction, all gases, 60-70 atm and 300 degrees with phosphoric acid catalyst
exothermic: lower temperature favours forward reaction - better yield low temperatures: slow rate of reaction so 300 degrees is compromise high pressures favour forward reaction and increase rate of reaction but very expensive - 60-70atm is compromise
two ways to work out a lattice energy
experimental - using born-haber cycle theoretical - calculations based on the purely ionic model of a lattice
what is elemental analysis?
experiments determine the masses or % compositions of different elements in a compound this data helps to work out empirical and molecular formulae knowing molecular formula helps work out structure of compound from different spectra
describe how iodine vapour is used to make colourless chemicals in a mixture visible
expose the chromatogram to iodine vapour - leave plate in a sealed jar with a few iodine crystals. iodine vapour is a locating agent i.e. it sticks to the chemicals on the plate and they'' show up as brown/ purple spots
the bigger k is, the ....... the reaction
faster
relationship between equilibrium constants and feasible reversible reactions
feasible reversible reactions have large equilibrium constants 1. an equilibrium constant is a measure of the ratio of the concs of products to reactants at equilibria for a reversible reaction at a specific temperature 2. reactions with -ve delta G, and so are theoretically feasible, have large values for their equilibrium constants (>1) 3. reactions with +ve delta G, and so not theoretically feasible, have small values for their equilibrium constants (<1) 4. this relationship is represented by the equation (In K = log, R = gas constant, 8.31)
state the 2 main ways in which colourless chemicals are made visible on the chromatogram
fluorescent dyes and iodine vapour
what does the dot and cross diagram of NaCl ignore
for Na: - Na starts as a giant lattice of metal ions - energy is required to separate the Na atoms in the giant lattice to gaseous ions = standard enthalpy of atomisation - energy is required for the 1st ionisation energy of sodium for Cl: - chlorine consists of Cl2 molecules - energy is required to break the bonds between Cl atoms in Cl2 to separate gaseous Cl atoms = standard enthalpy change of atomisation - energy is released in the first electron affinity of chlorine for the joining of Na+ and Cl-, energy is release = lattice energy
what does the relationship of -delta H / T mean
for a given quantity of energy, the higher in entropy is greater when the surroundings are cool than when they are hot adding energy to molecules in a cool system has a proportionally greater effect on the number of ways distributing matter and energy than adding the same quantity of energy to a system that is already very hot (if you already have a higher temp, adding more heat won't make much difference)
what happens when a weak acid reacts with a strong base
for every mole of OH- added, one mole of HA is used up and one mole of A- is formed
ammonia + propanoic anhydride
forms propanamide and ammonium propanoate
ammonia with acid
forms white solid
Rate = k[BrO3 −][Br −][H+] 2 (i) What is the overall reaction order?
fourth - you add up all of the orders
addition of chlorine to benzene
free-radical reaction in UV light. some types used as commercial insecticide but use is restricted because of concerns about its toxicity to humans
alkylation and acylation (Friedel-Crafts reactions)
friedel-crafts reaction in an important method for substituting an alkyl/ acyl group for a hydrogen in a benzene ring. in the reaction, a haloalkane or acyl chloride is refluxed with an arene with aluminium chloride catalyst.
acyl chlorides
functional group -COCl end in -oly chloride
what tends happens when you react 2 achiral things and why
get a racemic mixture of a chiral product because when 2 molecules react there's often an equal chance of forming each of the enantiomers
explain the zero order reaction for conc-time graphs?
gradient (rate) = constant therefore rate not changing as conc changes rate = k therefore gradient of line gives k for a zero order reaction
explain the zero order reaction for conc-time graphs
gradient = constant therefore rate not changing as conc changes rate = k therefore gradient of line gives k for a zero order reaction
Grignard reagent to alcohols
grignards reagent with aldehydes and ketones occurs in 2 steps: to aldehyde/ ketone add 1. in dry ether, grignard reagent 2. add dilute acid to solution forms an alcohol methanal --> primary alcohol. other aldehydes --> secondary alcohol. ketone --> tertiary alcohol
carboxylic acids with carbonates/ hydrogencarbonates to form salt + CO2 + H2O example
group 2: ethanoic acid + calcium carbonate → calcium ethanoate + water + carbon dioxide 2CH3COOH + CaCO3 → (CH3COO)2Ca + H2O + CO2
how to draw a 3D molecule
groups with largest Mr on wedges
what are electrochemical cell reactions called? are they reversible
half reactions and they are reversible
what direction does the half-reactions go
half-reaction with more positive electrode potential goes forward and more negative goes backwards
what can iron (III) bromide be described as?
halogen carrier: when they come close to the halogen molecule (in this case bromine) they have the ability to attract the Br-/Cl- ect. towards them, polarising the halogen molecule, allowing them to be carried by it --> electrophile for the reaction
why is methyl orange a hard indicator to use
hard to spot the point at which an orange colour marks the end point
explain the reduction of carboxylic acids. how can excess reducing agent be destroyed when the reaction is complete?
harder to reduce than carbonyl can be reduced to primary alcohol by LiAlH4 (powerful reducing agent) in dry ether add dilute acid
acidified dichromate(VI) ions
heating an aldehyde with acidified dichromate(VI) ions, you get a carboxylic acid
preparing aliphatic amines from halogenoalkanes
heating the aliphatic amines with halogenoalkanes in a sealed flask with excess ammonia in ethanol during step 1, ammonia acts as a nucleophile and step 2, it acts as a base to remove a proton from the salt initially formed
esterification
heating under reflux carboxylic acid with alcohol in presence of an acid catalyst e.g. H2SO4 or HCl e..g ethanoic acid + ethanol -> ethyl ethanoate + water condensation reaction
techniques used in the preparation and purification of a liquid organic compound
heating under reflux, extraction with a solvent in a separating funnel, distillation, drying with an anhydrous salt and boiling temperature determination
compare the boiling point of benzene and hexane
hexane has more H atoms therefore more electrons so stronger London forces - but spatial fit: benzene = planar and so can fit closer together therefore benzene has a higher bp as Lf are more strong
what is glacial ethanoic acid? what are the safety precautions
high concentration of ethanoic acid use in fume hood with goggles because it makes you cry
what are the types of column chromatography
high performance liquid chromatography gas chromatography
what does electrode potential tell us
how easily the half cell gives up electrons (oxidised)
what does the half-cell electrode potential measure
how easily the substance in the half-cell oxidise
Explain why the calculation of the pH of a solution of ethanoic acid gives a more accurate value than a similar calculation for hydrochloric acid
hydrochloric acid is a much stronger acid (ethanoic acid) OR hydrochloric acid has a (much) smaller pKa (than ethanoic acid) OR Ionization / dissociation of hydrochloric acid is (much) greater (than ethanoic acid) Approximation of negligible ionization invalid / incorrect OR [HCl]equilibrium not equal to [HCl]initial
why do fuel cells not need to be recharged?
hydrogen / reactant supplied continuously
phenol
hydrogen bonding gives stronger intermolecular forces than in benzene and so is solid at room temp dissolves slightly in water and more in alkalis e.g. NaOH forming a soluble ionic compound
acids as proton donors - Bronsted-Lowry theory
hydrogen chloride molecules give protons to water molecules when they dissolve in water producing hydrated H+ called oxonium ions. water acts as a base
preparing aliphatic amines from nitriles - hydrogenation
hydrogen gas with nickel catalyst
identify 1 major hazard associated with the use of a hydrogen-oxygen fuel cell in a vehicle?
hydrogen is flammable / explosive
Hydrogen-oxygen fuel cells are sometimes regarded as a source of energy that is carbon neutral. give 1 reason why this may not be true
hydrogen made using energy that is not 'carbon neutral'
Manganese(IV) oxide, MnO2, and manganate(VII) ions, MnO3-, react in alkaline solution to form manganate(VI) ions, MnO42- i) write the ionic equation for this reaction ii) give a reason why this reaction is not disproportionation
i) MnO2 + 2MNO4- + 4OH- --> 3MnO42- + 2H2O ii) there is not 1 species that is being oxidised and reduced or 2 different species are oxidised and reduced to form the same species
A student carried out a titration by adding 0.032 mol dm-3 potassium hydroxide solution to 25.0 cm3 of 0.024 mol dm-3 propanoic acid. A sketch graph of pH against volume of potassium hydroxide solution added is shown below. 14 7 0 pH Vol of KOH / cm3 (a) *(i) Describe and explain the behaviour of the solution formed in the region circled on the sketch graph. *(ii) Explain why the pH at the equivalence point of this titration is greater than 7.
i) buffering region Identification of species present responsible for buffering action - Both propanoic acid (weak acid) and propanoate (ions) (conjugate base) present On addition of OH− (in small quantities) H+ ions react with (the added) OH− and (the equilibrium) CH3CH2COOH ⇌ CH3CH2COO− + H+ shifts to the right ii) Propanoate ions present (at equivalence point) Propanoate (ions) react with water Forming hydroxide ions
To determine rate equation for the reaction, propanone is reacted with iodine in the presence of HCl(aq) at constant temp. CH3COCH3 + I2 → CH3COCH2I + HI Samples are withdrawn at known times, quenched with NaHCO3 (aq), and the iodine remaining titrated with a standard solution of sodium thiosulfate. The rate equation for the reaction is rate = k[CH3COCH3][H+ ][I2] (a) The graph of [I2] against time is a straight line, showing that the order of reaction with respect to iodine is zero. (i) Explain why the propanone and the hydrogen ions must be in large excess in this experiment in order to give this straight line. ii) What further experiment could be done to show that the order of reaction with respect to propanone is one? State the effect of this change on the graph.
i) ensures that [H+ ] and [propanone] are virtually constant, so that they do not affect the rate the [I2] changes OR so that the overall order (of reaction) is not determined OR otherwise a curved graph is obtained ii) double/ change the concentration of propanone slope/gradient of line doubles/ changes
Iodine reacts with propanone in the presence of an acid catalyst. CH3COCH3(aq) + I2(aq) --> 3COCH2I(aq) + HI(aq) An experiment was carried out to investigate the kinetics of this reaction by monitoring [I2]. The progress of the reaction was followed by mixing together the reagents, removing samples of the mixture every 5 mins, quenching the reaction and then titrating to find the concentration of iodine at a given time. (i) Suggest a suitable reagent with which you could titrate the iodine. (ii) State and explain how you would quench the reaction
i) sodium thiosulfate ii) add excess sodium hydrogen carbonate to remove excess acid
zero order reactions
if [X] changes and the rate stays the same, the order of reactions with respect to X is 0 so, if [X] doubles, the rate will stay the same the rate is also unaffected by changes in concentration of that reaction conc - time graph for zero order is straight as gradient = rate so rate = k
following rate of reaction using loss of mass
if a gas is given off, the system will loss mass you can measure this at regular intervals with a balance put glass wool in the top so gas can escape measure mt- mfinal - plot on y axis so it is proportional to concentration use mole calculations to work out how much gas you've lost and therefore how many moles of reactants are left
explain why some elements in the d block of the periodic table are poor heterogenous catalysts.
if a metal is to be a good catalyst for the addition of hydrogen, it must not adsorb the hydrogen so strongly that the hydrogen atoms become unreactive. this happens with tungsten. equally, if adsorption is too weak there are insufficient adsorbed atoms for the reactions to occur at a useful rate, and this is the case with silver. the strength of adsorption must have a suitable intermediate value, which is the case with nickel, platinum and palladium
SN1 mechanism
if it's an SN1 mechanism and you start with a single enantiomer reactant, the product will be a racemic mixture of 2 optical isomers of each other, so won't rotate plane-polarised light step 1: a group breaks off, leaving a planar (flat) ion step 2: the planar ion can be attacked by a nucleophile from either side resulting in 2 optical isomers
What evidence suggests that this reaction proceeds by more than one step?
if one of the concentrations is zero order - [X] is in chemical equation but not in rate equation / not in rate determining step (so is in a step other than rate determining step) if one concentration is second order - The number(s) (of particles) in the rate equation / rate-determining step do not match those in the equation for the reaction if there are more than 2 reactant particles in the overall equation - it is very unlikely that they will react at the same time - always answer with this
overall what does an increase or decrease in pressure cause
if pressure doubles, conc of all substances will double therefore, Q increases as numerator (NO2) will be affected > than the denominator due to the different powers in order to re-establish equilibrium, the numerator needs to decrease and denominator increase until Q = K therefore equilibrium shifts left.
what is the pattern of the pH of salts?
if salt is made of: - base is stronger than acid, pH >7 - acid is stronger than base, pH < 7 - strong base strong acid, pH = 7 the weaker the acid/ base, the stronger the conjugate which influences the pH of the salt
when does a reaction not occur with a negative delta G?
if the activation energy is very high or the compounds are not in standard state - values assume standard conditions
when can an endothermic reaction be feasible?
if the increase in delta Ssys is greater than the decrease in delta Ssurr
following rate of reaction using electrical conductivity
if the number of ions changes, so will the electrical conductivity e.g., C4H9Br(l) + H2O(l) --> C4H9OH(l) + H+ + Br-
second order
if the rate is proportional to [X]^2, then the order of reaction with respect to X is 2. if [X] triples, the rate will be 3^2 = 9 times faster
following rate of reaction using change in pH
if the reaction produces or uses up H+ ions, the pH of the solution will change. so you could measure the pH at regular intervals and read the absorbance
when can a reaction that does not tend to go at room temp become feasible?
if the temp increases because delta Ssurr decreases in magnitude as T increases. (e.g., methane + O2 doesn't react readily at room temp as it has a higher Ea)
CrO42- to Cr2O72-
if you add some acid to this yellow solution, you form an orange solution that contains dichromate(VI) ions. this is a reversible reaction, so an equilibrium exists between chromate(VI) and dichromate(VI) ions
reduction of nitrobenzene to phenylamine importance
important industrially in the preparation of azo dyes. tin is expensive and so in industry the cheaper metal iron is used
nitration of benzene
important reaction as it produces a range of important products including dyes and explosives e.g. TNT electrophilic substitution - H is replaced by NO2 55 degrees with conc nitric acid and conc sulfuric acid (to produce nitronium ion NO2+ - a very reactive electrophile) overall equation: HNO3 + H2SO4 -> NO2+ + HSO4- + H2O regenerate the H2SO4 anything about 55 degrees forms multiple substitutions
when is entropy 0
in a perfectly ordered crystal at 0K
the formation of zwitterions
in aq solution, carboxylic acid groups ionise producing H+ ions, whereas amino groups are basic and attract H+ ions. due to this, amino acids form ions in aq solution as the amino groups accept H+ ions and the acid groups give them away. The ions formed are, however, unusual in that they have both +ve and -ve charges.
amines forming a complex ion with Copper (II) ions
in copper(II) sulfate solution, the Cu2+ ions for [Cu(H2O)6]+2 complexes with water. this solution is blue adding a small amount of butylamine solution to copper(II) sufate solution you get a pale blue ppt - the amine acts as a base and takes 2 H+ ions from the complex. this leaves a pale blue ppt of copper hydroxide - see I for equation add more butylamine solution, and the ppt dissolves to form a deep blue solution. some of the ligands are replaced by butylamine molecules, which donate their lone pairs to form dative covalent bonds with the Cu2+ ions. - see II for equation
heterogenous catalyst
in different physical state from the reactant: - solid catalyst provide a surface for the reaction to take place on. catalyst is normally mesh/ a powder to increase surface area/ spread over inert support - heterogenous catalyst can be easily separated from the products and leftover reactants These molecules then get adhered to the suitable sites available for adsorption. The reactants, when bound to the surface have a higher probability of reacting with each other, and after the reaction, they form an intermediate compound. After this process, the intermediate compound gets desorbed from the surface, which again becomes available for adsorption for other molecules to come. - heterogeneous catalysts can be poisoned though. a poison is a substance that clings to the catalyst' surface more strongly than the reactant does, preventing the catalyst from getting involved in the reaction it's meant to be speeding up. for example, sulfur can poison the iron catalyst used in the Haber process impurities are less likely to desorb
how do fuel cells generate electricity from hydrogen and oxygen
in most cells chemicals that generate electricity are in electrodes and the electrolyte that form the cell in fuel cells, the chemicals are stored outside the cell. e.g. in alkaline hydrogen-oxygen fuel cell which are used in electric vehicles H2 and O2 (g) are fed into 2 separate platinum-containing electrodes which are separated by an anion-exchange membrane allowing anions (OH-) and water to pass through but not the gases. the electrolyte is an aq alkaline (KOH) solution
dimers
in pure liquid carboxylic acids, dimers form. this is when a molecule hydrogen bonds with 1 other molecule. this increases the size of the molecule, increasing intermolecular forces and so boiling point increases
why do magnesium halides have more covalent character in their ionic bonds than sodium halides?
in sodium halides, Na+ has a small charge so can't pull electrons from anion so charge is distributed evenly around ions - almost no polarisation in magnesium halides, the Mg2+ charge pulls the electrons from the anion, polarising the bond the greater the charge density of the cation, the poorer the match between experimental and theoretical values for lattice energy the more polarised a bond, the more covalent it gets
Titration Strong Acid - Weak Base
in titration of hydrochloric acid with ammonia solution, for example, the salt formed at the equivalence point is ammonium chloride. since ammonia is a weak base, the ammonium ion is an acid it has buffer region at the end
affect of pressure changes to equilibrium
increase: will shift where fewer molecules decrease: shift where more molecules not significant for equilibriums with only solids and liquids
for initial rates, why is it useful to be given further data
increases reliability
explain why the pH range (of which an indicator changes colour) is given roughly by pKa = +/- 1
indicator changes colour when [Hln] (acid) = [ln-] (salt) therefore pH = pklnd + log (I) - as log(I) = 0 pH = pkind but the human eye sees colour change when [Hln] = 10 x [ln] (few drop acid) therefore pH = pkind + log 0.1 ---> [salt] / [acid] = 1/10 = 0.1 pH = pkind -1 or when [ln-] = 10 x [Hln] (few drop alkali) therefore pH = pkind + log10 (which = 1) pH = pkind + 1 therefore you get a range same with buffers: work effectively when pH = pka if outside range do not have high enough reservoir of salt or acid
indicators in titration
indicator chosen must change colour completely in the pH range of the near vertical part of the pH curve - essential if the visible end-point is to correspond to the equivalence point when exact amounts are mixed the range of the equivalence point should be a 2pH range
Explain the meaning of the term perfect ionic model.
ions are perfect spheres only electrostatic attraction/ no covalent interaction
give a reason why the partial pressure of a solid is not include in a kp expression
it does not contribute to overall pressure
Why is ethanoic anhydride used in manufacturing of aspirin rather than ethanoyl chloride?
it does not form HCl gas - so is safer ethanoyl chloride reacts to vigorously for industrial use
what does the more reactive metal do in electrochemical cells?
it forms ions more readily and is oxidised becoming the anode where electrons flow from the less reactive metal becomes the cathode
hydrogen cyanide safety
it is a highly toxic gas. done in a solution to minimise this. do test in a fume cupboard with gloves
what is a universal indicator?
it is a mixture of indicators therefore shows many colours at different pHs
what is the advantage of using electrochemical cells for commercial source of electrical energy?
it is a source of portable energy
describe autocatalysis using the example of the homogeneous catalyst of Mn2+ in the reaction between C2O42- and MnO4-
it is an autocatalysis reaction as Mn2+ is a product of the reaction and acts as a catalyst of the reaction. this means that as the reaction progresses and the amount of the product increases, the reaction speeds up
why is the standard hydrogen electrode called the primary standard?
it is the standard electrode to which the potential of all electrodes are measured
why is benzene banned from labs at school?
it is toxic and a carcinogen
how electrode potentials are used to predict whether disproportionation reactions will happen
it is when an element is simultaneously oxidised and reduced. first combine the half-equations to create the equiation for the disproportionation then use Ecell = Ereduction- Eoxidation to work out the overall potential if positive value it could disproportionate in solution
why does phenylamine dissolve in conc HCl easily if it is only slightly soluble in H2O and how could you reform phenylamine after this reaction
it reactions with the H+ ions forming phenyl ammonium ions which are soluble in the aqueous mix add a strong base (NaOH) C6H5NH3+ + OH- --> C6H5NH2 (oily) + H2O
when using potassium cyanide, why do we need to ensure the area around is dry
it reacts with moisture to form the toxic gas, hydrogen cyanide
suggest 2 properties of platinum that make it suitable for use as an external electrical contact in the cell?
it's a conductor and it's inert
Calculating initial rate of reaction
k = (1.54 x 10−6) ÷ (0.1 x 0.15) = 1.0267 x 10-4 rate = 4.1 x 10−8
calculate pH of 0.1 mol dm-3 propanoic acid pKa = 4.871
ka = [H+]2 / [HA] [H+] = √ka[HA] √10^-4871 x 0.1 = 1.16 x 10-3 pH = 2.94
due to the delocalisation of benzene what was discovered about benzene
kekule realised that benzene had a ring structure with 6 C atoms each joined to 1 H atom. however, he thought that the ring contains 3 C=C bonds and 3 C-C bonds. this molecule would be a triene rather than a delocalised ring system
ammonia is stable in air but can be oxidised on the surface of a copper catalyst. an equation for this reaction is 4NH3(g) + 5O2(g) --> 6H2O(g) + 4NO(g) deltafH = -905kjmol-1 the catalyst is usually warmed to approximately 300degrees to start the reaction, but after a short reaction time the copper catalyst often melts. give a reason why the catalyst is warmed and why it melts
kinetic energy for particles to have energy greater or equal to activation energy melts because it is an exothermic reaction which releases energy as heat.
apart from equivalent H atoms, what else does not cause splitting?
labile protons: protons attached to a v electronegative atoms. they don't couple with the protons linked to neighbouring atoms. therefore the nmr peak for a proton in an OH group = single
what are the main advantages of a fuel cell over a rechargeable cell to provide power for an electric motor?
lack of pollution only waste product is water H2 +O2 supplied continuously more efficient operated without stopping to recharge
what does the extent of solubility of ionic salts depend on?
lattice enthalpy vs enthalpy of hydration (depends on size of ions)
how are electrochemical cells made?
made from 2 different metals dipped in salt solutions of their own ions and connected by a wire (the external circuit) with a salt bridge when drawing it the anode should always be on the left and cathode on the right
describe how fluorescent dyes are used to make colourless chemicals in a mixture visible
many TLC plates have a special fluorescent dye added to the silica or alumina layer that glows when UV light shines on it you can put the plate under a UV lamp and draw around the dark patches to show where the spots of chemical are
combining mass spectrometry with GC or HPLC
mass spec is used to identify substances from their mass/charge ratio whereas gas chromatography and HPLC separates mixtures into individual components but not v good at identifying putting HPLC or GC with mass spec can be extremely useful.
elastomers
materials made of long chain molecules that can be moulded --> shapes that return to original shape when pressure removed
plastics
materials made of long chain molecules that can be moulded --> shapes which are retained
composites
materials made up of 2 or more recognisable constituents, which all contribute to the properties
reaction quotient
mathematical relationship between the concentrations of the reactants and products
what is the advantage of a measuring cylinder and a pipette
measuring cylinder is quicker and measures a variety of volumes pipette is more accurate and can be used to extract samples from a reaction mixture
physical properties of amines
methylamine and ethylamine are gases at room temp and propylamine and butylamine are liquids - smell of fish like ammonia, alkyl amines with short chains dissolve readily in water, forming hydrogen bonds phenyl amine with large non-polar benzene ring is only slightly soluble
buffer solutions
mixtures of molecules and ions in solution which help to keep the pH more or less constant when small quantities of acid or alkali are added
outline the basic principle for all types of chromatography
mobile phase moves through/ over stationary phase the distance each substances moves up the plate depends on it's solubility in the mobile phase and retention time by the stationary phase components that are more soluble in the mobile phase will travel further up the plate. it's these differences in solubility and retention by the stationary phase that separate out the different substances
calculate the % of iron in a sample of steel wire if 1.51g of the wire was dissolved in excess dilute H2SO4 and solution made up to 250cm3 in a flask. 25cm3 of this solution was pipetted to a conical flask and required 25.45 cm3 of 0.02 moldm-3 kMnO4 for complete oxidation.
mol (MnO4-) = 0.02 x 25.45 / 1000 = 0.000509 mol (Fe2+ in 25cm3) = 5 x 0.000509 = 0.002545 mol (Fe2+ in 250cm3) = 10 x 0.002545 = 0.02545 mas (Fe2+) = 55.8 x 0.02545 = 1.42g % Fe = 1.42/ 1.51 x 100 = 94%
units for rate of reaction
mol dm-3 s-1 if time is in minutes instead of seconds it is min-1
partial pressure of a gas equation
mole fraction of gas x total pressure of the mixture
in column chromatography what could you use to get a better separation?
more than 1 eluent (the "carrier" portion of the mobile phase) can be used
asymmetric carbon atoms
most common chiral compounds are organic molecules which have a single chiral centre due to the carbon atom attached to 4 different groups
calculate pH of solution formed when 100cm3 of 0.2moldm-3 ethanoic acid (pka = 4.76) is added to 40 cm3 of 0.25 moldm-3 KOH
n(HA) = 100 x 0.2/1000 = 0.02 n(OH-) = 40 x 0.25 / 1000 = 0.01 therefore HA is in excess ...................HA + OH- --> A- + H2O before 0.02 0.01 after 0.01 0.01 therefore [HA] = [A-] therefore 1/2 of the HA molecules have reacted with OH- therefore ka = [H+] or pka = pH pH = pka = 4.76
calculate pH of solution when 30cm3 of 0.2moldm-3 ethanoic acid (pka = 4.76) is added to 100 cm3 of 0.1 moldm-3 NaOH
n(HA) = 30 x 0.2/1000 = 0.006 n(OH-) = 100 x 0.1 / 1000 = 0.01 XS mol (OH-) = 0.01-0.006 = 0.004 XS (OH-) = 0.004 / 130/1000 = 0.0308 XS [H+] = kw / [OH-] = 10^-14/ 0.0308 = 3.25 x 10^-13 pH = -log (3.25 x 10^-13) = 12.49
calculate pH of solution formed when 50cm3 of 0.5moldm-3 ethanoic acid (pka = 4.76) is added to 75 cm3 of 0.2 moldm-3 NaOH what is the assumption for this reaction
n(HA) = 50 x 0.5/1000 = 0.025 n(OH-) = 75 x 0.2 / 1000 = 0.015 therefore HA is in XS ...................HA + OH- --> A- + H2O before 0.025 0.015 change -0.015 after 0.01 0.015 left over [HA] = 0.01 / 125/1000 = 0.08 therefore formed [A-] = 0.015/ 125/1000 = 0.12 ka = [H+][A-] / [HA] [H+] = 10^-3.76 x 0.08 / 0.120 = 1.16 x 10^-3 pH = -log(1.16 x 10^-3) = 4.94 [HA]eq is determined by the amount of acid yet to be neutralised [I-] is determined by the amount of acid converted to the salt
explain the splitting pattern of a peak from protons bonded to an atom that is next to an atom with 2 protons
n+1 rule: 2+1 = 3 lines. the central line is twice as large as the other 2 this is because there are 3 energy states available to the 2 protons, depending on whether each proton is aligned with or against the magnetic field: both aligned one aligned, one not and vica versa (2) both against
does the molar volume of a gas change for gases under the same conditions
no
does the indicator affect the pH of the solution?
no as only 1/2 drops added
Can aldehydes and ketones form hydrogen bonds with other aldehyde or ketone molecules? what does this mean for the boiling point?
no as they do not have a polar O-H bond which means they have a lower boiling point than alcohols but have london forces and permanent dipole-permanent dipole bonds
can normal cell reactions be recharged and why?
no because the reactions cannot be reversed as one substance cannot be reduced and oxidised
what effect does a race mate have on plane polarised light
no effect - it is optically inactive as each enantiomer cancels out the effect of the other
what affects Rf calculations
no matter how big the paper is the composition of the paper, solvent, and temperature change can differ the Rf value
Domestic bleaches are dilute solutions of sodium chlorate(I). The amount of ClO- ions in a sample can be found by reacting it with excess acidified potassium iodide solution. ClO- + 2I- + 2H+ → I2 + Cl- + H2O The iodine produced is then titrated with standard sodium thiosulphate solution. • 10.0 cm3 of a domestic bleach was pipetted into a 250 cm3 volumetric flask and made up to the mark with distilled water. • A 25.0 cm3 portion of the solution was added to excess acidified potassium iodide solution in a conical flask. • This mixture was titrated with 0.100 mol dm-3 sodium thiosulphate solution, using starch indicator added near the end point. • The mean titre was 12.50 cm3 . The equation for the reaction between iodine and thiosulphate ions is 2S2O3 2- + I2 → S4O6 2- + 2I- Calculate the amount (moles) of chlorate(I) ions in 1.00 dm3 of the original bleach
no. moles 2− OS 32 used = 12.5 × 0. 1 / 1000 = 1. 25 × 10-3 (1) no. moles I2 = 1.25 × 10-3 / 2 = 6.25 × 10-4 (1) no. moles ClO- = no. moles 12 (1) no. moles ClO- in original 10 cm3 = 10 × 6.25 × 10-4 = 6.25 × 10-3 (1) no. moles ClO- in 1 dm3 = 100 × 6.25 × 10-3 (1) = 0.625
what are the main 3 types of cells
non-rechargeable, rechargeable and fuel cells
The initial attack by 2,4-dinitrophenylhydrazine, when it reacts, is by...
nucleophile the reaction is nucleophilic addition elimination
nylon
nylon is the collective name for polymers with aliphatic hydrocarbon sections linked by amide bonds. they are aliphatic polyamides in which the polar amide bonds are fixed and inflexible, but the non-polar hydrocarbon sections are free to flex. weak bonding and low mp
why does nuclear magnetic resonance (NMR) spectrometry only work with odd-numbered nuclei
odd numbered nuclei have a magnetic field around them. Even numbered nuclei do not NMR only works if nuclei have a magnetic field
high-performance liquid chromatography in the mobile phase
often a polar mixture e.g. methanol and water. it is forced through the column under high pressure. The mixture to be separated is injected into the stream of solvent and carried through the column as a solution mixture is separated as the different parts are attracted by different amounts to the solid, so they take different lengths of time to travel through the column. as liquid leaves the column, UV light is passed through it and is absorbed by different parts of the mixture as they come through, and a UV detector measures the UV light absorbed by the mixture. a graph (chromatogram) is produced
What are half cells?
one half of an electrochemical cell. they can be constructed of a metal dipped in its ions, or a platinum electrode with 2 aqueous ions
what makes a good salt bridge
one that won't form a ppt with any ions in the half cell kNO3 (+ all nitrates) are soluble NaCl is a worse salt bridge as might form a ppt
in strong acids and bases what is the enthalpy involved in neutralisation
only 1 - enthalpy when OH- and H+ ions react unlike neutralisation in weak acids and bases, there is no enthalpy of dissociation enthalpy with strong acids and bases as they fully dissociate. as a result, standard enthalpy of neutralisation is similar for all reactions of strong acids and bases
how to write a Kp expression for a heterogeneous equilibrium
only include gases
how does the splitting of peaks in proton NMR spectroscopy work?
only peaks of Hs bonded to Cs split e.g. -OH and -NH hydrogens aren't split splitting is caused by the non-equivalent H atoms that are bonded to neighbouring/adjacent carbon which interact with each other - this effect is called spin-spin coupling only hydrogen nuclei on adjacent C atoms affect each other they're split into 1 more than the no of H on the neighbouring C atoms - n + 1 rule
Suggest why a three step synthesis is likely to be 'greener' than a six step process
overall higher yield less loss of chemicals less waste of product
equation for the oxidation number of the metal ion
oxidation number of the metal ion = total oxidation number - sum of the charges of the ligands
what does (O) represent
oxidising agent
Henderson-Hasselbalch equation - used to work out the conc of salt and acid or base
pH = pKa + log [A-]/[HA] this equation relies on the fact that: [HA]= [HA] start and [A-] = [A-] start
how to find pKa from titration curve
pH at "half-equivalence" (for the first equivalence point) this is because at half-equivalence, [HA] = [A-] so from Ka equation, cancelling them out means Ka = [H+] so, -logKa = -log[H+] so, pKa = pH
Titration: Weak Acid - Strong Base
pH of pure acid can be calculated from Ka. however, when some strong alkali runs in from the burette, some of the ethanoic acid reacts to produce sodium ethanoate.
name the types of chromatography used in organic chemistry?
paper thin layer column gas-liquid
forming proteins from amino acids
peptides are made by linking amino acids in a peptide bond in a condensation reaction and further bonds are condensation polymerisation
how are ester used
perfumes and food flavourings solvents as they are polar and have low boiling points and evaporate easily which is useful in making glues plasticisers - esters are used to make plastics more flexible during the polymerisation process. they can leach out of the plastic over time and the plastic does become brittle
alcohols to chloroalkanes
phosphorus pentachloride - observe misty fumes of hydrochloric acid the reaction between alcohols and HCl is fastest if the alcohol is tertiary and slowest if primary
what affects entropy?
physical state: solid particles vibrate in fixed point - lowest entropy gas particles are random - highest entropy e.g. burning Mg(s) with O2(g) --> MgO(s) so entropy is lowered dissolving : dissolving a solid increases entropy and increases no. of particles more particles means more entropy: as there are more ways they and their energy can be arranged e.g. N2O4(g) -> 2NO2(g) - more moles so entropy increases
a buffer is made from using ethanoic acid and an sodium ethanoate. 1.20 moldm-3 of sodium ethanoate is used. what conc of ethanoic acid is required so that the buffer has a pH of 4.9? under these conditions, ka of ethanoic acid = 1.75 x 10-5
pka = -logka pka = -log(1.75 x 10-5) = 4.756... pH = pka + log( [A-] / [HA] ) 4.9 = 4.756 + log ([CH3COO-] / [CH3COOH]) log ([CH3COO-] / [CH3COOH]) = 4.9 - 4.756... = 0.143 10^0.143 = 1.39 you know that the salt fully dissociates so [salt] = [A-]. this lets you calculate [HA] at equilibrium, which is equal to [HA] at the start of the reaction (since ethanoic acid is a weak acid) so: 1.20 / [CH3COOH] = 1.39 [CH3COOH] = 1.20 / 1.39 [CH3COOH] = 0.86 mol dm-3
how to calibrate a pH probe
place the bulb of the pH meter into deionised water and allow the reading to settle. now adjust the reading to 7.0 do the same with a standard solution of pH4 and 10. make sure you rinse the probe with deionised water in between each reading
end point of titration
point at which colour change or pH change indicates enough solution has been added to react with the chemical in the flask
is a reaction feasible if the electrode potential of the cell positive or negative
positive
suggest 1 reason why waste disposal centres contain a separate section for cells and batteries?
prevent pollution of the environment by toxic substances recycling of valuable components
what type of amine is phenylamine
primary
what do primary, secondary and tertiary haloalkanes react by? why?
primary - only SN2 - loads of space around carbon for attack not SN1 - as carbocation forms = secondary =
oxidation of primary alcohols
primary alcohol --> aldehyde --> carboxylic acid using acidified potassium dichromate
amines as bases
primary amines, like ammonia, act as Bronsted-lowry bases. the lone pair of electrons on the N is a proton acceptor
how to measure the voltage between 2 half cells?
put a voltmeter in the external circuit. this is the cell potential, Ecell voltmeters also measure the direction of the current
explain the uses of quaternary ammonium salts?
quaternary ammonium salts with long chain alkyl groups are used as surfactants: - ionic head: dissolve in water - long alkyl chain: dissolves the grease
rate of reaction equation for second order
rate = k [reactant]^2
rate equation for zero order reactions
rate = k[reactant]^0 so, rate = k
using propanone and iodine
rate equation for this reaction is: rate = k[CH3COCH3][H+] using the rules for the RDS, here's what you can say about the reaction: 1. propanone and H+ are in the rate equation - so they, or something derived from them, must be in the RDS 2. iodine is not in the rate equation so it's not involved until after the RDS 3. the order of reaction for both propanone and H+ is 1 - so the RDS must use 1 molecule of each 4. H+ is a catalyst - so it must be regenerated in another step
the hydrolysis of haloalkanes by alkali is a nucleophilic substitution reaction. RX + OH- --> ROH + X- the mechanism of this reaction for primary haloalkanes is different from the mechanism for tertiary haloalkanes. describe how knowledge of the rate equations for the hydrolysis of haloalkanes provides evidence for the mechanisms of these reactions. (6)
rate equation: rate = k[RX][OH] for primary haloalkane rate = k[RX] for tertiary haloalkane primary reacts by SN2 tertiary reacts by SN1 for primary, the RDS is when the compound reacts with OH- ions via a transition state RX + OH− --> [HO---R---X]−and slow /rate determining step --> ROH + X− for tertiary, RDS is when the compound forms a carbocation in a step that only involves the haloalkane RX --> R+ + X− and slow /rate determining step
amine reactions with acyl chlorides
react with delta+ carbon atoms in the acyl chloride group. the reaction forms an N-substituted amide this reaction also forms HCl as a side product a reaction of this type is involved in making paracetamol
iodoform test
reactants: I2/NaOH (aq) - (made in situ - in the test tube) conditions: warm + test = pale yellow ppt Positive test for: ethanal, methyl ketones
carbonyl compound to hydroxynitrile
reactants: KCN/H+ (dilute sulfuric acid) nu addition and stereo chemistry due to molecule being planar about C=O bond reaction increases the number of carbon atoms in organic molecule you need both KCN and HCN because with just HCN there is insufficient conc of CN- ions
amines with acids - formation of salts
reaction is more rapid than with water amines react even more readily with acids than with water. the lone pair on the N rapidly accepts an H+ ion from the acid to form a substituted ammonium salt. when the vapour of gaseous amines react with hydrogen chloride gas, the product e.g. ethylammonium chloride forms white smoke which settles to a solid
what happens when diluting acid to conc of H+ and pH
reduce conc of H+ which increases pH
aldehydes and ketones to primary and secondary alcohols
reduction reducing agents e.g. NaBH4 (sodium borohydride) dissolved in methanol and water or use LiAlH4 in dry ether
carboxylic acids to primary alcohol
reduction using LiAlH4 in dry ether
preparing aliphatic amines from nitriles
reduction LiAlH4 in dry ether, followed by dilute acid
preparing aliphatic amines from nitriles
reduction of nitriles produces primary amines. Unlike the nucleophilic substitution reaction of ammonia with halogenoalkanes considered above, this reaction produces a pure product as no further reactions can occur reduction can be achieved in 2 ways: 1. hydrogenation using hydrogen gas with a nickel catalyst 2. reduction using LiAlH4 in ethoxyethane, followed by dilute acid LiAlH4 is too expensive for industrial use so you hydrogen
nitriles to carboxylic acid
refluxing the nitrile with dilute hydrochloric acid (speeds up reaction) and distil off the carboxylic acid HCl + 2H2O + RCN --> RCOOH + NH4Cl hydrolysis reaction or add NaOH and reflux and then before distilling add conc H2SO4 to acidify NaOH + H2O + RCN -> RCOONa + NH3
as temperature rises, entropy of a chemical...
rises. this is because increasing temperature raises the number of energy quanta to share between atoms. there are also jumps in entropy whenever there is a change of state as the energy is added to a change and also there are more ways of arranging atoms in a gas
phenol + alkali
salt and water e.g. sodium phenoxide is made when phenol reacts with sodium hydroxide
what is the solvent for NMR
samples of NMR are dissolved in solvents free of protons e.g. CCl4, CDCl3 D stands for deuterium which is H-2 (2 neutrons); a solvent is deuterated if it contains D
What d-block elements aren't transition elements and why?
scandium only forms 1 ion, Sc3+, which has an empty d subshell. Sc has the electronic configuration [Ar]3d1 4s2, so when it loses 3 electrons it forms [Ar] Zinc only forms one ion, Zn2+, which has a full d subshell.
fuel cells using alcohols instead of hydrogen
scientists are developing fuel cells that use hydrogen-rice fuels which are converted into H2 in the car by a reformer fuels include methanol and ethanol they're creating fuel cells using alcohol directly without reforming them to H2
What are buffers used for?
shampoo, medicine, water in pools, food and stabilise the pH of blood
definition of rate equation
shows how changes in concentrations of reactants affect the rate of a reaction
the nuclei aligned in the same direction are at a .... what does this create?
slightly lower energy level, so there is an energy gap between the nucleus that are aligned with or against
is the rate-determining step the fastest or slowest step in a multi-step reaction?
slowest
high-performance liquid chromatography in the stationary phase
small particles of a solid packed into a column. this is often silica bonded to various hydrocarbons
how does ionic radii affect enthalpy of hydration?
smaller ions have greater enthalpy of hydration smaller ions have higher charge density so attract water molecules more and have more exothermic enthalpy of hydration
explain why in a titration, the sides of the conical flask are washed with deionised water. also, how does it affect the titre results
so any drips of the substance is washed into solution so it can react, otherwise the titre would be too high. deionised water has no ions to react with the solution so no reaction occurs.
when working out the rate of a substance, why do the other substances have a much larger concentration
so the other substances remain roughly constant so that the substance you are measuring for is the only variable
what is a primary standard and why is kMnO4 a good one?
solution with a known concentration good because: - can' t gain or lose mass in air (doesn't react with ) H2O - has a high molecular mass therefore, low % error and weighing error - pure solid
methods of purifying organic compounds
solvent extraction use of drying agents recrystilisation
High resolution mass spectrometry and why are they useful
some mass spectrometers can measure atomic and molecular masses extremely accurately - to 4dp useful for identifying compounds that appear to have the same Mr when rounded to the nearest whole number e.g. propane and ethanal disadvantage - diff compounds have the same molecular formula so further tests are needed
using titration to find the % of copper in an alloy e.g. Cu2+ ions will oxidise iodide ions to iodine - this can find % of copper in alloy e.g. brass
stage 1: use sample of oxidising agent to oxidise as much iodide as possible a) dissolve weighed alloy in conc nitric acid and pour mix into 250cm3 flask and make up with deionised water b) pipette 25cm3 portion of dilute solution and transfer to flask. add sodium carbonate solution to neutralise any nitric acid - add until ppt forms. remove it by adding drops of ethanoic acid c) add excess potassium iodide solution to react with Cu2+ ions d) white ppt of copper (I) iodide forms stage 2: find out how many moles of iodine have been produced stage 3: calculate conc of oxidising agent a) no of moles of Cu in both 25cm3 and 250cm3 b) calculate mass of copper c) work out % of copper
steps to find concentration of a solution of the oxidising agent potassium iodate(V)
stage 1: use sample of oxidising agent to oxidise as much iodine as possible - measure 25cm3 of potassium iodate (V) solution (KIO3) (oxidising agent) and add to excess of acidified potassium iodide the iodate(V) ions oxidise some iodide ions to iodine stage 2: find how many moles of iodine have been produced by titrating the resulting solution with sodium thiosulfate (say the moles of I2 produced were 6.6x10-4) stage 3: look at original equation 25cm3 of potassium iodate(V) produced 6.6x10-4 moles of iodine. ratio of iodate(V) ions: iodine = 1:3 so 6.6x10-4 / 3 = 2.2x10-4 moles of iodate ions and so work out concentration
carbonyl compounds to alcohols
step 1: reactants: LiAlH4 conditions: in dry ether (solvent) LiAlH4 provides :H- (hydride ions) reduction reaction nucleophilic addition where the carbon atom in the carbonyl group is electron deficient because the electronegative oxygen draws electrons away
where does the equilibrium lie for strong and weak acids
strong - to the right weak - to the left
polyester fibre properties
strong, flexible and abrasion-resistant. terylene is used in clothes to make them crease-free and make them last longer. you can treat polyesters by stretching and heat-treating to make them stronger
alcohol to bromoalkane
substitution reaction HBr 50% concentrated H2SO4 needs to be tertiary alcohol nucleophilic substitution
how does some addition of benzene occur
sufficient energy needs to be provided and further addition occur until saturated compounds are formed
how do some addition reactions to benzene occur?
sufficient energy needs to be provided and further addition occurs until saturated compounds have formed
difference between kinetic stability and kinetic inertness
systems with no tendency to react as stable - delta S total is negative systems that could react but do not do so for a kinetic reason as kinetically inert
what is the only thing to affect kp?
temperature
what factors could change the Rf value in TLC chromatography
temperature solvent composition of the TLC plate
complexes with four-fold coordination
tetrahedral shape e.g. the [CuCl4]-2 complex, which is yellow, and the [Co(Cl)4]-2 complex ion, which is deep blue bond angles are 109.5 degrees
what does the addition of HCN in the presence of KCN helps to prove
that if the product of the addition is chiral, the outcome is a racemic mixture of the 2 optical isomers it is planar about the carbonyl carbon so the nucleophile has an equal chance of bonding to the C on either side of the plane
reactivity of benzene and phenol
the -OH group activates its benzene ring and makes it more reactive than benzene. a lone pair of electrons on the OH interacts with the delocalised electrons in the ring, releasing electrons into the ring and increasing electron density in the ring. this makes electrophilic attack easier for electrophilic substitution in milder conditions.
how is the Arrhenius equation used to calculate activation energy
the Arrhenius equation links the rate constant (k) with activation energy and temperature (T) put the Arrhenius equation into log form you can use this equation to create an Arrhenius plot by plotting In k against 1/T this will produce a graph with a gradient of -Ea/R once you know the gradient, you can find activation energy
why aren't: carboxylic acids, acyl chlorides, esters and amides which all contain a carbonyl group, carbonyl compounds
the C of the carbonyl group is also attached to another electronegative atom. these modify the properties of the group.
Explain why delta H for this reaction: CH3 COOH(l) + CH3 CH2 OH(l) CH3 COOCH2 CH3 (l) + H2 O(l) is not exactly zero.
the C-O and O-H bond enthalpies differ in different compounds bond enthalpies are average values of the enthalpies in different compounds
Glycolic acid, CH2OHCOOH, has an acid dissociation constant of 1.5 × 10-4 mol dm-3 compared with a value of 1.7 × 10-5 mol dm-3 for ethanoic acid. Give a possible explanation as to why the value of Ka for glycolic acid is approximately ten times larger than that of ethanoic acid
the O of the (extra) OH withdraws electrons weakens O-H bond in acid so H+ lost more easily
what is a secondary standard and why is it used?
the SHE is difficult to use (as it involves gas and H2 is flammable), so often a different standard is used. these standards are themselves calibrated against the SHE (this is the definition)
why are amines stronger bases than ammonia?
the alkyl group is e- releasing and increases the electron density on the lone pair on N. this makes the lone pair more attractive to protons than the lone pair on the N in ammonia. the equilibrium lies further to the right than the equilibrium involving ammonia
what does separation by column chromatography depend on?
the balance between solubility in the mobile phase and retention in the stationary phase
how is cell potential related to total entropy change and equilibrium constant
the bigger the cell potential, the bigger the total entropy change taking place during the reaction in the cell Also: as entropy and equilibrium constant (K) are linked we can show the second equation
how to work out Ecell (cell potential) from standard electrode potentials
the cell potential will always be a positive voltage because the more negative electrode potential value is being subtracted from more positive electrode potential value
explain the advantage of investigating reaction kinetics by measuring initial rates
the concs of all the reacting chemicals are known at the start before they have begun to be used up it is then only necessary to follow the concs of one of the reactants long enough to determine the initial rate from a conc-time graph
Kw
the constant in all aqueous solutions at 298K - makes it possible to calculate the pH of alkaline solutions Kw changes if the temperature changes in pure water = [H+]^2
the contact process for making sulfuric acid
the contact process gets its name from the contact between the reacting gases and the surface of the heterogeneous catalyst. the vanadium(V) oxide catalyst is effective because the metal can change its oxidation state reversibly. first the vanadium(V) oxide is reduced to vanadium(IV) oxide as it oxidises sulfur dioxide to sulfur trioxide. then the vanadium(IV) oxide is reoxidised to vanadium(V) oxide by oxygen in the mixture of reacting gases
what is partial pressure?
the contribution each gas in a mixture of gases makes to the total pressure
what happens when a lead-acid cell is recharged?
the current is reversed and the reactions are reversed. Pb2+ --> Pb and Pb2+ --> PbO2, SO42- --> electrolyte
why is benzene resistant to reactions
the delocalised system makes it very stable, hence it tends to undergo substitution reactions
what is a chemical shift?
the difference in radio frequency absorbed by the nuclei (H or C) in the molecule being analysed and that absorbed by the same nuclei in TMS. symbol is: δ and is measured in ppm a small amount of TMS is often added to samples to give a reference peak on the spectrum
why is the sign of ΔH not a totally reliable guide to the likely direction of change
the direction of change may depend on the conditions of temp and pressure - some exothermic reactions never occur because the rate is so slow and the mixture of reactants are effectively inert - some endothermic reactions occur readily under normal conditions
what happens to acids when they dissolve in water
the dissociate meaning they form ions in solution - extent of dissociation distinguishes strong and weak acid
the 6 electron delocalised pi system
the electrons in the pi system don't belong to any particular C atom - they are delocalised around the whole pi system. as the electrons are delocalised and more spread out, they will repel each other less making the molecule more stable.
standard lattice energy definition
the energy change when 1 mole of an ionic solid is formed from its gaseous ions under standard conditions (298K and 100kPa)
lattice energy
the energy given out when gaseous ions combine to make a solid lattice
1st electron affinity
the enthalpy change when 1 mole of electrons are added to 1 mole of neutral gaseous atoms to form 1 mole of gaseous 1- ions
enthalpy change of hydration definition
the enthalpy change when 1 mole of gaseous ions dissolves in water to form an infinitely dilute solution
enthalpy change of solution definition
the enthalpy change when 1 mole of solute dissolves in water to form an infinitely dilute solution
discharge equation
the equation for the reaction where the emf is used up
what does the ka show
the extent of which acids dissociates - measures strength
polarisability definition
the extent to which the electron cloud in a molecule/ ion can be distorted by a nearby electric charge
how does ionic charge affect enthalpy of hydration?
the greater the charge, the greater enthalpy of hydration. ions with high charge attract water molecules more as the electrostatic attraction is stronger. so, more energy is released when bonds are made, giving more exothermic enthalpy of hydration
how to spot a first order reaction using half life
the half life is t1/2 it can be shown mathematically from the rate equation that for a first order reaction: (see equation) where k is the rate constant this shows that, at a constant temperature, the half-life of a first order reaction is the same wherever it is measured on a conc-time graph. it is independent of the initial concentration
what is contradictory about the solubility of ionic salts in water? how do you explain this
the high mp of a salt (NaCl) shows that a large amount of energy is needed to separate the ions from a crystal the ions are strongly hydrated by polar water molecules - pulled away - the water molecules cluster around the ions and form ion-dipole interactions - the energy released when the water molecules bind to the ions (enthalpy of hydration) compensates for the energy needed to overcome the electrostatic attractions (lattice energy) holding the lattice
how does ionic charge affect lattice energy
the higher the charge, the more energy released when an ionic lattice forms due to stronger electrostatic force between the ions. more energy released means that the lattice energy will be more negative and more exothermic
what does the LiAlH4 or NaBH4 provide
the hydride ion
why is the lone pair on the N of a secondary amine more reactive than on a primary amine
the inductive effect of the extra alkyl group. so the secondary amine can also react with the halogenoalkane in a reaction which forms a tertiary amine it is possible to limit further reaction by using an excess of the primary amine so that there is a much greater chance of the primary amine rather than the secondary amine acting as nucleophile.
what is acylation?
the insertion of an acyl group into a compound in replace for a H atom
Why is phenylamine a weaker base than ammonia?
the lone pair in phenylamine is delocalised into the pi cloud of benzene ring and is less attractive to protons than the lone pair in ammonia. the lone pair is less electron dense therefore, the equilibrium for the reaction of phenylamine with water is further to the left than that of ammonia
what is the electromotive force?
the max voltage of a cell symbol = E unit = V
activation energy definition
the minimum energy needed in a collision between molecules if they are to react.
what state are zwitterions and why? are they soluble in water
the movement of H+ ions from the -COOH group to its -NH2 group occurs in solution before the solid amino acid crystallises out. this means that amino acids also exist as zwitterions in the solid state. this ionic character of amino acids accounts for their high solubility in water and their high melting temps
what does the order wrt reactant show
the no of molecules of the reactant that react in or before the RDS
the number of lines = why does this happen?
the number of inequivalent protons on adjacent C atoms + 1 (N+1 rule) there are n+1 energy states available to the proton(s), depending on which protons are aligned with or against the magnetic field
recrystallisation with 2,4-DNP
the orange ppt is a derivative of the carbonyl compound with can be purified by recrystallisation. each different carbonyl compound gives a crystalline derivative with a different melting point - measure the m.p. and compare to identify compound
carbon monoxide poisoning due to ligand exchange
the oxygen or water molecule in haemoglobin can be replaced in a ligand exchange reaction by CO, forming carboxyhaemoglobin. this is bad as CO forms strong dative covalent bonds with the iron ion and doesn't readily exchange with oxygen or water ligands, meaning the haemoglobin can't transport oxygen any more leading to CO poisoning.
what is an isoelectric point of an amino acid
the pH value at which it exists as a zwitterion
why is this not a good method to measure the ka of a weak acid: measuring the pH of a solution when the concentration is known
the pH values of dilute solutions are very susceptible to contamination e.g. dissolved CO2 from air
the main difference between carbon-13 NMR and proton NMR spectra
the peaks in proton NMR spectrum split according to how the hydrogen environments are arranged relative area of signals has no link to number of c-13 atoms
definition of order of a reactant species
the power to which the concentration of the species is raised in the rate equation
what is quenching
the process of stopping or slowing down the reaction in an aliquot
a mole fraction definition
the proportion of a gas mixture that is a particular gas
What is dynamic equilibrium?
the rate of the forward reaction is equal to the rate of the backward reaction occurring constantly. the concentrations of the reactants and products remain constant
which way are half reactions written in?
the reduction reaction goes in the forward direction which electrons added on the LHS
in NMR what does the area under the peak tell you
the relative number of H atoms in each environment e.g. if the area under 2 peaks is in ratio 2:1, there will be 2 H atoms in first environment for every one in second environment
what does a chromatogram show
the retention times of the components of the mixture - time taken for a substance to pass through the column and reach the detector. compare experimental retention times with reference books to identify substances in mixture
Steps of gas chromatography
the sample is injected into a stream of gas, which carries it through a coiled column coated with viscous liquid (oil) or a solid components of the mixture constantly dissolve in oil or adsorb onto the solid, evaporate back to gas and the re-dissolve as they travel through the column. different components in mixture can be identified by retention times (time to travel through column)
example of combining mass spec with GC
the sample is separated using gas chromatography, but instead of going to detector, the separated components are fed into mass spectrometer. the spectrometer produces a mass spectrum for each component, which can be used to identify each one and show what the original sample consisted of it is used in forensics to separate and detect illegal substances in samples
definition of the RDS
the slowest step in a multistep reaction
what happens when a nucleus with spin is placed in a magnetic field?
the small magnetic field generated by the nuclear spin can be aligned with or against the applied magnetic field
how does ionic size affect lattice energy
the smaller the ionic radii of the ions, the more exothermic (more negative) the lattice energy. smaller ions have a higher charge density and the ions sit closer together in the lattice
what happens in HNMR to give greater detail that does not happen in CNMR?
the spins (only seen in high res) of non-equivalent protons (those with different chemical environments) connecting to neighbouring C atoms interact with each other. this interaction = spin-spin coupling and splits the peak into a number of linear = splitting patterns
how are complimentary colours produced
the splitting of the d-orbitals in transition metals by ligands causes some frequencies of light to be absorbed by the complexes the rest of the frequencies of light are transmitted (or reflected) these transmitted/reflected frequencies combine to make the complement of the colour of the absorbed frequencies - this is the colour you see. a colour wheel shows complimentary colours - the complementary colours are opposite each other on the colour wheel. if there are no 3d electrons or the 3d subshell is full, then no electrons will jump, so no energy will be absorbed. if there's no energy absorbed, the compound will look white or colourless.
equation for reaction at hydrogen electrode
the standard hydrogen electrode is a reference electrode, and allows scientists to work out and compare the electrode potentials of whatever half-cell the hydrogen electrodes' connected to. the hydrogen half-cell has a value of 0.00V. this means the voltage reading will be equal to electrode potential of the other half-cells
describe the stationary and mobile phase in gas chromatography
the stationary phase is a solid/ solid coated by a viscous liquid e.g. oil, packed into a long tube the tube is coiled to save space and built into an oven the mobile phase is an unreactive carrier gas e.g. N2 or He
what does it tell you when the experimental and theoretical values of lattice energies are similar
the structure of the lattice is close to being purely ionic
total pressure of a gas mixture definition
the sum of all the partial pressures of the individual gases
half-life of a reaction definition
the time for the concentration of one of the reactants to fall by half
what is half-life (t1/2)
the time it takes for half of the reactant to be used up
hess's law
the total enthalpy change of a reaction is always the same, no matter which route is taken
what are a conjugate acid-base pair linked by
the transfer of a proton
what happens when a solid containing a transition metal ion is dissolved in water?
the transition metal ion forms an aqueous complex in solution (the metal ion will be surrounded by water ligands
what happens to E0 values when the conc of ions increases?
the value becomes more positive as the equilibrium shifts to the right (side with out electrons)
if you increase the temperature, what happens to Kw?
the value of Kw increases, as the endothermic reaction is favoured
what is the standard electrode potential of a half-cell?
the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode the SHE has an electrode potential of 0.00V
effect of temperature on neutrality of water
the water is neutral as [H+] = [OH-] as you can have a neutral solution that does not have a pH of 7
optical isomers have the same chemical and physical properties as each other except for...
their effect on plane polarised light and how they react with other chiral compounds
Amines as nucleophiles
their lone pair of electrons is attracted to any positive ion or positive centre in a molecule e.g. delta+ carbons in the C-Hal bond of halogenoalkanes. the protonated amine formed in the first step then loses a proton to form the secondary amine, butylmethyamine
when using chromatography to identify an amino acid, why might a tripeptide only show 2 amino acid spots
there are 2 of the same amino acids in the tripeptide
what are the units for the mole fraction?
there are none
why is 2nd electron affinity endothermic
there is repulsion when adding an electron to a negatively charged ion so more energy is needed to be added
what is a grignard reagent made from
they are an organomagnesium compound that are made by reacting a halogenoalkane with magnesium in dry ether
hazards of sodium and potassium oxides
they are corrosive
what happens to energetic stability of a substance when there is more disorder?
they are more energetically stable so particles will move to try to increase their entropy. this is why some reactions are feasible even when the enthalpy change is endothermic
what effect do optical isomers have on plane polarised light
they can rotate it and so are optically active. one enantiomer rotates it in one directions and the other enantiomer rotates it by the same amount in the opposite direction. the measurement is degrees of rotation
half-cells involving solutions of 2 aqueous ions of the same element e.g. Fe2+ (aq)/ Fe3+ (aq)
this conversion happens on the surface of the electrode Fe2+(aq) -> Fe3+(aq) + e- Fe3+(aq) + e- -> Fe2+(aq) as neither reactants nor products are solids, an electrode is needed to conduct electricity but be inert. platinum is good but expensive so graphite is often used. these reactions are reversible
measuring Kc for a reaction used to make an ester
this esterification is very slow at room temperature without catalyst diluting equilibrium mixture means reaction is slow enough to find equilibrium conc of ethanoic acid by titration without position shifting procedure: 1. mix measured quantities and allow equilibrium to form 2. titrate with NaOH to determine total amount of acid in sample at equilibrium: both HCl and CH3COOH 3. use equation for reaction and info from steps 1&2 to get equilib. concs.
are these molecules optical active and/or chiral
this molecule is an allene (2 double bonds and different groups attached to the terminal carbon. it is optically active but not chiral this molecule is a spiran. it is optically active (provided the 2 groups attached to terminal carbon are different) but not chiral
disadvantage of making primary aliphatic amines from haloalkanes
this reaction also produces secondary, tertiary and quaternary salts so we have an impure product this occurs as primary amines still have a lone pair of electrons on the nitrogen so also acts as a nucleophile reacting with any remain halogenoalkane. amine acts as nucleophile not ammonia due to the positive inductive affect so, preparing amines from nitriles is better as it produces a pure product
what is the function of the platinum electrode?
to allow transfer of electrons
when waiting for mixture to reach equilibrium, why is important to leave it in an airtight flask
to ensure its in a closed system to ensure nothing affects the equilibrium
what can redox titrations be used for?
to find the amount of reagent in a sample
the higher the change on the metal aqua ion, the more acidic the solution. why?
to lose H+, the O-H bond in a water ligand must break. to break, the electrons in the bond are pulled towards the O. the higher the charge on the metal ions and/or the smaller metal ion, the stronger the pull on the electrons away from the H and towards the O, breaking the O-H bond
why should the potential be measured under zero-current conditions?
to measure the full potential difference (emf), no current must be drawn from the cell - this is achieved by using a high resistance voltmeter.
what is the action of a buffer solution?
to remove any hydrogen or hydroxide ions that is added to it - otherwise pH will change
what can gas-liquid chromatography be used for?
to separate mixtures of volatile liquids
tests for aldehydes
tollen's reagent, Fehling's (benedict's) solution, acidifed dichromate(VI) ions
following rate of reaction using colour change
track colour change using a colorimeter a colorimeter measures absorbance (amount of light absorbed by the solution) the more concentrated the colour of solution, the higher the absorbance is for example, in the reaction between propanone and iodine, the brown colour fades so the absorbance of the solution will decrease. you can measure change in absorbance: 1. plot calibration curve - graph of known concentration of the coloured solution (in this case I2) plotted against absorbance 2. during experiment, take a small sample from you reaction solution at regular intervals and read the absorbance 3. use your calibration curve to convert the absorbance at each time point into a concentration
chlorination of 2-methyl propan-2-ol how do you separate the organic layer from the aqueous layer
transfer content to a separating funnel allow layers to separate and remove lower aqueous layer by opening tap
why do transition metals and their compounds make good heterogeneous catalysts
transition metals make good heterogeneous catalysts as they can use their partially filled d-orbitals to make weak bonds with the reactant molecules
enantiomers
two compounds that are optical isomers of each other
battery definition
two or more electrochemical cells connected in series
comparing addition and condensation polymers
type of reaction: addition: only addition reactions condensation: addition plus elimination type of links: addition: central chain consists of C atoms linked by c-c bonds. condensation: central chain consists of short aliphatic or aryl sections linked by ester or amide groups type of monomer: addition: monomers have molecules with C=C bonds. condensation: monomers have molecules with 2+ functional groups which may be same or different conditions for preparation: addition: requires an initiator with high temp and pressure unless using catalyst condensations: no initiators require with low temp and pressure condensation is biodegradable addition is not
half life second order reaction
unequal half-lvies
how to do cell notations
use a comma if the state does not change
electrochemical series
use the series to work out if a reaction is feasible - if it is the overall potential will be positive (on the left) the very positive electrode potentials mean that they are good at attracting electrons - so they are the best oxidising agents (on the right) the very negative potentials mean they are good at giving away electrons (by giving them something else which is reduced) - best reducing agents
how to calculate lattice energy
using born-haber cycle
oxidation numbers of vanadium
vanadium can exist as 4 oxidation numbers in solution - +2, +3, +4 and +5 when you switch between oxidation numbers, it's a redox reaction - ions are either oxidised (they lose electrons and their oxidation number increases) or reduced (they gain electrons and their oxidation number decreases)
explain E0 values and reactivity
very negative E0: most reactive metals (G1) - reacts as a reducing agent (is oxidised) - the reverse reaction of metal ions --> metal is least likely for reactive metals (most likely is metal --> metal ions) hence the more negative E0 - as the values become less negative the reactivity decreases least positive E0: relatively unreactive metals as reducing agent (transition metals) - but are readily reduced --> metals very positive E0: most reactive non-metals (halogens) - react as oxidising agents as positive of E0 increases, oxidising power increases
halogenoalkane to alcohol
warm aqueous KOH forms alcohols hydrolysis reaction all 3 types of halogenoalkane can be hydrolysed by heating with sodium hydroxide they all react using different mechanisms
why is LiAlH4 used with dry ether as the solvent
water would be explosive reaction dry ether = non-polar therefore aldehyde/ ketone is soluble
how to get 1 moldm-3 of H+ ions
we need 1 moldm-3 of HCl or 0.5 moldm-3 of H2SO4 as it is a diprotic acid
how to purify an ester
we separate the ester through distillation however this must be done in-situ as the reaction is reversible - must be done straight away to purify further, we add sodium carbonate solution to remove a carboxylic acid that remains. layer on top is ester so separate using separating funnel
Are carboxylic acids strong or weak acids?
weak
standard enthalpy changes of neutralisation reactions involving weak acid and weak base are less negative than those for strong base and acid because...
weak acids and bases are not fully ionised at the start and the solutions are not neutral at the equivalence point
Are phenols acidic or basic?
weak acids as they partially dissociate
what are indicators?
weak acids where HA (HIn) and A- (In-) are different colours
example and information about a weak acid and strong acid
weak: CH3COOH - backwards reaction favoured so not many H+ produced strong: HCl - forwards reaction favoured strongly. lots of H+ produced
