A-level Chemistry Edexcel Unit 6 (FULL)
expresssion for percentage uncertainty
(2x) +- /cm3 x 100
Iodoform Test method
- add I2 to alkaline solution til brown turns colourless - I2 and sodium hydroxide solution - add KI - positive = with carbonyl with methyl group = yellow ppt
Potassium manganate (VII)
- indicator not required for titration - self indicating = colourless to pink
Describe how to carry out the recrystallisation to obtain dry crystals of X.
1. Dissolve crystals in minimum volume of hot solvent 2. Filter through a preheated funnel and allow to cool 3. Filter using suction filtration and wash with minimum volume of cold solvent 4. Dry in a desiccator
Give reasons why it is often necessary to heat chemicals under reflux
1. It prevents escape of reactants and products 2. Enables flammable liquids to be heated safely 3. Allows for a complete reaction
outline how you would use the apparatus to prepare 100cm³ of 0.1moldm⁻³ solution of X starting from 1moldm⁻³ of X.
1. Use a pipette to measure 10cm³ of the 1moldm ⁻³ of X. 2. Transfer this to a 100cm³ volumetric flask. 3. Make the solution up to the mark with distilled water and then mix.
outline how to carry out solvent extraction
1. shake 2. release pressure 3. remove lower layer by opening the tap 4. repeat extraction with additional solvent
Recrystallization Method
1.) dissolve crystals in min. volume of hot ethyl ethanoate (solvent /water) 2.) filter to remove soluble impurities and allow to cool 3.) filter and wash with small amount of cold solvent (water bath and vacuum filtration) 4.) dry between filter paper or with warm oven
How to obtain pure crystals
1.) evaporate solution to crystallisation point/ solid begins to form 2.) allow to cool and dry crystals between filter paper/ warm in oven
Preparation of Solution for titration
1.) transer solution to volumetic flask 2.) add washings = spray with water and pour out 3.) make up to 250CM3 with distilled water/ dilute H2SO4 4.) and then mix
Flame Test Method
1.) use nichrome wire 2.) dip wire in conc HCl to clean 3.) dip wire in solid 4.) place in hot bunsen flame and observe flame colour
Test for ketone/ carbonyl
= 2,4-DNP = orange ppt
Electrode Diagrams: what must be added to both beakers of aq sol
= H2SO4 must be added to both beakers if only aq = needs H+
Test for COOH/OH
= PCl5 = misty white fumes (HCl)
Why add water before acid?
= acid reacts very exothermically with water = vigorous
Test for hydroxybenzioc
= acyl chloride > steamy fumes = Br (not benzoic) > white ppts
what is reflux
= as liquid vaporises, vapour is cooled and condensed back into liquid
test for C=C bond
= bromine water = decolourises
Na2S2O8 is added to MnS2O8
= colourless to purple
Why is solid white? (trans metal)
= d orbitals can't split therefore no colour as no ligand is present
Test for Ammonia
= damp red litmus paper turns blue = react with HCl gas > gives dense white smoke
test for Cl-
= dilute nitric acid and silver nitrate > white ppt = conc. H2SO4 > misty white fumes (HCl)
Advantage of vacuum filtration
= faster than other filtration = drys crystals at the same time > more filtration removed = stem can't be blocked
Hazard of H2
= flammable = explosive in air
when asked for safety avoid
= goggles = lab coats
checking burette to increase accuracy
= has no air bubble and part below tap is full
How to seal capillary tube?
= heat one end of tube with bunsen flame until glass melts
how to confirm decomposition is complete
= heat to constant mass
why use anti bumping granules
= make boiling smoother
confirm identity of purified product
= measure melting temp and compare to data book
high resolution NMR
= n+1 rule > n = adjacent H atom
why add starch when pale yellow and not at beginning
= prevent formation of insoluble complex
Why is reflux necessary? (2)
= prevents escape of volatile liquids = allows for complete reaction > increase rate of reaction/ increase yield of product
what change is seen when Na2S2O8 is added to KMnO4 2-
= remains purple
meaning of concordant
= results are within +- 0.1/0.2 cm3
Tollen's reagent
= silver nitrate and NH3 = condition > heat = positive > silver mirror (aldehyde)
what neutralises excess acid?
= sodium hydrogen carbonate = effervescence (CO2)
Advantage of quenching
= solid needs to dissolve in reaction mixture before quench
Calibrate pH meter
= use alkaline buffer solution and acidic buffer solution = then measure pH
why read from top of meniscus
= volume decrease > pipette is calibrated to be measured from bottom = volume stays the same > burette is measured by dif in two readings
Why Kc calculated with moles
= volumes all cancel
why water is added before , rather than after the acids
Acid reacts exothermically with water.
suggest why VO²⁺ ions are produced rather than the V³⁺ ions predicted when calculating the cell electrode potential.
Activation energy is too high.
NH₂-C=O
Amide
NH₂-CHR
Amine
suggest a chemical test that could be used to show that the purified product is a ketone
Brady's reagent , orange precipitate
Observation when a gas is evolved from a liquid
Bubbles.
absorbs water
CaCl2
suggest a change or addition to the steam distillation apparatus that would reduces the risk of the sample tested being flammable and toxic.
Carry out in a fume cupboard.
test for water
CoCl2 = blue to pink CuSO4 = white to blue
Expression for mass from volume and density
D x V = MASS
suggest a suitable way to remove the solvent
Distillation
equation for Ecell
E right - E left more positive = always right
Describe how pure crystals are obtained from a filtrate.
Evaporate the solution to crystallisation point and dry in a desiccator.
Explain why suction filtration is preferred to gravity filtration.
Faster rate of filtration and produces a drier product.
Suggest why the concentration of Iron(II) sulfate calculated is lower in real life.
Fe²⁺ oxidised to Fe³⁺ by oxygen in the air on standing
Observation when CuO is formed
Formation of a Black solid.
suggest the best way to obtain some pure 1-methyl-2,4-dinitrobenzene after the distillation.
Fractional distillation followed by recrystallisation
How would you show that the decomposition was complete
Heat to a constant mass.
explain the hazard associated when reducing Vanadium (V) oxide to Vanadium (III) oxide by passing hydrogen over the heated oxide.
Hydrogen gas is flammable.
Expression for mass from moles and molecular mass
MASS = MRMOL
Describe how to calibrate a pH meter.
Measure the pH of an alkaline buffer solution then measure the pH of an acidic buffer solution.
Solution to stop reaction
NaHCO3
what is the colour of a dilute aqueous solution of iodine.
Pale Brown
Suggest what would happen to the solution of iron(II) sulfate if was prepared using distilled water rather than dilute sulfuric acid as the solvent.
Pale green gas turns brown because Fe(OH)₃ is formed from oxidation of Fe²⁺ to Fe³⁺.
Explain the purpose of the gas outlet in the distillation apparatus.
Prevents pressure building up by allowing gases to escape.
suggest an advantage of using continuous monitoring method rather than a sampling method.
Reaction doesn't need to be quenched.
suggest why a water bath was used in the experiment rather than a Bunsen flame.
So that X doesn't melt.
Name the reagents that are required for the iodoform test
Sodium hydroxide and iodine solution .
identify the reagents required to prepare nitrous acid for the reaction that produces azo dye.
Sodium nitrite and hydrochloric acid.
state the type of impurities removed in suction filtration
Soluble impurities.
how species such as CH₃CHOH⁺ is formed from a molecule of compound X
Species is formed by loss of CH₃ group
Observation when a gas is evolved from a solid.
Steamy fumes.
Yellow Solid
Sulfur
suggest why the flammability of a benzoic acid would not present a significant risk in this experiment even if a Bunsen burner is used.
The amount of benzoic acid is small
explain whether or not the infrared spectra could be used to distinguish X from its isomer.
The fingerprint region could be used to distinguish compounds.
explain why in certain cases it is possible to calculate kc using moles rather than concentrations.
They all have the same mole ratio so all volumes cancel.
explain why when preparing nitrobenzene steps are taken so that the reaction mixture is always at or below room temperature
To ensure only mono-substitution
suggest why methanol is added to the cooled mixture of acyl chloride and X to prepare an ester.
To remove any residual Acyl chloride
Give the reason why water is added when preparing benzoic acid
To remove excess acyl chloride
expression for moles from volume and 24dm3
V/24dm3 mol-1/1000 = mols
how would reading from the top of the meniscus would affect the volume of the mixture delivered from the pipette.
Volume is lower , pipette is calibrated to be measured from the bottom of the meniscus.
how would reading from the top of the meniscus would affect the volume of solution delivered from the burette.
Volume is not changed because the volume from burette is the difference between two readings.
give reason why indicator X should be used for the titration
X range corresponds to the steep change in pH
Give the formula of the ion responsible for the deep colour when excess ammonia solution is added to a compound.
[Cu(NH₃)₄]²⁺
Explain in terms of change of state how the process of heating under reflux works
as liquid boils vapour is cooled and condenses back to liquid
test for CO2
bubble gas through lime water (CaCOH)2 turns cloudy
how would you dry an extract
by adding a drying agent of anhydrous calcium chloride and decant
Salt bridge
filter paper and KNO3
smallest titre have
greatest error uncertainty
Colour of [CuCl3]-
green
which piece of equipment should be used to measure the volumes used in the experiment
if not an excess use a burette/pipette as it allows accurate measurement, if in excess use a measuring cylinder as its faster and easier to use.
explain why generally working from high to low concentration would be less accurate
increases the possibility of contamination of ions due to residues from earlier experiments.
Describe the procedure for carrying out the iodoform test.
mix the reactants together and heat.
expression for gradient
rise/run
standard half cells
temp = 298K conc = 1 mol dm-3 pressure = 100 Kpa
explain without calculation why the titre at time zero is 20cm³ rather than 0cm³
the acid/catalyst reacts with the base
Justify why using half-life value the order of reaction is first order.
the half-lives are similar if values from graph and same if in general terms.
what is the purpose of adding sodium hydrogen carbonate as a drying agent
to neutralise unreacted acid
how to prepare Tollens reagent
use silver nitrate solution and ammonia and heat
