Chemistry A level-periodic table and enthalpy module
What is meant by the term exothermic?
(a reaction) that releases energy/ (a reaction) that releases heat/ a reaction with a negative ∆H
What are standard conditions?
101 kPa AND 298 K, 1 mol dm^-3
Write the equation for the reaction of magnesium with oxygen
2Mg + O2 → 2MgO
What is a radical ?
A species with an unpaired electron
Explain how atmospheric methane molecules can contribute to global warming.
Absorbs IR radiation Bonds vibrate
Write the formula of the Group 2 carbonate which decomposes at the highest temperature
BaCO3 OR RaCO3
What is the most reactive metal in group 2
Barium
Use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic.
Bond breaking absorbs energy AND bond making releases energy More energy released than absorbed
In a catalytic converter, nitrogen monoxide reacts with carbon monoxide. Outline the stages that allow nitrogen monoxide and carbon monoxide to react in a catalytic converter.
CO and NO are adsorbed (onto surface) carbon dioxide and nitrogen are made CO2 and N2 desorbs (from the surface) OR products desorbs (from the surface)
Choose a reaction that you have studied that is catalysed by chlorine radicals. Write down an equation for the overall reaction and show how chlorine radicals are not consumed by the overall reaction
Cl + O3 → ClO + O2 ClO + O → Cl + O2 overall: O3 + O → 2O2 OR Cl + CH4 → CH3 + HCl CH3 + Cl2 → CH3Cl + Cl overall: CH4 + Cl2 → CH3Cl + HCl
Define enthalpy change of formation
Enthalpy change when 1 mole of a compound is formed from its elements under standard conditions
Suggest how strontium carbonate is converted into strontium oxide
HEAT
Suggest two reasons why standard enthalpy changes of combustion determined experimentally are less exothermic than the calculated theoretical values.
Heat released to the surroundings ALLOW heat loss or Incomplete combustion Non-standard conditions
3 physical properties of group 2 metals
High melting points and boiling points low density form colourless(white compounds)
Ethane reacts with bromine in the presence of ultraviolet radiation to form many organic products. (i) Two of these products are bromoethane and hydrogen bromide. Describe the mechanism of the reaction between ethane and bromine that forms bromoethane and hydrogen bromide. Include in your answer • the type of bond fission that occurs • equations for each step of the reaction • the name of each step of the reaction.
Homolytic fission Initiation: Br2 --> 2Br Propagation: Br + C2H6 --> HBr + C2H5 C2H5 + Br2 --> C2H5Br + Br Termination: Br + C2H5 --> C2H5Br OR Br + Br --> Br2 OR C2H5 + C2H5 --> C4H10
Explain why the first ionisation energy of B is less than that of Be.
In B, electron being removed is at a higher energy / In Be, electron being removed is at a lower energy An s electron is lost in Be AND a p electron is lost in B
What is the trend in hydroxide solubility down group two?
Increases down the group Mg(OH)2 is slightly soluble Be(OH)2 is a very strong alkaline
Does reactivity increase or decrease down group two? Why?
Increases, electrons are lost more easily due to larger atomic radius and more shielding
Which group 2 element reacts very slowly with water
Mg
Old samples of magnesium oxide become contaminated with magnesium carbonate. Suggest how this contamination takes place.
MgO has reacted with CO2
What is Mg(OH)2 used for?
Milk of magnesia-antacid to treat indigestion or heartburn
Explain why the boiling points increase down the alkane homologous series.
More surface contact OR bigger molecules More London forces
Give two reasons why there are many organic products of the reaction between bromine and ethane.
More than one C-H bond can be substituted OR multisubstitution can occur OR more than one substitution can happen Lots of termination steps termination steps can give products that will also react with (bromine) radicals
In the stratosphere, NO catalyses the breakdown of ozone. Write two equations to show how NO catalyses this breakdown
NO + O3 --> NO2 + O2 NO2 + O --> NO + O2
Describe and explain, using equations, how the concentration of ozone in the stratosphere is maintained.
O3 --> O2 + O AND O + O2 --> O3 rate of ozone decomposition (almost) equals rate of ozone formation
Compare the manufacture of H2O2 from hydrogen and oxygen (and aquathropine) with the manufacture from barium peroxide. Explain the advantages of the manufacture of H2O2 from hydrogen and oxygen.
Oxygen comes from air No poisonous materials formed OR no poisonous materials involved No waste products formed / atom economy is 100% Anthraquinone is regenerated OR Anthraquinone acts as a catalyst
Describe one difference you might observe if strontium, instead of magnesium, was reacted with dilute hydrochloric acid.
Quicker OR more vigorous OR gets hotter
It is not possible to measure the enthalpy change of formation of N2O(g) directly. Suggest why it is not possible.
Reaction gives NO OR reaction gives NO2 OR reaction gives a mixture of oxides OR activation energy too high OR rate of reaction is too slow
Explain the meaning of the term endothermic.
Reaction in which energy enters the system/ taken from the surroundings
CaCO3(s) → CaO(s) + CO2(g) ∆H = +180 kJ mol-1 Suggest two reasons why it is necessary to heat the calcium carbonate strongly to achieve decomposition.
Reaction is endothermic To overcome activation energy Or to break covalent bonds
Describe and explain the trend in reactivity of Group 2 elements with chlorine as the group is descended.
Reactivity increases (down the group) Increasing size mark Atomic radius increases OR There are more shells Increased shielding mark There is more shielding Nuclear attraction (to outermost electron) decreases Increased nuclear charge is outweighed by increased shielding/distance Easier to remove (outer) electron(s) OR Ionisation energy decreases
What is meant by an homologous series?
Series having same functional group and a general formula
A student added an excess of hydrochloric acid to an old sample of magnesium oxide that is contaminated with magnesium carbonate. State two observations that the student would make.
Solid disappears Fizzing
Explain why increasing the temperature of H2O2(aq) increases the rate of decomposition. As part of your answer, you should add a second curve and any necessary labels to the Boltzmann distribution of molecular kinetic energies shown below.
The correct curve for higher temperature (peak is lower and shifts to right, activation energy has more molecules under it. Activation energy does not change OR clearly labelled on the diagram More molecules have energy above the activation energy
enthalpy change of reaction
The energy change associated with a given reaction (enthalpy change when) the number of moles of reactants, as specified in the (balanced) equation react together
enthalpy change of combustion.
The enthalpy change for the complete combustion of 1 mol (of a substance)
State le Chatelier's principle.
The position of equilibrium will shift so as to minimise the effect of any change in conditions
Explain why the actual conditions used in the chemical industry might be different from high pressures and low temperatures
Too expensive to use a high pressure Too slow to use a low temperature
Describe how hydrogen can be used in the manufacture of margarine
Unsaturated (vegetable) oils OR oils containing C=C bonds (reacted with hydrogen) in the presence of a nickel catalyst
Explain why xenon has a lower first ionisation energy than neon.
Xe has a bigger atomic radius OR Xe has more shells Xe has more shielding The nuclear attraction decreases Less energy required to remove outer electrons
Define catalyst
a substance that increases the rate of reaction by lowers the activation energy (for a reaction) by providing an alternative route without being used up.
Why is it important to life on the Earth's surface that the concentration of ozone in the stratosphere is maintained?
absorbs (harmful) UV
Ethene and steam can be converted into ethanol. Name the catalyst used in this reaction
acid i.e H2SO4
Suggest two reasons why the standard enthalpy change of combustion of methanol will be different from actual value
actual bond enthalpies may be different from average values conditions are not standard / methanol/ water is a liquid under standard conditions
Explain why first ionisation energies show a general increase across Period 3, Na-Ar.
atomic radii decreases number of protons in the nucleus increases nuclear attraction increases requiring more energy to remove outer electron
Explain why barium is more reactive than calcium.
barium atoms are larger barium atoms have more shielding this outweighs the increase in nuclear charge barium electrons are lost more easily /less energy required
Which group 2 element doesn't react with wate
berylium
Which group 2 metal oxide is insoluble in water
beryllium oxide
Why do bond enthalpies have positive values?
bond breaking is endothermic/ energy has to be put in to break a bond
What is meant by homogeneous (in catalytic terms)?
catalyst is in the same state/ phase as reactants
Suggest two ways in which the use of catalysts helps chemical companies to make their processes more sustainable.
catalyst not used up in reaction reactions take place at lower temperatures with lower activation energy OR use less fuel less carbon dioxide emitted into atmosphere so fossil fuels last longer different reactions can be used with better atom economy or less waste less hazardous chemicals catalysts or enzymes can generate specific products
Describe what you would expect to see when magnesium ribbon is added to an excess of dilute hydrochloric acid.
effervescence Mg disappears
Define the term bond enthalpy.
energy needed to break 1 mole of bonds in the substance in the gaseous state
The student looked in a text book and found that the actual value for the standard enthalpy change of combustion of propan-1-ol was more exothermic than the experimental value. Suggest two reasons for the difference between this value and the one he obtained experimentally.
heat losses thermal capacity of beaker ignored conditions were non-standard combustion could be incomplete propan-1-ol evaporates water evaporates
The calculated value of ΔHc from this experiment is different from the value obtained from data books. Apart from heat loss, suggest two reasons for the difference.
incomplete combustion non-standard conditions evaporation of alcohol/water specific heat capacity of beaker/apparatus not taken into account
It is very difficult to determine the standard enthalpy change of formation of hexane directly. Suggest a reason why.
many different hydrocarbons would form OR activation energy too high OR reaction too slow OR they don't react together
What is oxidised and reduced in a group 2 reaction with water
metal oxidised, one hydrogen from each water is reduced
Explain why increasing the concentration of H2O2(aq) increases the rate of decomposition.
more particles per (unit) volume ALLOW particles are closer together more successful collisions per second
The reaction between nitrogen and oxygen in a car engine does not reach a dynamic equilibrium. Suggest why not
not a closed system
Conditions were used that should have given a conversion of A of 39%. In the manufacture, using these conditions, it was found that only 20% conversion was achieved. Suggest why the conversion was much less than theory suggested.
not enough time was allowed for the equilibrium to establish/ other products were formed
Explain, using an enthalpy profile diagram and a Boltzmann distribution, how the presence of a catalyst increases the rate of reaction.
one activation energy labelled on enthalpy profile diagram idea that activation energy is lowered catalyst has a different reaction pathway or two curves drawn on profile QWC - correct diagram of reaction profile for endothermic or exothermic reaction with products and reactants at different heights - y axis labelled as energy or enthalpy Drawing of Boltzmann distribution AND axes labelled (number of) molecules and energy More molecules with energy above activation energy with a catalyst more successful collisions
State one important manufacturing process in which hydrogen is used.
production of margarine/ammonia/Haber process
Explain how the catalyst can increase the rate of decomposition of H2O2(aq).
provides an alternative pathway with a lower activation energy
Give two features of a reversible reaction, when a dynamic equilibrium has been set up.
rate of forward reaction = rate reverse reaction concentrations remain constant closed system needed
Describe and explain the trend, down the group, in the reactivity of the Group 2 elements with water.
reactivity increases (down the group) atomic radii increase OR there are more shells there is more shielding the nuclear attraction decreases OR Increased shielding and distance outweighs the increased nuclear charge easier to remove (outer) electrons
What type of reaction takes place between group two elements and oxygen
redox reactions
What effect does a catalyst have on the rate of a reaction, and how does it achieve this effect? (4 marks)
speeds up a reaction • provides an alternative route or forms an intermediate of some sort • lowers activation energy (can be read into a label on a Boltzmann distribution) • so more molecules have E > activation energy or more collisions are successful • weakens bonds in the reactants
What can Ca(OH)2 be used for?
to neutralise soil
Products when group 2 elements react with water
x-hydroxide + hydrogen gas
