Chemistry Paper 2 June 2024

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1. AThe dissociation of water is endothermic, so raising the temperature shifts the equilibrium H2O <=> H+ + OH- to the right, increasing [H+][OH-] and hence Kw.
2. BThe dissociation of water is exothermic, so raising the temperature shifts the equilibrium to the right, increasing Kw.
3. CThe dissociation of water is endothermic, so raising the temperature shifts the equilibrium to the left, which increases the concentration of undissociated water and therefore increases Kw.
4. DRaising the temperature increases the viscosity of water, which allows more ions to dissociate and raises Kw.

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1. AT2 is higher: increasing temperature always produces a taller, narrower peak as molecules become more uniform in energy.
2. BT1 is higher: the peak of T1 lies at a lower energy value, and a higher temperature always corresponds to a lower average molecular energy.
3. CT1 is higher: a taller peak means more molecules have enough energy to react, which only happens at a higher temperature.
4. DT2 is higher: raising the temperature gives molecules a wider spread of energies, shifting the peak to a higher energy value while lowering and broadening the curve.

1. AA chemical reaction occurs whenever reacting particles collide, regardless of their energy or orientation, since any collision transfers enough energy to react.
2. BA chemical reaction occurs when reacting particles are heated until they move fast enough to break their own bonds before any collision takes place.
3. CA chemical reaction occurs only when reacting particles collide with each other with at least the activation energy and with the correct orientation.
4. DA chemical reaction occurs only when reacting particles collide with the walls of the container with enough energy to react.

1. AIt adds chlorine gas to the water to kill any remaining bacteria and viruses before it is supplied to consumers.
2. BIt removes suspended solid particles by causing them to clump together and sink to the bottom of the tank.
3. CIt reduces the hardness of the water by removing dissolved calcium and magnesium ions.
4. DIt increases the dissolved oxygen content of the water and strips out dissolved gases such as hydrogen sulfide and carbon dioxide, improving taste and odour.

1. AIt dissolves and removes dissolved organic pesticides and herbicides that aeration cannot remove.
2. BIt kills bacteria and other micro-organisms present in the water, acting in place of chlorination.
3. CIt acts as a coagulant/flocculant: the Al3+ ions neutralise the charge on suspended colloidal particles, causing them to clump together and settle out.
4. DIt raises the pH of the water, neutralising any acidity picked up from the source before the water is distributed.

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1. AThe Al-O bond is a coordinate (dative) bond, which makes Al2O3 react only with bases and never with acids.
2. BAl2O3 is purely ionic, like Na2O, so it reacts with acids in the same way but is completely unreactive towards bases of any strength.
3. CAl3+ is small and highly charged, so it polarises the oxide ion strongly, giving Al2O3 bonding partway between ionic and covalent; this lets it react as a base with acids and as an acid with strong bases.
4. DAl2O3 is purely covalent, like SiO2, so it cannot react with acids at all and only dissolves in concentrated alkalis.

1. AThey are called average because they represent the mean of the bond's dissociation energy and its formation energy, which are always different.
2. BEach bond energy value is an average of the bond's energy measured at several different temperatures, since bond strength changes noticeably with temperature.
3. CThe values are averaged over one mole of bonds because a single bond is too small for its energy to be measured directly by any instrument.
4. DA given bond type (e.g. C-H) has a slightly different actual bond energy in each different molecule it occurs in, so the quoted value is an average taken across many different compounds.

1. AThe fluorine atoms in F2 are so small that the non-bonding lone pairs on each atom are close enough to repel each other strongly, weakening the F-F bond; this repulsion is much less significant in the larger Cl-Cl bond.
2. BFluorine has a lower nuclear charge than chlorine, so its nucleus attracts the shared bonding electrons less strongly, making the F-F bond weaker.
3. CThe F-F bond is longer than the Cl-Cl bond, and longer bonds are always weaker regardless of any other factor.
4. DChlorine atoms are more electronegative than fluorine atoms, so chlorine forms a stronger, more polar covalent bond with itself.

1. AHydrogen iodide has a much weaker (less thermally stable) H-I bond than the strong H-Cl bond in hydrogen chloride, so HI readily decomposes back into H2 and I2, whereas HCl does not decompose under the same conditions.
2. BIodine is a stronger oxidising agent than chlorine, so the forward reaction with iodine is much faster and reaches equilibrium sooner than with chlorine.
3. CChlorine gas is denser than iodine vapour, so the chlorine reaction proceeds to completion because the heavier gas cannot escape and re-form as reactants.
4. DThe reaction with iodine only reverses because iodine is a solid at room temperature, while chlorine, being a gas, can never revert to reactants.

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1. AGeometric (cis-trans) isomerism, because rotation about the bond from that carbon to the ring is restricted, fixing the methyl and isopropyl groups on opposite sides.
2. BOptical isomerism, because that carbon is a chiral (asymmetric) centre attached to four different groups, so drug D and its mirror image are non-superimposable enantiomers.
3. CStructural (chain) isomerism, because the four different groups on that carbon can be rearranged into a different carbon skeleton with the same molecular formula.
4. DPositional isomerism, because the methyl group could equally be attached at a different point along the side-chain without changing the molecule's identity.

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1. AIt reduces the boiling point of the intermediate, which is required so that step 3's acid hydrolysis can be carried out at room temperature.
2. BIt converts a non-polar molecule into a polar one, which is only needed so that the final product, drug D, will dissolve in water.
3. CIt removes a chiral centre from the molecule, which is necessary before drug D can be classified as optically inactive.
4. DIt extends the carbon chain by one carbon atom, which is essential because the target carboxylic acid side-chain, -CH2CH2COOH, has one more carbon than the starting chloroalkane.

1. AThe purple KMnO4 solution turns bright orange as the ring itself is oxidised to a quinone.
2. BA white precipitate forms as the carboxylic acid side-chain is reduced back to an alcohol.
3. CNo visible change occurs, because KMnO4 cannot oxidise any part of an aromatic compound.
4. DThe purple KMnO4 solution is decolourised (turns colourless/pale) as it oxidises the alkyl side-chain(s) on the benzene ring.

1. ACarbon dioxide and water only, since acidified KMnO4 completely combusts any organic side-chain attached to a ring.
2. BBenzene-1,4-dicarboxylic acid: acidified KMnO4 oxidises any alkyl side-chain that has a benzylic hydrogen all the way down to -COOH directly on the ring, converting both substituents into carboxylic acid groups.
3. CBenzene-1,4-diol, since KMnO4 hydroxylates the ring positions para to each existing substituent.
4. DThe starting ethylbenzene derivative is fully restored, since KMnO4 only re-forms the original alkyl chains.

1. ABoiling water increases the degree of cross-linking between nylon chains, making the fabric permanently stiff and unable to be worn again.
2. BBoiling water reacts directly with unseen benzene rings in nylon, converting them into much more brittle aromatic ketones.
3. CBoiling water can hydrolyse the amide linkages and disrupts the hydrogen bonding between polymer chains, weakening the fibres and causing the fabric to lose its shape/strength.
4. DNylon is completely insoluble and totally unreactive, so boiling it only wastes energy without damaging the fabric in any way.

1. AIt cools the exhaust gases before they leave the vehicle, which is its only function, since exhaust gas composition is unaffected by it.
2. BIt catalyses the conversion of harmful exhaust gases (carbon monoxide, oxides of nitrogen and unburnt hydrocarbons) into less harmful gases (carbon dioxide, nitrogen and water), reducing air pollution from the vehicle.
3. CIt increases the concentration of carbon monoxide in the exhaust gas so that the engine's fuel economy is improved.
4. DIt filters out solid soot particles only, trapping them permanently so that no gases at all can pass through into the atmosphere.

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1. ACarbon dioxide, nitrogen and water (vapour).
2. BCarbon monoxide, nitrogen dioxide and unburnt hydrocarbons.
3. CCarbon dioxide, oxygen and sulfur dioxide.
4. DNitrogen, hydrogen and carbon monoxide.

1. A2CxHy + O2 -> 2xC + yH2O
2. BCxHy + (x + y/4)O2 -> xCO2 + (y/2)H2O
3. CCxHy + O2 -> xCO + (y/2)H2
4. DCxHy + xO2 -> xCO2 + yH2

1. ACatalytic cracking uses a solid catalyst (e.g. a zeolite) at a moderate temperature and pressure and tends to give branched-chain and aromatic products, while steam cracking uses no catalyst, mixes the feed with steam at a very high temperature, and is used mainly to produce short-chain alkenes such as ethene.
2. BCatalytic cracking and steam cracking are two names for exactly the same industrial process, differing only in which country's oil industry uses each name.
3. CCatalytic cracking only breaks down alkenes into alkanes, while steam cracking only breaks down alkanes into alkenes.
4. DCatalytic cracking uses steam as the catalyst at low temperature, while steam cracking uses a solid metal catalyst at very high temperature.

1. AIt converts short-chain alkanes into longer-chain alkanes, since long-chain fuels are in far higher demand than short-chain ones.
2. BIt converts longer-chain hydrocarbon fractions, which are in lower demand, into shorter-chain, more useful products such as petrol, and into alkenes used as feedstock for the petrochemical (plastics) industry, better matching supply to demand.
3. CIt separates crude oil into fractions purely by their different densities, without changing any of the molecules present.
4. DIt removes sulfur-containing impurities from crude oil fractions, which is its only industrial purpose.

1. AThey have no d electrons at all, so only their fixed number of s electrons can ever be involved in bonding.
2. BTheir large atomic radius means electrons are never lost, so transition elements exist only in the zero oxidation state.
3. CTheir 3d and 4s electrons are close in energy, so different numbers of electrons from both sub-shells can be lost or involved in bonding under different conditions.
4. DTheir outer electrons are held so weakly that they are always completely removed, giving only one possible, very high oxidation state.

1. AThey have unusually low melting points, which allows them to melt during a reaction and mix more thoroughly with the reactants.
2. BTheir variable oxidation states let them form intermediate species with reactants, providing an alternative reaction pathway of lower activation energy, and/or they adsorb reactants onto their surface, weakening bonds and increasing local reactant concentration.
3. CTheir fixed, single oxidation state makes them chemically inert, so they can sit in a reaction mixture without ever being consumed or altered.
4. DTheir compounds are always brightly coloured, and it is this colour itself that provides the catalytic effect.

1. AIncineration burns away combustible/organic material, greatly reducing the bulk volume of waste, leaving a smaller, more concentrated residue that is cheaper and easier to store and dispose of safely.
2. BIncineration converts the radioactive isotopes present into completely different, non-radioactive elements.
3. CIncineration is required only to sterilise the waste of any living micro-organisms before storage.
4. DIncineration destroys all radioactivity in the waste, so the ash produced is no longer radioactive at all.

1. ADilution chemically converts the radioactive isotopes into stable, non-radioactive isotopes of the same element.
2. BDilution is done purely to increase the total volume of waste, making it easier for regulators to detect before disposal.
3. CDiluting lowers the concentration/activity of the radioactive material to below the level considered safe by regulation, reducing the radiation dose and harm to organisms and the environment it is released into.
4. DDilution speeds up the radioactive decay of the isotopes present, shortening their half-lives before release.