A-LevelChemistryReaction kinetics9701/23 · May/June 2025 · Q37 Marks

Different hydrocarbon mixtures produced from fractional distillation of crude oil have different uses.

(a)(i)State the compound that is heated with long-chain hydrocarbons to produce more useful smaller alkanes and alkenes.[1]
(a)(ii)Describe how photochemical smog is produced during the combustion of petrol in an internal combustion engine.[2]
(b)(i)Name a catalyst for reaction 1.[1]
(b)(ii)Define activation energy, EA·[1]
(b)(iii)The Boltzmann distribution for the reaction mixture in reaction 1 is shown in Fig. 3.1. Use the Boltzmann distribution to explain the effect of adding a catalyst on the rate of reaction. [Fig. 3.1][2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.

[1] Al₂O₃/aluminium oxide
[2] • unburned hydrocarbons • react with NO/NO₂ • (to) produce PAN (– a component of photochemical smog) Any 2 points = 1 mark, all 3 points = 2 marks
[1] nickel / platinum
[1] minimum energy required for a collision to be effective
[2] M1 area under the curve with energy ≥ EA using a catalyst is larger so • greater proportion of particles with sufficient energy to react M2 rate faster because higher frequency of successful collisions

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