AQA A-Level Chemistry Paper 3, 2021: Question 20

1 mark · Easy difficulty · Multiple Choice

This question explores the effect of a catalyst on a reversible chemical reaction. You're asked to consider how catalysts influence both the forward and reverse reaction rates, without changing thermodynamic quantities such as enthalpy change or the equilibrium constant.

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Question

AQA A-Level Chemistry Paper 3, 2021: Question 20
Question text

20 Which statement about the addition of a catalyst to an equilibrium mixture is correct?

[1 mark]

A The activation energy for the reverse reaction increases.

B The equilibrium constant for the forward reaction increases.

C The rate of the reverse reaction increases.

D The enthalpy change for the forward reaction decreases.

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2021: Question 20

20 C 1 The rate of the reverse reaction increases.

How to answer it

Multiple Choice Explained: Effect of Catalyst on Equilibrium

Correct Answer: C – The rate of the reverse reaction increases

Why? A catalyst speeds up both the forward and reverse reactions equally by providing an alternative pathway with lower activation energy. This means both reaction rates increase, but the position of equilibrium and enthalpy change remain unchanged.

Why A is incorrect:

The activation energy for the reverse reaction increases – this is incorrect because a catalyst lowers the activation energy for both forward and reverse reactions.

Why B is incorrect:

The equilibrium constant increases – incorrect because a catalyst does not affect the equilibrium constant. It only changes how quickly equilibrium is reached.

Why D is incorrect:

The enthalpy change decreases – incorrect because the catalyst does not affect thermodynamic properties like enthalpy change; it only influences the reaction pathway (i.e. kinetics).

Topics

Physical Chemistry · 3.1.6 Chemical Equilibria, Le Chatelier's Principle, and Kc

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.