AQA A-Level Chemistry Paper 3, 2023: Question 25

1 mark · Easy difficulty · Multiple Choice

Effect of Catalysts on Equilibrium This question examines the properties and effects of catalysts in equilibrium reactions, specifically addressing their impact on reaction rates and equilibrium constants without altering the thermodynamic properties of the reaction.

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Question

AQA A-Level Chemistry Paper 3, 2023: Question 25
Question text

25 Which statement about catalysts used in reactions at equilibrium, at a

constant temperature, is correct?

[1 mark]

A They are always used in the solid state.

They increase the rate of the forward reaction but

B

decrease the rate of the reverse reaction.

C They have no effect on the value of the equilibrium constant.

D They make the forward reaction more exothermic.

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 25

25 C 1 (AO2) They have no effect on the value of the equilibrium constant.

How to answer it

Catalysts in Equilibrium Reactions

Question

Which statement about catalysts used in reactions at equilibrium, at a constant temperature, is correct?

  • A: They are always used in the solid state.
  • B: They increase the rate of the forward reaction but decrease the rate of the reverse reaction.
  • C: They have no effect on the value of the equilibrium constant.
  • D: They make the forward reaction more exothermic.

Correct Answer: C

The correct statement is C: They have no effect on the value of the equilibrium constant. Catalysts increase the rates of both the forward and reverse reactions equally, without altering the position of equilibrium or the equilibrium constant, which depends only on temperature.

Teacher Tip: Emphasise to students that catalysts only affect the kinetics of a reaction (i.e., the rate) but not the thermodynamics (e.g., equilibrium constant or enthalpy changes).

Explanation of Other Options

  • A: Catalysts can exist in any phase (solid, liquid, or gas), depending on the reaction mechanism.
  • B: Catalysts do not favour one direction over the other; they affect both forward and reverse reactions equally.
  • D: Catalysts do not change the enthalpy of a reaction or make the reaction more or less exothermic/endothermic.

How Catalysts Work

Catalysts provide an alternative reaction pathway with a lower activation energy, increasing the number of particles that can react at a given temperature. This applies equally to both the forward and reverse reactions in equilibrium.

Final Answer

The correct answer is:

C: They have no effect on the value of the equilibrium constant.

Topics

Physical Chemistry · 3.1.5 Kinetics

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.