AQA A-Level Chemistry Paper 3, June 2025: Question 9

1 mark · Easy difficulty · Multiple Choice

Identify which statement correctly describes the effect of using a catalyst in a reversible reaction.

Practise this question

Question

Multiple-choice question 09 asking: 'Which statement about the use of a catalyst in a reaction is correct?' followed by four options with selection lozenges: A 'The activation energy of the reverse reaction increases.', B 'The equilibrium constant increases.', C 'The rate of the reverse reaction increases.', D 'The enthalpy change for the reaction decreases.'
Question text

09 Which statement about the use of a catalyst in a reaction is correct?

[1 mark]

A The activation energy of the reverse reaction increases.

B The equilibrium constant increases.

C The rate of the reverse reaction increases.

D The enthalpy change for the reaction decreases.

Mark scheme

Show the mark scheme Mark scheme for question 09 indicating the correct answer is C ('The rate of the reverse reaction increases.') worth 1 mark (AO1).

09 C 1 (AO1) The rate of the reverse reaction increases.

How to answer it

AQA A-Level Chemistry • Physical Chemistry

Effect of Catalysts on Reaction Rates and Equilibria

What this question tests

This question assesses your fundamental knowledge of how a catalyst operates in reversible chemical reactions (Specification 3.1.5 & 3.1.6). Specifically, it tests your ability to distinguish between kinetic effects (rate of reaction, activation energy) and thermodynamic properties (equilibrium constant K, enthalpy change ΔH).

Question 09 Breakdown

1 Mark (AO1)

Multiple Choice: Statements about the use of a catalyst

✅ Correct Answer: C

The rate of the reverse reaction increases.

A catalyst provides an alternative reaction pathway with a lower activation energy (Ea). Because this pathway is available in both directions, it lowers the activation energy of both the forward and reverse reactions by the exact same amount. Consequently, the rates of both the forward and reverse reactions increase equally.

🧠 Option-by-Option Elimination

  • A is incorrect: The catalyst lowers the activation energy for both directions; it never increases it.
  • B is incorrect: The equilibrium constant (Kc or Kp) is only altered by changes in temperature. A catalyst speeds up the approach to equilibrium but does not alter the position of equilibrium.
  • C is correct: By lowering Ea in reverse, a greater proportion of reverse collisions have E ≥ Ea, speeding up the reverse reaction.
  • D is incorrect: Enthalpy change (ΔH) is a state function depending only on the energy of initial reactants and final products; the catalyst leaves ΔH completely unchanged.

💡 Key Knowledge Recap

  • Definition: A catalyst increases the rate of a chemical reaction without being consumed, by offering an alternative route with lower activation energy (Ea).
  • Equal Effect: In a reversible reaction, Ea(forward) and Ea(reverse) decrease by identical amounts.
  • Equilibrium Yield: Yield is unaffected because rates of forward and reverse reactions increase by the identical factor.
  • What a catalyst does NOT change:
    • ΔH (enthalpy change)
    • ΔG (Gibbs free energy change)
    • Kc or Kp (equilibrium constant)
    • Equilibrium yield / composition

❌ Common Misconceptions & Traps

  • "Catalysts only speed up the forward reaction": Many students forget that a reversible reaction proceeds via the exact same transition state backwards. Lowering the energy barrier lowers it from both sides.
  • Confusing Rate with Yield: A common error is assuming a catalyst increases the amount of product formed. It only reduces the time required to reach dynamic equilibrium.
  • Believing catalysts change Kc: Only temperature changes the numerical value of the equilibrium constant.

📐 Visualizing the Reaction Profile

On an enthalpy profile diagram:

  • The peak height (transition state) is lowered.
  • Distance from reactants to peak = Ea(forward) → decreases.
  • Distance from products to peak = Ea(reverse) → decreases.
  • Difference between reactants and products = ΔH → remains identical.

Topics

Physical Chemistry · 3.1.5 Kinetics · 3.1.6 Chemical Equilibria, Le Chatelier's Principle and Kc

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.