Explain why the value of the equilibrium constant, Kc, for a reversible reaction changes when the temperature is increased, but remains unchanged when a catalyst is added to the equilibrium mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Kc is the ratio of the rate constants for the forward and reverse reactions, kf/kr, or equivalently the ratio of the concentrations of products to reactants at equilibrium. Both kf and kr are temperature‑dependent because they are governed by the Arrhenius equation. When the temperature is increased the rate constant for the endothermic direction rises more than that for the exothermic direction, so the ratio kf/kr changes and therefore Kc changes.
A catalyst provides an alternative pathway with a lower activation energy for both the forward and reverse reactions. It increases kf and kr by the same factor, so the ratio kf/kr – and hence Kc – remains unchanged. The catalyst only speeds the attainment of equilibrium, not its position.
A catalyst provides an alternative pathway with a lower activation energy for both the forward and reverse reactions. It increases kf and kr by the same factor, so the ratio kf/kr – and hence Kc – remains unchanged. The catalyst only speeds the attainment of equilibrium, not its position.
Examiner tips
- State that Kc = kf/kr and that kf, kr are temperature‑dependent; explain the differential effect on endothermic vs exothermic directions; mention that a catalyst increases both kf and kr equally; keep answer concise and use correct terminology.
Common mistakes
- Confusing Kc with the rate constant of a single step; claiming the catalyst changes Kc; not explaining the differential temperature effect on forward and reverse rates.
Mark scheme (4 marks)
- Kc depends on the ratio of rate constants for forward and reverse reactions (or on the relative energies/Gibbs energy of reactants and products), which are temperature-dependent.
- Increasing temperature increases the rate of the endothermic direction more than the exothermic direction (or vice versa), so the forward and reverse rate constants change by different amounts, altering Kc.
- A catalyst increases the rate of both the forward and reverse reactions equally (by providing an alternative pathway of lower activation energy for both directions).
- Because both rate constants are increased by the same factor, their ratio kf/kr is unchanged, so Kc remains constant; equilibrium is reached more quickly but at the same position.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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