A student claims that adding a catalyst to a reaction mixture at equilibrium will increase the value of Kc because the forward reaction is speeded up more than the reverse reaction. Evaluate this claim.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The claim is incorrect – a catalyst does not change Kc. A catalyst lowers the activation energy of both the forward and reverse reactions by the same amount, so the rate constants kf and kr are increased by the same factor. Consequently the ratio kf/kr, which equals Kc, remains unchanged. Kc depends only on temperature; the catalyst merely allows equilibrium to be reached more quickly, not altering the thermodynamic position of equilibrium.
Examiner tips
- State that Kc is unchanged by a catalyst; cite the equal lowering of activation energy for both directions.
- Explain that the ratio kf/kr stays the same, so equilibrium concentrations are unaffected.
- Mention that Kc depends only on temperature, not on kinetic factors.
Common mistakes
- Claiming Kc increases because the forward rate is faster; ignoring that the reverse rate is equally affected.
- Confusing the speed of reaching equilibrium with the position of equilibrium.
- Failing to mention that Kc is a thermodynamic constant dependent only on temperature.
Mark scheme (4 marks)
- The student's claim is incorrect / the value of Kc does NOT change when a catalyst is added.
- A catalyst lowers the activation energy of both the forward and reverse reactions equally / by the same amount.
- Because both rates increase by the same factor, the ratio of rate constants (kf/kr) remains unchanged, so the position of equilibrium / the equilibrium concentrations are not affected.
- Kc depends only on temperature; the catalyst allows equilibrium to be reached more quickly but does not alter the thermodynamics of the system.
Key terms in this question
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
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