# 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.

> IB DP Chemistry Higher Level (2023 syllabus) — R2.3 How far? The extent of chemical change · Evaluate · 4 marks

## Mark scheme (4 marks)

1. The student's claim is incorrect / the value of Kc does NOT change when a catalyst is added.
2. A catalyst lowers the activation energy of both the forward and reverse reactions equally / by the same amount.
3. 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.
4. 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

- [catalyst](https://www.gradenine.co.uk/glossary/catalyst)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Evaluate" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-claims-that-adding-a-d6fddc12) · Published by Druglandscape Ltd.