# 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.

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

## Mark scheme (4 marks)

1. 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.
2. 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.
3. 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).
4. 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

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

## Related

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