# Explain why, for a reversible reaction at equilibrium, increasing the concentration of a reactant causes the equilibrium position to shift, and state what happens to the value of the equilibrium constant, K, when this change is made at constant temperature.

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

> A reversible reaction reaches dynamic equilibrium in a closed container at constant temperature. A chemist then injects additional reactant into the container.

## Mark scheme (4 marks)

1. At equilibrium, the rates of the forward and reverse reactions are equal; adding reactant increases the concentration of reactant species.
2. The increased reactant concentration causes the rate of the forward reaction to increase (more than the rate of the reverse reaction), so the system is no longer at equilibrium.
3. The equilibrium position shifts to the right (towards products) to partially counteract the increase in reactant concentration, consistent with Le Chatelier's principle.
4. The value of the equilibrium constant K does not change, because K depends only on temperature and temperature is held constant.

## Key terms

- [equilibrium position](https://www.gradenine.co.uk/glossary/equilibrium-position)
- [reversible reaction](https://www.gradenine.co.uk/glossary/reversible-reaction)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" 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/explain-why-for-a-reversible-reaction-ba7e11ee) · Published by Druglandscape Ltd.