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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A reversible reaction reaches dynamic equilibrium in a closed container at constant temperature. A chemist then injects additional reactant into the container.
Model answer (4 marks)
At equilibrium the forward and reverse rates are equal. Adding more reactant raises its concentration, so the forward rate increases more than the reverse rate, breaking equilibrium. The system responds by shifting the equilibrium to the right, forming more products, in line with Le Chatelier’s principle. The equilibrium constant K remains unchanged because it depends only on temperature, which is constant.
Examiner tips
- Use the term ‘Le Chatelier’s principle’ to show understanding of the shift.
- Mention that the forward rate increases more than the reverse rate.
- State that K is unchanged because temperature is constant.
- Keep the answer concise and use correct chemical terminology.
Common mistakes
- Failing to explain that the forward rate increases more than the reverse rate.
- Saying the equilibrium shifts to the left or that K changes.
- Using vague language like ‘the system changes’ without specifying the shift direction.
Mark scheme (4 marks)
- At equilibrium, the rates of the forward and reverse reactions are equal; adding reactant increases the concentration of reactant species.
- 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.
- The equilibrium position shifts to the right (towards products) to partially counteract the increase in reactant concentration, consistent with Le Chatelier's principle.
- The value of the equilibrium constant K does not change, because K depends only on temperature and temperature is held constant.
Key terms in this question
equilibrium position · reversible reaction
Related
- All IB DP Chemistry Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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