A reversible reaction takes place in a closed container. After some time, the system reaches equilibrium. A student adds more reactant to the closed container. Describe and explain what happens to the position of equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The added reactant increases its concentration.
1. The system responds by shifting the equilibrium to the right (forward reaction) to consume the excess reactant.
2. This shift is driven by Le Chatelier’s principle – the system opposes the change.
3. Consequently, more products are formed and their concentration rises.
4. The concentration of reactants falls (relative to immediately after the addition) until a new equilibrium is established.
1. The system responds by shifting the equilibrium to the right (forward reaction) to consume the excess reactant.
2. This shift is driven by Le Chatelier’s principle – the system opposes the change.
3. Consequently, more products are formed and their concentration rises.
4. The concentration of reactants falls (relative to immediately after the addition) until a new equilibrium is established.
Examiner tips
- Use the phrase ‘shift to the right’ and mention Le Chatelier’s principle.
- Show the cause (increase in reactant) and effect (more products).
- Keep the answer concise – 4 points, one sentence each.
Common mistakes
- Confusing the direction of the shift (saying it moves left).
- Failing to mention Le Chatelier’s principle as the reason for the shift.
- Not stating that the reactant concentration decreases after the shift.
Mark scheme (4 marks)
- The position of equilibrium shifts towards the products (forward reaction is favoured)
- This is because the system acts to counteract the change / oppose the increase in concentration of reactant
- The concentration of products increases / more products are formed until a new equilibrium is reached
- The concentration of reactants decreases (compared to immediately after addition) as equilibrium is re-established
Key terms in this question
reversible reaction · equilibrium · position of equilibrium
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Describe and explain" question →
- Decode the mark scheme abbreviations →
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