Nitrogen and hydrogen react together in a reversible reaction to form ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. Explain how each of the following changes would affect the position of equilibrium and the yield of ammonia: increasing the pressure; increasing the temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Increasing the pressure shifts the equilibrium to the right, i.e. towards the side with fewer moles of gas.
Because there are 4 moles of gas on the left (N₂ + 3H₂) and only 2 moles on the right (2NH₃), the reaction that produces fewer moles of gas is favoured when the pressure is increased.
Thus the yield of ammonia increases.
Increasing the temperature shifts the equilibrium to the left, i.e. in the endothermic direction.
Since the forward reaction is exothermic, the system opposes the increase in temperature by favouring the reverse reaction, so the yield of ammonia decreases.
Because there are 4 moles of gas on the left (N₂ + 3H₂) and only 2 moles on the right (2NH₃), the reaction that produces fewer moles of gas is favoured when the pressure is increased.
Thus the yield of ammonia increases.
Increasing the temperature shifts the equilibrium to the left, i.e. in the endothermic direction.
Since the forward reaction is exothermic, the system opposes the increase in temperature by favouring the reverse reaction, so the yield of ammonia decreases.
Examiner tips
- Use the phrase "shifts the equilibrium to the right/left" to show understanding of Le Chatelier’s principle.
- Show the moles of gas on each side to justify the pressure effect.
- Explain that the forward reaction is exothermic to justify the temperature effect.
- Mention the change in yield explicitly for each condition.
Common mistakes
- Confusing the direction of the shift (right vs left) for pressure or temperature.
- Failing to state the number of moles of gas on each side.
- Not linking the exothermic nature of the forward reaction to the temperature effect.
Mark scheme (5 marks)
- Increasing pressure shifts the equilibrium to the right / towards the side with fewer moles of gas
- Because there are 4 moles of gas on the left and 2 moles of gas on the right, so the reaction that produces fewer moles of gas is favoured
- Increasing pressure increases the yield of ammonia
- Increasing temperature shifts the equilibrium to the left / in the endothermic direction
- Because the forward reaction is exothermic, the system opposes the increase in temperature by favouring the endothermic (reverse) reaction, so the yield of ammonia decreases
Key terms in this question
reversible reaction · position of equilibrium · yield · exothermic
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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