Nitrogen and hydrogen react in a closed container to form ammonia according to the equation: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The reaction reaches equilibrium. Explain how the value of Kp changes, if at all, when (i) the temperature is increased and (ii) the pressure is increased, given that the forward reaction is exothermic.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The industrial production of ammonia involves a reversible reaction between nitrogen and hydrogen gases. At equilibrium, all three gases are present in the closed system.
Model answer (5 marks)
(i) Increasing the temperature shifts the equilibrium to the endothermic side, i.e. the reverse reaction. The proportion of ammonia decreases, so the equilibrium constant Kp decreases.
(ii) Changing the pressure does not alter the value of Kp; it only shifts the position of equilibrium but Kp remains the same.
(ii) Changing the pressure does not alter the value of Kp; it only shifts the position of equilibrium but Kp remains the same.
Examiner tips
- Use the term 'endothermic' for the reverse reaction. Mention that Kp is temperature‑dependent but pressure‑independent. Show the link between equilibrium shift and Kp value. Keep the answer concise and use correct chemical symbols.
Common mistakes
- Saying Kp increases with temperature. Confusing Kp with the position of equilibrium. Forgetting that pressure changes only the equilibrium position, not Kp.
Mark scheme (5 marks)
- Kp does not change when pressure is increased
- Kp only changes when temperature changes (not pressure)
- Increasing temperature causes Kp to decrease
- Because the forward reaction is exothermic, increasing temperature favours the reverse (endothermic) reaction
- The shift towards reactants means the proportion of ammonia decreases, so Kp decreases (linking position of equilibrium to Kp value)
Key terms in this question
Kp · equilibrium · 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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