The industrial production of ammonia uses the reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. Explain how increasing the temperature and increasing the pressure each affect the position of equilibrium in this reaction, and explain why a catalyst is used in this process.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ammonia is manufactured on a large scale using the Haber process. The reaction vessel is a closed system, and the conditions used must be carefully chosen to produce ammonia efficiently.
Model answer (5 marks)
Increasing the temperature shifts the equilibrium toward the endothermic direction, i.e. the reverse reaction (N₂+3H₂→2NH₃) is exothermic, so a higher temperature favours the reactants and the yield of NH₃ decreases.
Increasing the pressure shifts the equilibrium toward the side with fewer gas moles. The product side has 2 moles of gas versus 4 moles of reactants, so higher pressure favours NH₃ formation and the yield increases.
A catalyst is used because it provides an alternative pathway with a lower activation energy, increasing the rate of both forward and reverse reactions so that equilibrium is reached more quickly. It does not alter the equilibrium position.
Increasing the pressure shifts the equilibrium toward the side with fewer gas moles. The product side has 2 moles of gas versus 4 moles of reactants, so higher pressure favours NH₃ formation and the yield increases.
A catalyst is used because it provides an alternative pathway with a lower activation energy, increasing the rate of both forward and reverse reactions so that equilibrium is reached more quickly. It does not alter the equilibrium position.
Examiner tips
- State the effect of temperature on the exothermic forward reaction and the resulting yield. Mention the pressure effect via moles of gas. Explain the catalyst’s role in rate, not equilibrium position.
Common mistakes
- Confusing the temperature effect (claiming higher temperature increases NH₃). Saying the catalyst changes the equilibrium position. Omitting the moles‑of‑gas argument for pressure.
Mark scheme (5 marks)
- Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
- So the yield / amount of ammonia decreases when temperature is increased
- Increasing pressure shifts the equilibrium position towards the side with the smaller number of moles of gas
- So increasing pressure increases the yield / amount of ammonia produced
- A catalyst increases the rate of reaction by providing a different reaction pathway with a lower activation energy, so equilibrium is reached more quickly (but it does not change the position of equilibrium)
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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- Decode the mark scheme abbreviations →
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