The production of ammonia in the Haber process involves the following reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. A chemical engineer is considering changing the conditions used in the Haber process. Explain how increasing the temperature and increasing the pressure each affect the position of dynamic equilibrium in this reaction.

OCR A-Level Chemistry A (H432) — 5.1 Rates, equilibrium and pH · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

The Haber process is used industrially to manufacture ammonia. The reaction is carried out in a closed system, meaning the reaction reaches dynamic equilibrium.

Model answer (5 marks)

Increasing the temperature adds heat to the system. Because the forward reaction is exothermic, heat is a product. Le Chatelier’s principle predicts that the equilibrium will shift to consume the added heat, i.e. to the endothermic direction. Thus the equilibrium moves to the left, giving less NH₃.

Increasing the pressure favours the side with fewer gas molecules. The left side has 4 mol N₂+H₂, the right side has 2 mol NH₃. Therefore the equilibrium shifts to the right, producing more NH₃.

Both shifts are explained by Le Chatelier’s principle, which states that the equilibrium position moves to counteract the imposed change.

Examiner tips

  • Use the exact wording ‘shift to the left’ or ‘shift to the right’ to show understanding of Le Chatelier’s principle.
  • Mention that the forward reaction is exothermic and the reverse is endothermic to justify the temperature effect.
  • Show the change in moles of gas to justify the pressure effect.
  • Keep the answer concise – 5 marks can be earned with two short, clear sentences for each condition.

Common mistakes

  • Saying the temperature shift produces more NH₃ (wrong direction).
  • Confusing the number of moles on each side (e.g. stating 3 mol on the right).
  • Failing to mention that the shift is a response to the imposed change (Le Chatelier’s principle).

Mark scheme (5 marks)

  1. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
  2. So increasing temperature shifts the equilibrium to the left / towards the reactants, producing less ammonia
  3. Increasing pressure shifts the equilibrium position towards the side with the smaller number of moles of gas
  4. The right-hand side has 2 moles of gas compared to 4 moles on the left, so equilibrium shifts to the right / towards the products
  5. Both changes are explained by Le Chatelier's principle — the equilibrium position shifts to counteract the imposed change

Key terms in this question

dynamic equilibrium · reversible reaction

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