The manufacture of ammonia uses the Haber process, which involves the following reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol A student claims that using a very high temperature will always give the best yield of ammonia. Evaluate this claim, using Le Chatelier's principle where appropriate.

Edexcel A-Level Chemistry (9CH0) — 10.1 Dynamic equilibria and Le Chatelier · Evaluate · 5 marks · View as Markdown

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

The Haber process is a major industrial reaction used to produce ammonia for fertilisers and other chemicals. The reaction reaches a dynamic equilibrium under industrial conditions.

Model answer (5 marks)

The reaction is exothermic (ΔH = –92 kJ mol⁻¹). By Le Chatelier’s principle, increasing temperature favours the endothermic direction, i.e. the reverse reaction, so the equilibrium shifts to the left and the equilibrium yield of NH₃ decreases. Therefore a very high temperature cannot give the best yield – a lower temperature gives a higher equilibrium yield.
However, at very low temperatures the rate of reaction is extremely slow, so the system takes a long time to reach equilibrium. In practice a compromise temperature (≈450 °C) is used industrially to give a reasonable rate while maintaining an acceptable yield.

Examiner tips

  • Mention the sign of ΔH and its effect on equilibrium position
  • Explain the rate–yield trade‑off
  • State the industrial compromise temperature
  • Use correct chemical equations and units

Common mistakes

  • Saying a high temperature always increases yield without noting the exothermic nature
  • Confusing equilibrium position with reaction rate
  • Omitting the need for a compromise temperature

Mark scheme (5 marks)

  1. The reaction is exothermic (forward reaction releases heat / ΔH is negative)
  2. By Le Chatelier's principle, increasing temperature shifts the equilibrium in the endothermic direction (reverse reaction), reducing the yield of ammonia
  3. The student's claim is therefore incorrect regarding yield — a lower temperature gives a higher equilibrium yield of ammonia
  4. However, a very low temperature makes the rate of reaction too slow (and the reaction would take too long to reach equilibrium)
  5. A compromise / moderate temperature (around 450 °C) is used industrially to balance an acceptable rate with a reasonable yield

Key terms in this question

Le Chatelier's principle · yield

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