Ammonia is produced in the Haber process using the following reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. Explain what happens to the position of equilibrium when the temperature is increased, and what effect this has on the yield of ammonia.

OCR GCSE Chemistry A: Gateway Science (J248) — C5.3 Equilibria · Explain · 4 marks · View as Markdown

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

Ammonia is produced industrially via the Haber process. The reaction is reversible and reaches dynamic equilibrium under certain conditions.

Model answer (4 marks)

Increasing the temperature adds heat, which is a reactant for the endothermic reverse reaction. According to Le Chatelier’s principle the equilibrium shifts to the reverse side, breaking down NH₃ into N₂ and H₂. Consequently the amount of NH₃ present at equilibrium – the yield – decreases.

Examiner tips

  • Use the term ‘endothermic reverse reaction’ to show understanding of the reaction direction.
  • Explain that heat is a reactant for the reverse reaction – this shows application of Le Chatelier’s principle.
  • State the effect on yield explicitly – ‘decreases’ – to match the mark scheme.
  • Keep the answer concise and use correct chemical symbols and stoichiometry.”]

Mark scheme (4 marks)

  1. An increase in temperature causes the equilibrium position to shift in the direction of the endothermic reaction
  2. The endothermic direction is the backward/reverse reaction (breaking down ammonia)
  3. This is because Le Chatelier's principle states the equilibrium moves to counteract the change / to absorb the extra heat energy
  4. The yield of ammonia decreases because more ammonia is converted back into nitrogen and hydrogen

Key terms in this question

reversible reaction

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