The Haber process is used to manufacture ammonia. The reaction is a reversible reaction that reaches dynamic equilibrium. Explain what happens to the rate of the forward reaction and the rate of the backward reaction as the system moves towards dynamic equilibrium, and state what this means for the concentrations of ammonia, nitrogen and hydrogen once dynamic equilibrium is reached.

Edexcel GCSE Chemistry (1CH0) — 5.3 Dynamic equilibria, calculations involving volumes of gases (Chem only) · Explain · 4 marks · View as Markdown

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

In the Haber process, nitrogen and hydrogen react together to form ammonia. The reaction is reversible and is carried out at 450°C and 200 atm using an iron catalyst.

Model answer (4 marks)

As the reaction proceeds the rate of the forward reaction decreases because the concentrations of the reactants, N₂ and 2H₂, are being consumed.
The rate of the backward reaction increases because the concentration of the product, NH₃, is increasing.
At dynamic equilibrium the rate of the forward reaction equals the rate of the backward reaction.
Consequently the concentrations of N₂, H₂ and NH₃ remain constant once dynamic equilibrium is reached.

Examiner tips

  • Use the word ‘decreases’ for the forward rate and ‘increases’ for the reverse rate. State that the rates become equal at equilibrium. Mention that the concentrations of all species are constant at equilibrium.
  • Include the catalyst only if asked; it does not affect the rates directly in the explanation.

Common mistakes

  • Confusing the direction of the rate changes (e.g. saying the forward rate increases). Forgetting to state that the concentrations are constant at equilibrium. Using vague terms like ‘slow down’ instead of ‘decrease’ or ‘speed up’ instead of ‘increase’.

Mark scheme (4 marks)

  1. As the reaction proceeds, the rate of the forward reaction decreases (as reactants are used up)
  2. The rate of the backward reaction increases (as products build up)
  3. At dynamic equilibrium the rate of the forward reaction equals the rate of the backward reaction
  4. The concentrations of nitrogen, hydrogen and ammonia remain constant at dynamic equilibrium

Key terms in this question

dynamic equilibrium · forward reaction · backward reaction · concentration · reversible reaction

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