The Haber process is used to manufacture ammonia. The reaction reaches a dynamic equilibrium. Explain what is meant by the term 'dynamic equilibrium' and describe how changing the pressure affects the position of this equilibrium. The equation for the reaction is: N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Edexcel GCSE Chemistry (1CH0) — 9.3 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).

The Haber process operates at high temperature and high pressure. The reaction is reversible and reaches a state of dynamic equilibrium inside the reactor vessel.

Model answer (4 marks)

A dynamic equilibrium is a state in which the forward and reverse reactions continue to occur at the same rate, so the concentrations of reactants and products remain constant.

In the Haber process, increasing the pressure favours the side with fewer gas molecules. Because the products (2 NH₃) contain fewer moles of gas than the reactants (4 moles of N₂ + H₂), the equilibrium shifts to the right, producing more ammonia.

The shift reduces the total pressure by decreasing the number of gas molecules in the system.

Examiner tips

  • Use the term "dynamic equilibrium" and explain both reactions continue and rates are equal. Show the effect of pressure using the mole count (4 → 2) to justify the shift. Keep the answer concise and use correct chemical formulae.
  • common_mistakes
  • :
  • Confusing static with dynamic equilibrium. Failing to mention that both reactions still occur. Not linking the pressure change to the difference in gas moles.

Mark scheme (4 marks)

  1. The forward and reverse reactions are both still occurring (at equilibrium)
  2. The rate of the forward reaction equals the rate of the reverse reaction
  3. Increasing pressure shifts the equilibrium to the right / towards the products (ammonia)
  4. Because there are fewer moles of gas on the right-hand side (2 moles) than the left-hand side (4 moles), so the system reduces pressure by favouring the side with fewer gas molecules

Key terms in this question

dynamic equilibrium · pressure

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