Carbon monoxide reacts with steam in a reversible reaction: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g). The forward reaction is exothermic. A mixture of these gases is allowed to reach equilibrium in a sealed container. Explain how each of the following changes affects the position of equilibrium and the yield of hydrogen: (i) increasing the temperature; (ii) increasing the pressure; (iii) adding more steam.

AQA A-Level Chemistry (7405) — 3.1.6 Chemical equilibria, Le Chatelier and Kc · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

(i) Increasing the temperature shifts the equilibrium to the left (the endothermic direction) because the forward reaction is exothermic. Therefore the yield of hydrogen decreases.

(ii) Increasing the pressure has no effect on the position of equilibrium because the number of moles of gas is the same on both sides of the equation.

(iii) Adding more steam shifts the equilibrium to the right (the forward reaction is favoured). Therefore the yield of hydrogen increases.

Examiner tips

  • Use the correct Le Chatelier principle for temperature (exothermic → left), pressure (equal moles → no shift), and concentration (adding reactant → right).
  • Show the effect on H₂ yield directly after each point.

Common mistakes

  • Confusing the direction of the temperature shift (writing right instead of left).
  • Assuming pressure changes affect the equilibrium when the moles are equal.
  • Failing to link the shift to the hydrogen yield.

Mark scheme (5 marks)

  1. (i) Increasing temperature shifts the equilibrium to the left / in the endothermic direction
  2. (i) Therefore the yield of hydrogen decreases
  3. (ii) Increasing pressure has no effect on the position of equilibrium because there are equal numbers of moles of gas on both sides
  4. (iii) Adding more steam shifts the equilibrium to the right / forward reaction is favoured
  5. (iii) Therefore the yield of hydrogen increases

Key terms in this question

position of equilibrium · yield · exothermic

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