The production of hydrogen for fuel cells involves the following reversible reaction carried out in a closed system: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g). This reaction is endothermic in the forward direction. A chemical engineer wants to increase the yield of hydrogen. Explain how changes in temperature and pressure would each affect the position of equilibrium and the yield of hydrogen.

OCR A-Level Chemistry A (H432) — 5.1 Rates, equilibrium and pH · Explain · 5 marks · View as Markdown

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

Engineers optimising hydrogen production must balance the effects of temperature and pressure on the position of equilibrium. The reaction CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g) is endothermic in the forward direction and involves a change in the number of moles of gas.

Model answer (5 marks)

1. Increasing the temperature favours the endothermic forward reaction, so the equilibrium shifts to the right.
2. This shift produces more CO and 3 H₂, so the hydrogen yield increases.
3. Decreasing the pressure favours the side with more gas molecules.
4. The right‑hand side contains 4 moles of gas compared with 2 moles on the left, so the equilibrium moves to the right.
5. Both effects are explained by Le Chatelier’s principle, which predicts a shift that counteracts the imposed change.

Examiner tips

  • Use the word ‘shift’ and state the direction (right) for both temperature and pressure changes. Include the reason (endothermic or greater moles) and the outcome (more H₂). Mention Le Chatelier’s principle to show understanding of the underlying concept.

Common mistakes

  • Saying the temperature change shifts equilibrium to the left (wrong direction). Forgetting that the right side has more moles of gas. Not linking the shift to an increase in hydrogen yield.

Mark scheme (5 marks)

  1. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction (forward reaction)
  2. This increases the yield of hydrogen / more hydrogen is produced
  3. Decreasing pressure shifts the equilibrium position towards the side with the larger number of moles of gas
  4. This is because the right-hand side has more moles of gas (4 moles) than the left-hand side (2 moles)
  5. These changes are explained by Le Chatelier's principle — the equilibrium shifts to counteract the imposed change

Key terms in this question

closed system · reversible reaction

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