The production of hydrogen for fuel cells involves the reversible reaction between methane and steam: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g). The forward reaction is endothermic. A chemical engineer wants to maximise the yield of hydrogen. Explain how changes in temperature and pressure each affect the position of equilibrium in this reaction, and state the direction of shift in each case.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Engineers must balance maximising hydrogen yield with the costs and practicalities of industrial conditions. The reaction CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g) is carried out in a closed system.
Model answer (5 marks)
Increasing the temperature favours the endothermic forward reaction, so the equilibrium shifts to the right, giving more H₂.
Decreasing the pressure favours the side with more gas molecules. The products side has 4 mol gas versus 2 mol on the reactants side, so the equilibrium shifts to the right, increasing H₂ yield.
Decreasing the pressure favours the side with more gas molecules. The products side has 4 mol gas versus 2 mol on the reactants side, so the equilibrium shifts to the right, increasing H₂ yield.
Examiner tips
- Use Le Chatelier’s principle to link temperature to endothermic direction; state the shift and its effect on H₂. Use gas‑mole reasoning for pressure; give the shift direction and its effect on H₂.
Common mistakes
- Confusing the effect of temperature (writing that it shifts left for an endothermic reaction). Forgetting that decreasing pressure favours the side with more moles of gas, or stating the wrong side.
Mark scheme (5 marks)
- Increasing temperature shifts the equilibrium position in the direction of the endothermic (forward) reaction
- This is because Le Chatelier's principle states the system acts to counteract the increase in temperature / the system absorbs the extra heat energy by favouring the endothermic direction
- So increasing temperature increases the yield of hydrogen (more products formed)
- Decreasing pressure shifts the equilibrium position towards the side with the larger number of moles of gas — the right-hand side (products side has 4 moles of gas vs 2 moles on the reactants side)
- So decreasing pressure increases the yield of hydrogen (equilibrium shifts to the right, producing more products)
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
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- Decode the mark scheme abbreviations →
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