# 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.

> OCR A-Level Chemistry B: Salters (H433) — 5.1 Rates, equilibrium and pH · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. Increasing temperature shifts the equilibrium position in the direction of the endothermic (forward) reaction
2. 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
3. So increasing temperature increases the yield of hydrogen (more products formed)
4. 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)
5. So decreasing pressure increases the yield of hydrogen (equilibrium shifts to the right, producing more products)

## Key terms

- [reversible reaction](https://www.gradenine.co.uk/glossary/reversible-reaction)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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