The manufacture of methanol involves the reversible reaction: CO(g) + 2H₂(g) ⇌ CH₃OH(g). The forward reaction is exothermic. Industrial chemists must choose conditions carefully to maximise the yield of methanol. Explain how changing temperature and pressure affect the position of equilibrium in this reaction, and state the direction each change shifts the equilibrium.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Methanol is an important industrial solvent and fuel additive. It is produced from carbon monoxide and hydrogen gases in a closed system, allowing dynamic equilibrium to be established.
Model answer (5 marks)
1. Increasing the temperature supplies extra heat.
2. The reaction is exothermic, so the system favours the endothermic reverse reaction to absorb the heat.
3. Therefore the equilibrium shifts to the left, reducing methanol yield.
4. Decreasing the temperature removes heat, so the forward exothermic reaction is favoured.
5. Lower temperature shifts the equilibrium to the right, increasing methanol production.
6. Increasing pressure favours the side with fewer gas molecules.
7. The reactants contain 3 moles of gas, the product only 1 mole.
8. Thus higher pressure shifts the equilibrium to the right, towards methanol.
2. The reaction is exothermic, so the system favours the endothermic reverse reaction to absorb the heat.
3. Therefore the equilibrium shifts to the left, reducing methanol yield.
4. Decreasing the temperature removes heat, so the forward exothermic reaction is favoured.
5. Lower temperature shifts the equilibrium to the right, increasing methanol production.
6. Increasing pressure favours the side with fewer gas molecules.
7. The reactants contain 3 moles of gas, the product only 1 mole.
8. Thus higher pressure shifts the equilibrium to the right, towards methanol.
Examiner tips
- Use the term "exothermic" and "endothermic" explicitly. Show the link to Le Chatelier’s principle when discussing temperature. Mention the change in the number of gas moles for pressure. Keep each point concise to fit the 5‑mark structure.
Common mistakes
- Confusing the direction of the temperature shift (writing that higher T favours the forward reaction). Forgetting to state the number of gas moles on each side when explaining pressure effects. Using vague language like "more methanol" without linking to equilibrium shift.
Mark scheme (5 marks)
- Increasing temperature shifts the equilibrium position towards the endothermic (backward/reverse) reaction
- Because the system counteracts the change by absorbing the extra heat energy (Le Chatelier's principle)
- Decreasing temperature shifts the equilibrium position towards the exothermic (forward) reaction, increasing methanol yield
- Increasing pressure shifts the equilibrium towards the side with fewer moles of gas
- The equilibrium shifts to the right (towards methanol) when pressure is increased, because the reactant side has 3 moles of gas and the product side has only 1 mole of gas
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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