Methanol is manufactured industrially using the following reversible reaction carried out at high pressure and a temperature of around 250 °C in the presence of a copper-based catalyst: CO(g) + 2H₂(g) ⇌ CH₃OH(g) ΔH = −90 kJ/mol Explain the effect on the position of equilibrium, and on the yield of methanol, of (i) increasing the pressure and (ii) increasing the temperature. In your answer, refer to Le Chatelier's principle.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Le Chatelier’s principle states that when a system at equilibrium is disturbed, the equilibrium shifts to oppose the change.
(i) Increasing the pressure favours the side with fewer moles of gas. The reaction has 3 mol gas on the left and 1 mol gas on the right, so the equilibrium shifts to the right, producing more CH₃OH. The yield of methanol therefore increases.
(ii) The forward reaction is exothermic (ΔH = −90 kJ mol⁻¹). Raising the temperature adds heat, so the system responds by shifting to the endothermic reverse reaction to absorb the excess heat. The equilibrium therefore moves to the left, reducing the amount of methanol formed. The yield of methanol decreases.
(i) Increasing the pressure favours the side with fewer moles of gas. The reaction has 3 mol gas on the left and 1 mol gas on the right, so the equilibrium shifts to the right, producing more CH₃OH. The yield of methanol therefore increases.
(ii) The forward reaction is exothermic (ΔH = −90 kJ mol⁻¹). Raising the temperature adds heat, so the system responds by shifting to the endothermic reverse reaction to absorb the excess heat. The equilibrium therefore moves to the left, reducing the amount of methanol formed. The yield of methanol decreases.
Examiner tips
- State Le Chatelier’s principle first.
- Explain the pressure effect using moles of gas.
- Explain the temperature effect using the sign of ΔH.
- Link each shift to the change in methanol yield.
Common mistakes
- Confusing the direction of the temperature shift (claiming it moves right).
- Ignoring the moles of gas when discussing pressure.
Mark scheme (5 marks)
- States Le Chatelier's principle: when a system at equilibrium is subjected to a change, the equilibrium shifts to oppose that change.
- Increasing pressure shifts the equilibrium to the right / towards the side with fewer moles of gas.
- Therefore increasing pressure increases the yield of methanol.
- Increasing temperature shifts the equilibrium to the left because the forward reaction is exothermic, so the system opposes the increase in temperature by favouring the endothermic (reverse) reaction.
- Therefore increasing temperature decreases the yield of methanol.
Key terms in this question
Le Chatelier's principle · position of equilibrium · yield
Related
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