Ethanol can be produced by the hydration of ethene. The reaction is reversible and reaches dynamic equilibrium in a closed system. The equation for the reaction is: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) ΔH = −46 kJ/mol. Explain the effect on the position of equilibrium and on the yield of ethanol when the temperature is decreased and when the pressure is increased.

Edexcel A-Level Chemistry (9CH0) — 10.1 Dynamic equilibria and Le Chatelier · Explain · 5 marks · View as Markdown

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

Ethanol can be produced industrially by reacting ethene with steam. The reaction is exothermic in the forward direction and reaches dynamic equilibrium in a closed system: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g) ΔH = −46 kJ/mol

Model answer (5 marks)

1. Decreasing the temperature removes heat from the system. The forward reaction is exothermic (ΔH = −46 kJ mol⁻¹), so the system responds by producing more heat – i.e. shifting the equilibrium to the right, towards ethanol.
2. This shift increases the concentration of ethanol in the closed system, so the yield of ethanol rises.
3. Increasing the pressure favours the side with fewer gas molecules. The left side has 2 mol gas (C₂H₄ + H₂O) and the right side has 1 mol gas (C₂H₅OH). Thus the equilibrium shifts to the right.
4. Consequently, the yield of ethanol is increased when the pressure is raised.
5. In both cases the equilibrium is moved towards the product side, giving a higher ethanol yield.

Examiner tips

  • Use the words ‘shift to the right’ and ‘increase the yield’ to show understanding of Le Chatelier’s principle.
  • Mention the exothermic nature of the forward reaction when discussing temperature.
  • State the change in the number of gas moles when discussing pressure.
  • Keep the answer concise – 5 points can be covered in 4–5 short sentences.

Common mistakes

  • Saying the equilibrium shifts to the left when temperature is lowered – wrong direction for an exothermic forward reaction.
  • Confusing the effect of pressure with the effect of temperature – e.g. claiming pressure has no effect because the reaction is reversible.
  • Failing to mention the change in the number of gas moles when discussing pressure.

Mark scheme (5 marks)

  1. Decreasing temperature shifts the equilibrium to the right (towards the products / in the forward direction)
  2. Because the forward reaction is exothermic, so lowering temperature causes the equilibrium to shift in the direction that releases heat / opposes the change (Le Chatelier's principle)
  3. Decreasing temperature increases the yield of ethanol
  4. Increasing pressure shifts the equilibrium to the right (towards the products / in the forward direction)
  5. Because there are fewer moles of gas on the right-hand side (2 moles of gas on the left, 1 mole on the right), so increasing pressure favours the side with fewer gas molecules, increasing the yield of ethanol

Key terms in this question

dynamic equilibrium · position of equilibrium · yield

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