Ethanol is manufactured industrially by the hydration of ethene. The reaction takes place in a closed container and reaches a dynamic equilibrium. Explain what is meant by the term 'dynamic equilibrium' and describe how increasing the concentration of ethene in the closed container would affect the position of this equilibrium.

Edexcel GCSE Chemistry (1CH0) — 9.3 Dynamic equilibria, calculations involving volumes of gases (Chem only) · Explain · 4 marks · View as Markdown

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

Ethene reacts with steam to produce ethanol: C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g). The reaction is carried out in a closed container and reaches a dynamic equilibrium.

Model answer (4 marks)

At dynamic equilibrium the forward and reverse reactions continue to occur but at the same rate, so the concentrations of reactants and products remain constant.

Increasing the concentration of ethene increases the rate of the forward reaction. The system responds by shifting the equilibrium to the right, producing more ethanol, to reduce the excess ethene.

This shift occurs because the system opposes the change in concentration, restoring balance between the forward and reverse rates.

Examiner tips

  • Use the term ‘dynamic equilibrium’ and state both reactions still occur and rates are equal. Show the effect of adding ethene: shift to the right, more product. Explain the reason: system opposes the change in concentration.
  • common_mistakes
  • :
  • Saying the equilibrium stops or that the reaction becomes irreversible. Forgetting to mention that both reactions still occur. Not explaining why the shift happens (opposing the change).

Mark scheme (4 marks)

  1. The forward and reverse reactions are both still occurring (at equilibrium)
  2. The rate of the forward reaction equals the rate of the reverse reaction
  3. Increasing the concentration of ethene causes the equilibrium position to shift to the right / towards the products
  4. This is because the system acts to reduce the increased concentration of ethene / opposes the change

Key terms in this question

dynamic equilibrium · concentration

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