A sealed flask contains the following reversible reaction at dynamic equilibrium: H₂(g) + I₂(g) ⇌ 2HI(g). The temperature of the flask is then increased. Explain what is meant by the term 'dynamic equilibrium' and describe what happens to the equilibrium position when the temperature is increased, given that the forward reaction is exothermic.

Pearson Edexcel International GCSE Chemistry (4CH1) — 3.3 Reversible reactions and equilibria · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

At dynamic equilibrium the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of H₂, I₂ and HI remain constant.

When the temperature is increased the equilibrium shifts to the endothermic side. Because the forward reaction is exothermic, the reverse (endothermic) reaction is favoured.

Consequently the concentration (yield) of HI decreases and the concentrations of H₂ and I₂ increase.

Examiner tips

  • Use the definition of dynamic equilibrium – equal forward and reverse rates, constant concentrations. Show that heating favours the endothermic direction. State the effect on HI concentration.
  • Use the exact wording: "the yield of HI decreases" to match the mark scheme.

Common mistakes

  • Confusing dynamic equilibrium with static equilibrium. Saying the reaction stops or that temperature has no effect. Using the wrong direction for the temperature shift (i.e. saying HI increases).

Mark scheme (4 marks)

  1. At dynamic equilibrium the rate of the forward reaction equals the rate of the backward reaction
  2. The concentrations of reactants and products remain constant (at dynamic equilibrium)
  3. Increasing temperature shifts the equilibrium position in the direction of the endothermic reaction
  4. The yield of HI decreases / the concentration of HI decreases (because the backward/reverse reaction is favoured)

Key terms in this question

dynamic equilibrium · reversible reaction · equilibrium position

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