When copper sulfate solution is mixed with excess anhydrous white copper sulfate powder in a sealed container, a reversible reaction occurs. Explain what happens to the rate of the forward and backward reactions over time, and describe the conditions that exist once dynamic equilibrium has been reached.

Edexcel GCSE Chemistry (1CH0) — 4.2 Reversible reactions and equilibria · Explain · 4 marks · View as Markdown

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

Anhydrous copper sulfate is a white powder. When water is added, it forms blue hydrated copper sulfate. This reaction is reversible. A student places anhydrous copper sulfate in a sealed container with a small amount of water and observes the system over time.

Model answer (4 marks)

The forward reaction (CuSO₄·xH₂O → CuSO₄ + xH₂O) starts fast because there is a large excess of anhydrous CuSO₄ and water. As the reaction proceeds the amount of water and hydrated salt decreases, so the forward rate falls.
The backward reaction (CuSO₄ + xH₂O → CuSO₄·xH₂O) starts very slow or zero because initially there is almost no hydrated salt. As the forward reaction produces hydrated CuSO₄, the backward rate rises.
At dynamic equilibrium the forward and backward rates are equal.
At equilibrium the concentrations of CuSO₄·xH₂O, CuSO₄ and H₂O are constant, but they are not equal – the system is in a steady state.

Examiner tips

  • Use the word ‘rate’ and ‘concentration’ correctly; show the change over time. Mention that the forward rate decreases while the backward rate increases. State that at equilibrium the rates are equal and concentrations are constant. Keep the answer concise – 4 marks only.
  • common_mistakes
  • :
  • Saying the concentrations are equal at equilibrium. Forgetting that the backward rate starts at zero or very low. Mixing up which reaction is forward and which is backward.

Mark scheme (4 marks)

  1. The rate of the forward reaction starts high and decreases over time (as reactants are used up)
  2. The rate of the backward reaction starts at zero/low and increases over time (as products build up)
  3. At dynamic equilibrium, the rate of the forward reaction equals the rate of the backward reaction
  4. At dynamic equilibrium, the concentrations of reactants and products remain constant (not equal)

Key terms in this question

reversible reaction · dynamic equilibrium · backward reaction

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