Explain why, in a closed system, a reversible reaction eventually reaches a state where the concentrations of reactants and products remain constant, even though the reaction has not stopped.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Hydrogen iodide can be formed from hydrogen and iodine gases in a sealed container: H₂(g) + I₂(g) ⇌ 2HI(g). When hydrogen and iodine are first mixed, no hydrogen iodide is present.
Model answer (4 marks)
At the start only the forward reaction occurs because the reverse reaction requires HI, which is absent.
As HI is produced the forward rate decreases and the reverse rate increases.
Equilibrium is reached when the forward rate equals the reverse rate.
At equilibrium both reactions continue at the same rate, so concentrations remain constant – a dynamic equilibrium.
As HI is produced the forward rate decreases and the reverse rate increases.
Equilibrium is reached when the forward rate equals the reverse rate.
At equilibrium both reactions continue at the same rate, so concentrations remain constant – a dynamic equilibrium.
Examiner tips
- Use the key terms: forward, reverse, rate, equilibrium, dynamic equilibrium.
- Show the sequence: start → change in rates → equality of rates → constant concentrations.
- Keep the answer concise – 4 points, one sentence per point.
Common mistakes
- Saying the reaction stops at equilibrium; it continues but at equal rates.
- Using ‘static equilibrium’ instead of ‘dynamic equilibrium’.
Mark scheme (4 marks)
- At the start, only the forward reaction occurs / the forward reaction is faster than the reverse reaction initially
- As the reaction proceeds, the rate of the forward reaction decreases and the rate of the reverse reaction increases
- Equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction
- Both forward and reverse reactions continue at equal rates, so concentrations do not change / the system is in dynamic equilibrium
Key terms in this question
reversible reaction · closed system
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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