Explain what is meant by a reversible reaction reaching a state of dynamic equilibrium in a closed system.

Cambridge International IGCSE Chemistry (0620) — 6.3 Reversible reactions and equilibrium · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

A reversible reaction is one that can proceed in both the forward and reverse directions.
The system must be closed so that no substances can enter or leave.
At equilibrium the rate of the forward reaction equals the rate of the reverse reaction.
Consequently the concentrations of reactants and products remain constant (though they need not be equal).

Examiner tips

Use the exact wording from the mark scheme; include all four points.
Show the logical sequence: reversible → closed system → equal rates → constant concentrations.
Avoid extra explanation or examples that are not asked for.

Common mistakes

Omitting that the system must be closed.
Confusing equilibrium with a static state; not mentioning equal rates.
Writing that concentrations are equal, rather than just constant.

Mark scheme (4 marks)

A reversible reaction is one that can proceed in both the forward and reverse directions
The system must be closed so that no substances can enter or leave
The rate of the forward reaction equals the rate of the reverse reaction
The concentrations of reactants and products remain constant (not necessarily equal)

Key terms in this question

reversible reaction · dynamic equilibrium · closed system