Reversibility and dynamic equilibria
4 marks · NormalThe reversible reaction between hydrogen and iodine to form hydrogen iodide can reach dynamic equilibrium in a sealed tube.
State what is meant by dynamic equilibrium.
Describe two conditions necessary for dynamic equilibrium to occur in this reaction.
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(a) The forward and backward reactions occur at the same rate and concentrations of reactants and products remain constant
(b) The system must be closed/sealed; temperature and pressure must be constant; or both reactions must still occur
Common mistakes: Saying rates are equal without mentioning forward/reverse reactions; Not specifying system must be closed/sealed
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Marking points
B1 (a) · 1 mark · States forward and reverse reactions occur at same rate
B2 (a) · 1 mark · States concentrations of reactants and products remain constant
B3 (b) · 1 mark · Identifies system must be closed/sealed (no escape of substances)
B4 (b) · 1 mark · Gives another relevant condition (e.g., temperature and pressure constant; both reactions can still happen)
Worked solution
a) At dynamic equilibrium, the forward and backward reactions take place at the same rate. As a result, the concentrations of all reactants and products remain constant.
b) The reaction must take place in a sealed container, so no substances can enter or leave. The temperature and pressure must remain constant; also, both reactions continue to happen.