Reversibility and dynamic equilibria

2 marks · Easy

Explain why the concentration of reactants does not change once a reversible reaction has reached dynamic equilibrium in a sealed container.

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Because the forward and backward reactions happen at the same rate, so reactants are converted to products as quickly as products are converted back to reactants, keeping concentrations constant.

Also accept: At dynamic equilibrium, the rates of the two reactions are equal so the amount of each substance does not change.

Common mistakes: The reactions stop happening.

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Marking points

M1 · 1 mark · Identifies that forward and backward reactions occur at the same rate.

A1 · 1 mark · Explains that concentrations remain constant because of these equal rates.

Worked solution

At equilibrium, the forward and backward reactions take place at the same rate. This means that reactants are made into products as quickly as products are turned back into reactants, so their concentrations do not change.