Reversible Reactions and Dynamic Equilibrium — AQA GCSE Chemistry

In a reversible reaction, the products can react to reform the reactants. In a closed system this leads to equilibrium.

Reversible reactions

A reversible reaction is one in which the products can react together to form the original reactants again. It is shown with the ⇌ symbol: A + B ⇌ C + D

Energy changes

If the forward reaction is exothermic, the reverse reaction is endothermic by the same amount of energy (and vice versa).

Example

Heating hydrated copper sulfate (blue) drives off water to form anhydrous copper sulfate (white); adding water reverses it and releases heat: hydrated CuSO₄ ⇌ anhydrous CuSO₄ + water

Dynamic equilibrium

When a reversible reaction takes place in a closed system (nothing enters or leaves), it reaches a dynamic equilibrium:

• The forward and reverse reactions are still happening, but at the same rate.
• The concentrations of reactants and products stay constant (they are not necessarily equal).

Exam tips

• Reversible reactions use the ⇌ symbol.
• If the forward reaction is exothermic, the reverse is endothermic by the same amount.
• Equilibrium is dynamic — both reactions continue at equal rates.
• It requires a closed system so nothing escapes.