Reversible ReactionsOxford AQA IGCSE Chemistry: Revision notes

A reversible reaction is a reaction where the products can react together to re-form the original reactants. This is shown in an equation using the ⇌ symbol instead of a single arrow, showing that the reaction proceeds in both the forward and reverse directions.

General form: A + B ⇌ C + D

A key example of a reversible reaction is the interconversion of copper sulfate:

CuSO4·5H2O (blue, hydrated) ⇌ CuSO4 (white, anhydrous) + 5H2O

• The forward reaction (removing water by heating) is endothermic — blue hydrated copper sulfate turns to white anhydrous copper sulfate
• The reverse reaction (adding water) is exothermic — white anhydrous copper sulfate turns back to blue hydrated copper sulfate, releasing heat

This colour change makes the reaction useful as a simple test for the presence of water.

When a reversible reaction takes place in a closed system (nothing can enter or leave), the forward and reverse reactions eventually occur at exactly the same rate. At this point, the reaction has reached dynamic equilibrium.

At equilibrium:

• the forward and backward reactions are still happening, but at equal rates
• the concentrations of reactants and products remain constant (though not necessarily equal to each other)
• the reaction appears to have stopped, but at a molecular level it has not

The relative amounts of reactants and products present at equilibrium depend on the conditions (temperature, pressure, concentration) under which the reaction takes place. Changing these conditions can shift the position of equilibrium to favour either the forward or reverse reaction, which is important in industrial processes such as the Haber process and the Contact process.