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In some chemical reactions, the products of the reaction can react together to produce the original reactants. These reactions are called reversible reactions. They can be represented in the following way:

A + B ⇌ C + D

The symbol ⇌ has two half arrowheads, one pointing in each direction. It is used in equations that show reversible reactions:

• the forward reaction is the one that goes to the right
• the backward reaction is the one that goes to the left

The reaction mixture may contain reactants and products, and their proportions may be changed by altering the reaction conditions.

Example

Ammonium chloride

Ammonium chloride is a white solid. It breaks down when heated, forming ammonia and hydrogen chloride. When these two gases are cool enough, they react together to form ammonium chloride again.

This reversible reaction can be modelled as:

ammonium chloride ⇌ ammonia + hydrogen chloride

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

Energy changes in reversible reactions

If a reaction is exothermic in one direction, it will be endothermic in the other direction. The same amount of energy is transferred in both the forwards and reverse reaction.

Copper sulfate

Blue copper sulfate is described as hydrated. The copper ions in its crystal lattice structure are surrounded by water molecules. This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate. This reaction is reversible:

hydrated copper sulfate ⇌ anhydrous copper sulfate + water

CuSO₄.5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)

The forward reaction is endothermic and the reverse reaction is exothermic.

Dynamic equilibrium

When a reversible reaction happens in a closed container, it reaches a dynamic equilibrium. At equilibrium:

• the forward and backward reactions are still happening
• the forward and backward reactions have the same rate of reaction
• the concentrations of all the reacting substances remain constant