Reversible reactions

In principle, all chemical reactions are reversible reactions. The products could be changed back into the original reactants using a suitable reaction. This is not obvious when a reaction ‘goes to completion’, a situation in which very little or no reactants are left. Examples of reactions that go to completion are:

It is more obvious in reactions that do not go to completion that the reaction is reversible. The reaction mixture may contain reactants and products, and their proportions may be changed by altering the reaction conditions.

Two examples

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)

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

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

Write the balanced equation for the forward reaction in the breakdown of ammonium chloride.

NH4Cl(s) → NH3(g) + HCl(g)

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)

A bunsen burner heats a bowl of hydrated copper(II) sulfate.

Bunsen burner heats a bowl of hydrated copper(II) sulfate

Dynamic equilibrium

When a reversible reaction happens in a closed system, such as a stoppered flask, it reaches a dynamic equilibrium. At equilibrium:

  • the forward and backward reactions are still happening
  • the forward and backward reactions have an equal rate of reaction
  • the concentrations of all the reacting substances remain constant and do not change

Nitrogen reacts with hydrogen to produce ammonia: N2(g) + 3H2(g) ⇌ 2NH3(g)

Write the balanced equation for the backward reaction.

2NH3(g) → N2(g) + 3H2(g)