Reversible reactions

In principle, all chemical reactions are reversible reactions. This means that the products can be changed back into the original reactants. This is not obvious when a reaction 'goes to completion', where 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. This is the case when the reaction mixture contains both reactants and products.

Examples of reversible reactions

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
Question

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.

A reversible reaction

1. Bunsen burner heats a bowl of hydrated copper sulfate

Dynamic equilibrium

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

  • the forward and backward reactions are still happening
  • the rates of the forward and backward reactions are the same
  • the concentrations of the reactants and products remain constant (they do not change)
Question

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)

Note that equilibrium can only be achieved if none of the reactants or products can escape. For example if calcium carbonate is heated in a sealed container, breakdown occurs and the following equilibrium is made:

CaCO3(s) ⇌ CaO(s) + CO2(g)

If this reaction is carried out in an open test tube the breakdown goes to completion:

CaCO3(s) → CaO(s) + CO2(g)

This occurs as the carbon dioxide gas escapes and the backwards reaction cannot occur.