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A balanced ionic equation shows the reacting ions in a chemical reaction. These equations can be used to represent what happens in precipitation reactions.

Half equations

A half equation is used to represent what happens when atoms or ions gain or lose electrons. In half equations:

• electrons are shown as e^-
• the numbers of atoms of each element must be the same on both sides
• the total charge on each side must be the same (usually zero)

These are half equations for some reactions where positive ions gain electrons:

Na⁺ + e^- → Na

Pb²⁺ + 2e^- → Pb

2H⁺ + 2e^- → H₂

Worked example

Balance the half equation for the formation of aluminium during electrolysis: Al³⁺ + e^- → Al.

The balanced half equation is: Al³⁺ + 3e^- → Al (because three negatively charged electrons are needed to balance the three positive charges on the aluminium ion).

These are half equations for some reactions where negatively charged ions lose electrons:

2Cl^- → Cl₂ + 2e^-

2O^2- → O₂ + 4e^-

Ionic equations for precipitation reactions

In a typical precipitation reaction, two soluble reactants form an insoluble product and a soluble product.

For example, silver nitrate solution reacts with sodium chloride solution. The products are insoluble solid silver chloride and sodium nitrate solution:

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

The Na⁺ ions and NO₃^- ions remain separate in the sodium nitrate solution and do not form a precipitate. This means these can be ignored when writing the ionic equation. Only how the solid silver chloride forms needs to be shown:

Ag⁺(aq) + Cl^-(aq) → AgCl(s)

In a balanced ionic equation:

• the number of positive and negative charges on each side of the arrow are the same
• the numbers of atoms on each side of the arrow are the same