Equations with ions - Higher

Half equations

A half equation models the change that happens to one substance in a chemical reaction. They are usually used to model the reactions that happen at electrodes during electrolysis.

Reactions at cathodes

A cathode is a negatively charged electrode. Positively charged ions gain electrons at a cathode, forming neutral atoms or molecules. The ions, charges and atoms must balance. For example:

Na+ + e- → Na

Ca2+ + 2e- → Ca

2H+ + 2e- → H2


Balance this half equation: Al3+ + e- → Al

Al3+ + 3e- → Al

Reactions at anodes

An anode is a positively charged electrode. Negatively charged ions lose electrons at an anode, forming neutral molecules. For example:

2Cl- → Cl2 + 2e-

2O2-→ O2 + 4e-


Balance this half equation: Br- → Br2 + e-

2Br- → Br2 + 2e-

Ionic equations

A balanced ionic equation shows the reacting ions in a chemical reaction. These equations are often used to model what happens in a precipitation reaction. In these reactions, two soluble reactants form an insoluble product and a soluble product.

For example, silver nitrate solution reacts with sodium chloride solution. Insoluble solid silver chloride and sodium nitrate solution form:

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. Ions that remain separate during a reaction are called spectator ions. This means you can ignore them when you write the ionic equation. You only need to model how the solid silver chloride forms:

Ag+(aq) + Cl-(aq) → AgCl(s)

In a balanced ionic equation:

  • the number of positive and negative charges is the same
  • the numbers of atoms of each element on the left and right are the same

Explain why this ionic equation is balanced:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

There are the same numbers of atoms of each element on both sides of the equation. The total charge on both sides is also the same (zero).


Balance this ionic equation, which models the formation of a silver carbonate precipitate:

Ag+(aq) + CO32-(aq) → Ag2CO3(s)

2Ag+(aq) + CO32-(aq) → Ag2CO3(s)


Balance this ionic equation, which models the formation of an aluminium hydroxide precipitate:

Al3+(aq) + OH-(aq) → Al(OH)3(s)

Al3+(aq) + 3OH-(aq) → Al(OH)3(s)

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