Half Equations & Ionic Equations (AQA GCSE Chemistry): Revision Note

Half equations & ionic equations

Higher tier only

How to write a half equation

• Half equations and ionic equations are specific types of equations for showing some of the fine details going on in chemical reactions

• Half equations are used to show what happens to the electrons in reactions where atoms, molecules or ions are gaining or losing electrons

• They are called half equations, because they represent only half of what is happening in a reaction involving electron transfer

  • One species gains electrons

  • Another species loses electrons

• Examples of half equations are:

Pb²⁺ + 2e^- → Pb

2Br^-  → Br₂ + 2e^-

How to write an ionic equation

• Ionic equations are used to indicate what happens to ions during reactions

• They help to simplify complicated processes where many substances are present, but only certain ions are actually reacting with each other

• For example, we can use ionic equations to show what happens when an acid neutralises and an alkali:

HCl + NaOH  → NaCl + H₂O

• Written out as an ionic equation would be

H⁺ + OH^- → H₂O

• This is because sodium and chloride ions were present at the beginning and also at the end of the reaction, so they are unchanged

• Ions which are present but do not take part in reactions are called spectator ions