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
