Redox is the study of electron transfer. Six skills sit inside it, and they depend on each other in a fixed order.
This page sits under SPM Chemistry. Below you will find the order to study, one short example that uses several skills, and where to start.
How do the six skills connect?
Oxidation numbers come first because they tell you which atom gains or loses electrons. Half-equations then write that transfer for each species, and cells and electrolysis use the half-equations.
- Assigning oxidation numbers gives the change in each atom.
- Identifying oxidation and reduction names what is oxidised, reduced and which agent is which.
- Writing half-equations balances the electrons.
- Predicting electrolysis products applies half-equations when electricity drives the change.
- Explaining simple chemical cells applies them when the reaction drives electricity.
- Interpreting corrosion and prevention applies them to rusting and protection.
What does one example look like across the skills?
A worked example: a strip of magnesium is placed in silver nitrate solution, and grey silver appears on it.
Oxidation numbers: Mg goes from 0 to +2 and Ag goes from +1 to 0. Magnesium is oxidised and silver ions are reduced.
Half-equations: Mg → Mg²⁺ + 2e⁻ and Ag⁺ + e⁻ → Ag. Doubling the second gives Mg + 2Ag⁺ → Mg²⁺ + 2Ag, with the charges balanced at 2+ on both sides.
If the same two metals are connected as a cell, magnesium is the negative terminal and the half-equations are unchanged. Only the setting is different.
Who should start where?
Match your difficulty to the lesson:
- You cannot tell which species is oxidised: start with oxidation numbers.
- You can name oxidation and reduction but equations do not balance: start with half-equations.
- You can write half-equations but mix up the electrodes: read the cell and electrolysis lessons together.
- You know the ideas but lose marks on long answers: go straight to the redox practice set.
Energy changes in reactions are covered separately in thermochemistry.
Need a teacher to check the order?
A teacher can ask you to explain one redox change aloud and quickly see which of the six skills is missing. If you would like that, see online one-to-one Chemistry tuition and the one-hour trial class (from RM50).