Oxidation and reduction · Year 12
Oxidation and reduction always happen together, so these are called redox reactions. At first they are described with oxygen: a substance that gains oxygen is oxidised and one that loses oxygen is reduced. The fuller idea uses electrons: oxidation is loss of electrons and reduction is gain of electrons (OIL RIG). Half equations show the electrons, and at A-level and in CAPS oxidation numbers keep track of them.
Key facts
- Oxidation is gain of oxygen or loss of electrons. Reduction is loss of oxygen or gain of electrons.
- OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
- In a displacement reaction the more reactive metal's atoms lose electrons (oxidised) and the less reactive metal's ions gain them (reduced): Mg + Cu²⁺ → Mg²⁺ + Cu.
- A half equation shows one change with its electrons: Mg → Mg²⁺ + 2e⁻ (oxidation), Cu²⁺ + 2e⁻ → Cu (reduction). The charges balance on each side.
- An oxidising agent takes electrons and is reduced; a reducing agent gives electrons and is oxidised.
- Oxidation numbers: elements are 0, O is usually −2, H is +1, and they add up to the charge. Oxidation makes the oxidation number go up; reduction makes it go down.