Definitions
OIL
Oxidation Is Loss (of electrons)
RIG
Reduction Is Gain (of electrons)
Oxidation States (Numbers)
Rules for Assigning Oxidation States
- Free elements = 0 (e.g. Na, O₂, S₈)
- Monoatomic ion = charge (e.g. Na⁺ = +1, Cl⁻ = −1)
- Oxygen = −2 (except peroxides = −1)
- Hydrogen = +1 (except metal hydrides = −1)
- Fluorine = always −1
- Sum of oxidation states in a compound = 0; in a polyatomic ion = charge
Half-equations
Example: Mg + CuSO₄
Oxidation: Mg → Mg²⁺ + 2e⁻ (Mg loses electrons)
Reduction: Cu²⁺ + 2e⁻ → Cu (Cu²⁺ gains electrons)
Overall: Mg + Cu²⁺ → Mg²⁺ + Cu
Mg is the reducing agent (it causes Cu²⁺ to be reduced). Cu²⁺ is the oxidising agent.
Think About It
In the reaction: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻, identify the oxidising and reducing agents.
Cl₂ is the oxidising agent (it gains electrons, is reduced). Fe²⁺ is the reducing agent (it loses electrons, is oxidised: +2 → +3).