Redox in terms of electrons (Higher tier)
At Higher tier, oxidation and reduction are defined by electron transfer, not oxygen. This more general definition covers reactions where no oxygen is involved.
The OIL RIG mnemonic
Oxidation Is Loss (of electrons). Reduction Is Gain (of electrons).
A reaction in which one species is oxidised and another reduced is a redox reaction — they always happen together (electrons have to go somewhere).
✦Worked example— Example: Mg + Cu²⁺ → Mg²⁺ + Cu
A magnesium strip is dipped into copper sulfate solution. Brown copper deposits onto the magnesium; the blue colour fades.
Split into two half-equations:
- Mg → Mg²⁺ + 2e⁻ (oxidation: Mg loses electrons)
- Cu²⁺ + 2e⁻ → Cu (reduction: Cu²⁺ gains electrons)
Adding them: Mg + Cu²⁺ → Mg²⁺ + Cu. Electrons cancel.
✦Worked example— Example: 2Na + Cl₂ → 2NaCl
In ionic terms: Na atoms lose electrons; Cl atoms gain.
- Na → Na⁺ + e⁻ (oxidation)
- ½Cl₂ + e⁻ → Cl⁻ (reduction; or Cl₂ + 2e⁻ → 2Cl⁻)
Sodium is oxidised; chlorine is reduced.
Displacement reactions are redox
When a more reactive metal displaces a less reactive metal from a salt solution: Zn + CuSO₄ → ZnSO₄ + Cu
The Zn loses 2 e⁻ (oxidised) and Cu²⁺ gains 2 e⁻ (reduced). The sulfate is a spectator ion — it doesn't change.
How to write a half-equation
- Identify the species changing (atom → ion or vice versa).
- Balance atoms.
- Add electrons to balance charge.
Example: Cl₂ → 2Cl⁻ needs 2 electrons on the LEFT to balance charge: Cl₂ + 2e⁻ → 2Cl⁻.
✦Worked example— Worked example: identifying redox
In Fe + CuSO₄ → FeSO₄ + Cu, identify the species oxidised and reduced.
- Fe(0) → Fe²⁺(+2): lost 2 e⁻ → oxidised.
- Cu²⁺(+2) → Cu(0): gained 2 e⁻ → reduced.
⚠Common mistakes
- Forgetting OIL RIG. Oxidation = lose; Reduction = gain.
- Putting electrons on the wrong side. They go on the same side as the more positive species (so the equation balances).
- Not balancing charge. Charges and atoms both must balance.
- Saying "Cu²⁺ is oxidised" — Cu²⁺ gains electrons to become Cu, so it's reduced.
Links
Builds on C4.1 (oxygen-based redox). Used heavily in C4.9–C4.12 (electrolysis half-equations) and C8.5 (reactions used in tests).
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