Oxidation, reduction and redox reactions
Oxidation is gain of oxygen or loss of hydrogen. Reduction is loss of oxygen or gain of hydrogen. When both happen together, it is a redox reaction.
Activity: heating copper powder
- 1Take about 1 g of copper powder in a china dish and heat it.
- 2Observe: the brown copper powder gets coated with a black substance.
- 3★2Cu(s) + O₂(g) → 2CuO(s)On heating. Copper gains oxygen, so it is oxidised to black copper(II) oxide
- 4Now pass hydrogen gas over the hot black copper oxide.The black coat turns brown again
- 5★CuO(s) + H₂(g) → Cu(s) + H₂O(l)On heating. Copper oxide loses oxygen (reduced); hydrogen gains oxygen (oxidised)
★ key step
Spot what is oxidised and what is reduced
| Reaction | Oxidised (gains O or loses H) | Reduced (loses O or gains H) |
|---|---|---|
| CuO + H₂ → Cu + H₂O | H₂ becomes H₂O | CuO becomes Cu |
| ZnO + C → Zn + CO | C becomes CO | ZnO becomes Zn |
| 2PbO + C → 2Pb + CO₂ | C becomes CO₂ | PbO becomes Pb |
| MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ | HCl becomes Cl₂ (loses hydrogen) | MnO₂ becomes MnCl₂ (loses oxygen) |
Oxidising agent vs reducing agent
Oxidation and reduction always happen together. If one substance gains oxygen, another must lose it.
Your body runs on oxidation: glucose from rice and roti is oxidised in your cells to give energy. In industry, carbon reduces metal oxides to give metals such as zinc and iron.
2 marks: “Identify the substance oxidised and the substance reduced: ZnO + C → Zn + CO.” Carbon is oxidised to CO (it gains oxygen). Zinc oxide is reduced to zinc (it loses oxygen).
3 marks: “What happens when copper powder is heated in air, and hydrogen is then passed over the hot product? Write both equations.” Brown copper turns black: 2Cu + O₂ → 2CuO (copper is oxidised). Hydrogen over hot CuO turns it brown again: CuO + H₂ → Cu + H₂O (CuO is reduced).
GO for oxidation: Gain of Oxygen. LO for reduction: Loss of Oxygen. Hydrogen works the other way round.