Redox & Electron Transfer
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
Oxidation and Reduction in Terms of Electrons
- Oxidation is the loss of electrons
- Reduction is the gain of electrons
OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons)
Worked Example: Magnesium and Oxygen
- 2Mg + O₂ → 2Mg²⁺O²⁻
- Mg → Mg²⁺ + 2e⁻ (magnesium is oxidised - loses electrons)
- O₂ + 4e⁻ → 2O²⁻ (oxygen is reduced - gains electrons)
Oxidation and Reduction in Terms of Oxidation Number
- Oxidation is an increase in oxidation number
- Reduction is a decrease in oxidation number
Fe²⁺ → Fe³⁺: the oxidation number rises from +2 to +3, so iron is oxidised (it has also lost an electron, consistent with the electron-transfer definition).
Rules for Assigning Oxidation Numbers
- An element in its uncombined state has oxidation number 0 (e.g. O₂, Fe, Cl₂)
- A monatomic ion's oxidation number equals the charge on the ion (e.g. Fe²⁺ is +2)
- The oxidation numbers in a neutral compound sum to 0
- The oxidation numbers in an ion sum to the charge on that ion
Oxidising and Reducing Agents
- An oxidising agent oxidises another substance and is itself reduced
- A reducing agent reduces another substance and is itself oxidised
Worked Example: Zinc and Copper Sulfate
- Zn + CuSO₄ → ZnSO₄ + Cu
- Zn → Zn²⁺ + 2e⁻ (zinc is oxidised, so zinc is the reducing agent)
- Cu²⁺ + 2e⁻ → Cu (Cu²⁺ is reduced, so it is the oxidising agent)
Worked Example: Manganate(VII) and Iron(II)
- Question: MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺. Which ion is reduced, and what is the reducing agent? [1]
- Answer: Mn in MnO₄⁻ goes from +7 to +2 in Mn²⁺ - a decrease in oxidation number, so MnO₄⁻ is reduced. Fe²⁺ goes from +2 to +3 (oxidised), so Fe²⁺ is the reducing agent (it is itself oxidised, causing MnO₄⁻ to be reduced).
Remember: the oxidising agent gets reduced, and the reducing agent gets oxidised.
Colour-Change Tests for Redox
| Test | Reagent | Colour change | What it shows |
|---|---|---|---|
| For reducing agents | acidified potassium manganate(VII), KMnO₄ | purple → colourless | KMnO₄ is reduced (it acts as an oxidising agent) |
| For oxidising agents | potassium iodide, KI | colourless → brown | I⁻ is oxidised to I₂ (2I⁻ → I₂ + 2e⁻) |
Purple KMnO₄ fading to colourless confirms a reducing agent; colourless KI turning brown confirms an oxidising agent
Common Mistakes
- ❌ Confusing which agent gets oxidised and which gets reduced
- ✓ the substance NAMED as the agent describes what it does to the OTHER substance - a reducing agent reduces something else, and is itself oxidised
- ❌ Assuming oxidation number always matches the number of electrons directly transferred in a single step
- ✓ use the change in oxidation number (e.g. Mn: +7 → +2 is a change of 5) to work out electrons transferred per ion, then balance the full equation accordingly
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