Lesson 4.13 · AQA Combined Science

Electron transfer and half-equations

Higher Tier only. This is an optional extension after the core lessons. Go to exam practice.

OIL RIG

Oxidation Is Loss; Reduction Is Gain of electrons. Electrons have negative charge. A metal atom loses electrons to become a positive ion. In a redox reaction, one species loses the electrons that another gains.

Write half-equations

Zn → Zn²⁺ + 2e⁻ (oxidation).
Cu²⁺ + 2e⁻ → Cu (reduction).
Add and cancel the electrons: Zn + Cu²⁺ → Zn²⁺ + Cu. Sulfate ions stay unchanged and are spectator ions, so leave them out of the ionic equation.

Electrodes: atoms and charge must balance

Cathode: gain electrons = reduction.
Cu²⁺ + 2e⁻ → Cu
2H⁺ + 2e⁻ → H₂
Al³⁺ + 3e⁻ → Al
Anode: lose electrons = oxidation.
2Cl⁻ → Cl₂ + 2e⁻
4OH⁻ → O₂ + 2H₂O + 4e⁻
2O²⁻ → O₂ + 4e⁻ (molten oxide).
Check the number of each atom and the total charge on both sides.

Metals reacting with acids are redox

For Mg + 2H⁺ → Mg²⁺ + H₂, magnesium is oxidised: Mg → Mg²⁺ + 2e⁻. Hydrogen ions are reduced: 2H⁺ + 2e⁻ → H₂. Chloride ions in hydrochloric acid are spectators.

Remember

OIL RIG. Cathode = reduction; anode = oxidation. Balance atoms and total charge.

Electron transfer and half-equations

OIL RIG

Write half-equations

Electrodes: atoms and charge must balance

Metals reacting with acids are redox

Remember

Try it

Look at the diagram.

Case closed!