CC3.4 — Electrolysis (Edexcel 1SC0, Higher tier)

Electrolysis principles

Electrolysis uses electrical energy to break down an ionic compound (electrolyte). Requires the electrolyte to be molten or dissolved in water (ions must be free to move).

• Cathode (negative electrode): reduction — cations (positive ions) gain electrons.
• Anode (positive electrode): oxidation — anions (negative ions) lose electrons.

Molten electrolyte (e.g. lead bromide)

Only ions from the compound are present:

• Cathode: Pb²⁺ + 2e⁻ → Pb (lead deposits)
• Anode: 2Br⁻ → Br₂ + 2e⁻ (bromine gas produced)

Aqueous electrolyte (e.g. copper sulfate solution)

Both ions from the compound AND H⁺/OH⁻ from water are present.

Rules for predicting products:

• Cathode: the less reactive metal/hydrogen is discharged. If a metal less reactive than hydrogen is present → metal deposited (e.g. Cu). If only a metal more reactive than H → hydrogen gas.
• Anode: if halide ions (Cl⁻, Br⁻, I⁻) present → halogen gas. Otherwise → oxygen gas (from OH⁻).

Example — copper sulfate solution with inert electrodes:

• Cathode: Cu²⁺ + 2e⁻ → Cu
• Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻ (or 2H₂O → O₂ + 4H⁺ + 4e⁻)