Electrolysis

Oxidation and reduction in terms of electrons

• Oxidation is loss of electrons. Reduction is gain of electrons. (mnemonic: OIL RIG)
• At the anode, anions lose electrons → oxidation
• At the cathode, cations gain electrons → reduction
• A half-equation writes out the change at one electrode, showing the species, the electrons (e⁻), and the product
• The charge on each side of a half-equation must balance

Molten lead(II) bromide

• Cathode: Pb²⁺ + 2e⁻ → Pb   (reduction)
• Anode: 2Br⁻ → Br₂ + 2e⁻   (oxidation)

Aqueous sodium chloride

• Cathode: 2H⁺ + 2e⁻ → H₂
• Anode: 2Cl⁻ → Cl₂ + 2e⁻

Dilute sulfuric acid

• Cathode: 2H⁺ + 2e⁻ → H₂
• Anode: 2H₂O → O₂ + 4H⁺ + 4e⁻
• Water (not OH⁻) is the oxidised species because the solution is acidic and contains almost no OH⁻

Aqueous copper(II) sulfate

• Cathode: Cu²⁺ + 2e⁻ → Cu
• Anode: 2H₂O → O₂ + 4H⁺ + 4e⁻