Electrolysis

Electrolysis is the decomposition of an ionic compound, molten or in solution, by passing electricity through it.

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

Molten compounds

Molten lead(II) bromide: lead forms at the cathode (Pb²⁺ + 2e⁻ → Pb) and bromine at the anode (2Br⁻ → Br₂ + 2e⁻).

Aqueous solutions

Water also provides H⁺ and OH⁻ ions.

• At the cathode, hydrogen forms unless the metal is less reactive than hydrogen (e.g. copper).
• At the anode, oxygen forms unless a halide is concentrated (then chlorine, bromine or iodine forms).

Concentrated sodium chloride (brine): hydrogen at the cathode, chlorine at the anode, sodium hydroxide left in solution.

Uses

• Purifying copper: impure copper anode, pure copper cathode, copper(II) sulfate electrolyte.
• Electroplating: the object to be plated is the cathode.
• Extracting aluminium from bauxite (dissolved in molten cryolite).

Check yourself

1. What forms at each electrode when molten zinc chloride is electrolysed?
2. Why must ionic compounds be molten or dissolved?
3. In copper plating a spoon, which electrode is the spoon?