Electrolysis (Years 10–11) — Science

Splitting ionic compounds with electricity — electrodes, the aqueous rules, half-equations, electroplating and extracting aluminium.

Electricity can pull compounds apart. Electrolysis is the breakdown of an ionic compound — when molten or dissolved — using a direct current. It lets us extract reactive metals that ordinary chemistry can't, plate cheap metal with a shiny coat, and purify copper for wiring.

Why this matters Without electrolysis there would be no aluminium aeroplanes, no chrome-plated taps and no purified copper for the wires in your home. It is one of chemistry's great industrial processes — and a favourite for Paper 4 half-equations.

The cell and its parts

Electrolyte — the molten or dissolved ionic compound that conducts because its ions are free to move.
Electrodes — the two rods dipped into the electrolyte, usually inert graphite or platinum.
Cathode — the negative electrode; positive ions (cations) go there and gain electrons (reduction).
Anode — the positive electrode; negative ions (anions) go there and lose electrons (oxidation).

A simple electrolysis cell — d.c. supply, two electrodes, one electrolyte.

Molten vs aqueous

With a molten compound there are only two kinds of ion, so the products are simply the metal (at the cathode) and the non-metal (at the anode). With an aqueous solution, water adds H⁺ and OH⁻ ions, so we need rules.

Electrode	Aqueous rule
Cathode	Hydrogen is given off unless the metal is less reactive than hydrogen (e.g. copper, silver), in which case the metal is deposited.
Anode	A halide (Cl−, Br−, I−) gives off that halogen; otherwise oxygen is given off (from OH−).

Opposites attract: + ions to the − cathode, − ions to the + anode.

Half-equations, electroplating and aluminium

A half-equation shows the electrons gained or lost at one electrode. For molten lead(II) bromide, PbBr₂:

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

Extraction of aluminium. Aluminium is too reactive to extract by heating with carbon, so it is electrolysed from molten aluminium oxide (Al₂O₃) dissolved in cryolite to lower the melting point. Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻. The hot oxygen slowly burns the carbon anodes to CO₂, so they must be replaced.

Electroplating coats a cheap metal object with a thin layer of a better one (e.g. silver on cutlery). The object is the cathode, the plating metal is the anode, and the electrolyte contains ions of the plating metal.

Common exam mistake: mixing up the electrodes. Remember the cathode is always negative in electrolysis and attracts the positive metal ions. Also, oxidation happens at the anode — don't say a metal is “made” at the anode.

Memory hooks: “PANC” — Positive Anode, Negative Cathode. And “OIL RIG” — Oxidation Is Loss, Reduction Is Gain (of electrons). Exam technique: for half-equations, balance the atoms first, then add electrons to balance the charge.