Electrolysis

Electrolysis uses electricity to break down an ionic compound that is molten or dissolved (the electrolyte). Positive ions (cations) move to the negative cathode and gain electrons (reduction). Negative ions (anions) move to the positive anode and lose electrons (oxidation). In aqueous solutions, water's $\mathrm{H^+}$ and $\mathrm{OH^-}$ compete: hydrogen forms at the cathode unless the metal is less reactive than hydrogen; oxygen forms at the anode unless a halide is concentrated.

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Section 1 of 14

Learning objectives

Describe electrolysis and name the electrodes and electrolyte.
Predict products for molten and aqueous electrolytes.
Write half-equations at each electrode.

Pause and think

Why must sodium chloride be molten or dissolved before electrolysis can work?

Section 2 of 14

Core idea

Start hereSplitting salt with a battery

Pass a current through salty water and bubbles appear at both electrodes: one gas pops with a lit splint, the other bleaches litmus. Electricity has pulled a compound apart.

The big idea

In electrolysis, mobile ions move to the oppositely charged electrode. Cations gain electrons at the cathode (reduction); anions lose electrons at the anode (oxidation).

1. Molten compounds

Worked exampleMolten lead(II) bromide

Molten $\mathrm{PbBr_2}$ is electrolysed with graphite electrodes.

Goal: Products and half-equations.
Strategy: Cation to cathode, anion to anode.

$\mathrm{Pb^{2+}}+2e^-\rightarrow\mathrm{Pb}$
At the cathode: grey lead forms.
$\mathrm{2Br^-}\rightarrow\mathrm{Br_2}+2e^-$
At the anode: brown bromine vapour.

AnswerLead at the cathode, bromine at the anode.

✓ CheckElectrons lost = electrons gained ✓.

2. Aqueous solutions

Predicting products in solution

Drawing diagram…

Water supplies H⁺ and OH⁻ ions that compete.

Key takeawayConcentrated brine Products: hydrogen (cathode), chlorine (anode), sodium hydroxide (left in solution).

Edge caseActive electrodes With copper electrodes in

\mathrm{CuSO_4}

, the copper anode dissolves instead of oxygen forming. That's how copper is purified.

Pause and think

Why do the volumes of hydrogen and oxygen from dilute sulfuric acid come out in the ratio 2 : 1?

Summary

Cathode: reduction; anode: oxidation.
Molten: metal + non-metal.
Aqueous: reactivity and concentration decide.

Electrolyte: Molten or dissolved ionic compound that conducts.
Cathode: Negative electrode.
Anode: Positive electrode.

HistoryFaraday. Michael Faraday, a bookbinder's apprentice turned scientist, invented the words electrode, anode, cathode, ion and electrolyte in 1834.

Fun factAluminium was once precious. Before electrolysis made it cheap, aluminium cost more than gold. Napoleon III served his most honoured guests on aluminium plates.

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