Writing Half-Equations — Electrolysis | IGCSE Chemistry

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 7% of your exam marks.

stable

Low

Stable7%

Electrode products, half-equations and OILRIG tested in most series.

Oxidation and reduction in terms of electrons

is loss of electrons. is gain of electrons. (mnemonic: OIL RIG)
At the anode, anions lose electrons → oxidation
At the cathode, cations gain electrons → reduction
A 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)
Observations: silvery-grey molten lead collects at the cathode and red-brown bromine vapour is released at the anode

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⁻

Exam tip

Writing and checking a half-equation at each electrode

What comes up: "Write a half-equation for the reaction at the anode / cathode during the electrolysis of [substance]."

Write: Place the species being oxidised or reduced on the left, the product on the right, then balance charges by adding electrons (e⁻) to the positive side. At the cathode electrons are gained (add e⁻ to the left); at the anode electrons are lost (add e⁻ to the right). Check: the total charge on each side must be equal.

Watch out: The mark scheme awards a second mark specifically for both state symbols being correct for molten-salt half-equations (for example Pb²⁺(l) + 2e⁻ → Pb(l or s)); the equation can still score the first mark even if state symbols are wrong, but losing this second mark is common. For aqueous systems the mark scheme states that state symbols are ignored even if incorrect, so focus on getting the formula and electron count right. If both half-equations are correct but written in the wrong order (anode answer given for cathode and vice versa), only one mark is awarded — label each one clearly.