4CH1

Electrolysis

Principles of Chemistry · 2 question types

Practise

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
Electrode processes in an electrolysis cell — positive cations gain electrons at the negative cathode (reduction) while negative anions lose electrons at the positive anode (oxidation)
Source: Electrolysis diagram by Save My Exams

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
Electrolysis of molten lead(II) bromide — Pb²⁺ ions gain 2 electrons at the cathode to form lead metal, and Br⁻ ions each lose 1 electron at the anode, pairing up to form Br₂ molecules
Source: Electrolysis diagram by Save My Exams

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.