IGCSE · Practice questions · Electrolysis of aqueous solutions

Pick the winner.

Six original Cambridge-style questions on electrolysis of aqueous solutions: brine, dilute acid, halide solutions, copper(II) sulfate and the half-equations at each electrode.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Two rules, one per electrode.

Cathode: metal only if less reactive than hydrogen; otherwise hydrogen.
Anode: halogen from a concentrated halide; otherwise oxygen.
Tests: hydrogen pops, oxygen relights a glowing splint, chlorine bleaches damp litmus.
Extended: electrons on the left at the cathode, on the right at the anode.

Recall

[4 marks]

Concentrated aqueous sodium chloride is electrolysed using inert electrodes. Name the product at each electrode, give a test for each, and name the substance left in the solution.

Cathode: hydrogen; a lighted splint gives a squeaky pop. ✓
Anode: chlorine; it bleaches damp litmus paper. ✓
Sodium hydroxide is left in solution. ✓

Explain

[2 marks]

Explain why hydrogen, and not sodium, is formed at the cathode in the question above.

The solution contains Na⁺ ions and H⁺ ions (from water). ✓
Sodium is more reactive than hydrogen, so hydrogen ions are discharged instead. ✓

Analysis

[3 marks]

Dilute sulfuric acid is electrolysed. Name the gas at each electrode and explain why the volume of gas at the cathode is twice that at the anode.

Cathode: hydrogen. Anode: oxygen. ✓
The overall change is the splitting of water, 2H₂O → 2H₂ + O₂. ✓
Two molecules of hydrogen form for every molecule of oxygen, so the volume of hydrogen is twice that of oxygen. ✓

Analysis

[2 marks]

(Extended) Predict the product at the anode when (a) concentrated aqueous potassium iodide and (b) dilute aqueous potassium iodide are electrolysed with inert electrodes.

(a) iodine (a concentrated halide solution gives the halogen). ✓
(b) oxygen (a dilute halide solution gives mainly oxygen). ✓

Explain

[4 marks]

(Extended) Aqueous copper(II) sulfate is electrolysed using carbon electrodes. Describe what is seen at each electrode and in the solution, and write the half-equation at the cathode.

Cathode: a pink-brown layer of copper forms. ✓
Anode: bubbles of a colourless gas, oxygen. ✓
The blue colour of the solution fades as Cu²⁺ ions are removed. ✓
Cu²⁺ + 2e⁻ → Cu. ✓

Analysis

[2 marks]

(Extended) Write the half-equations for the reactions at the cathode and the anode during the electrolysis of concentrated hydrochloric acid.

Cathode: 2H⁺ + 2e⁻ → H₂. ✓
Anode: 2Cl⁻ → Cl₂ + 2e⁻. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.