Electrolysis – IGCSE Chemistry Definition

Electrolysis is one of the most heavily tested topics in the electrochemistry section of the 0620 syllabus. You need to explain the process, predict the products at each electrode, write half-equations, and know the industrial applications. It appears on Papers 1, 2, 4, and 6.

The 0620 definition

Electrolysis is the decomposition (breaking down) of an ionic compound, either molten or in aqueous solution, by passing a direct electric current through it.

Requirements for electrolysis

1. An electrolyte — a molten or dissolved ionic compound containing free-moving ions
2. Two electrodes — usually carbon (graphite), connected to a direct current (d.c.) power supply
3. A complete electrical circuit

How electrolysis works

1. The power supply pushes electrons around the external circuit
2. Cations (positive ions) migrate to the cathode (negative electrode) and gain electrons — reduction
3. Anions (negative ions) migrate to the anode (positive electrode) and lose electrons — oxidation
4. The electrolyte is decomposed into its elements or other products

Predicting products

Molten electrolytes (simpler)

Only two ions present — the metal cation goes to the cathode, the non-metal anion goes to the anode.

Electrolyte	Cathode product	Anode product
Molten PbBr₂	Lead	Bromine
Molten NaCl	Sodium	Chlorine
Molten Al₂O₃	Aluminium	Oxygen

Aqueous electrolytes (Supplement)

Water provides extra ions (H⁺ and OH⁻), so selective discharge rules apply:

• At the cathode: metal deposited if less reactive than hydrogen; otherwise hydrogen gas
• At the anode: halide ion discharged if concentrated; otherwise oxygen from OH⁻

Industrial applications

• Extraction of aluminium: electrolysis of molten aluminium oxide (bauxite dissolved in cryolite)
• Purification of copper: impure copper anode dissolves; pure copper deposited at cathode
• Electrolysis of brine: produces chlorine (anode), hydrogen (cathode), and sodium hydroxide (solution)

Worked exam question

Dilute sulfuric acid is electrolysed using carbon electrodes. (a) Name the product at the cathode. (1) (b) Name the product at the anode. (1) (c) Write the half-equation at the cathode. (1) (d) Explain why the volume of gas at the anode is half the volume at the cathode. (2)

Mark scheme

(a) Hydrogen [1]

(b) Oxygen [1]

(c) 2H⁺ + 2e⁻ → H₂ [1]

(d) The half-equation at the anode is 4OH⁻ → 2H₂O + O₂ + 4e⁻; 4 moles of electrons produce 1 mol O₂ but only 2 moles of electrons produce 1 mol H₂ [1]; so twice the volume of hydrogen is made compared to oxygen [1]

Common exam mistakes

• Using alternating current (a.c.) instead of direct current (d.c.). Electrolysis requires d.c. so that each electrode maintains its polarity.
• Confusing products of molten vs aqueous electrolytes. Molten NaCl gives sodium and chlorine. Aqueous NaCl gives hydrogen and chlorine (sodium is too reactive to be discharged from solution).
• Forgetting that the electrolyte must contain free ions. Solid ionic compounds do not conduct because ions are fixed in the lattice.