Electrolysis

Key Definition Electrolysis is the decomposition (breakdown) of an ionic compound, when molten or in aqueous solution, using a direct electric current.

Covalent compounds cannot be electrolysed because they contain no free ions or free electrons to carry charge.

Ionic compounds conduct electricity only when molten or dissolved in water, because the ions must be free to move from the fixed lattice to carry charge to the electrodes.

• Cations (positive ions) migrate to the cathode (negative electrode), where they are discharged by gaining electrons (reduction)
• Anions (negative ions) migrate to the anode (positive electrode), where they are discharged by losing electrons (oxidation)

Products of electrolysis

The products of electrolysis depend on the electrolyte.

Molten lead(II) bromide

• Lead (Pb) is produced at the cathode
• Bromine (Br₂) is produced at the anode

Cathode (reduction): Pb²⁺(l) + 2e⁻ → Pb(l) Anode (oxidation): 2Br⁻(l) → Br₂(g) + 2e⁻

Aqueous solutions

Aqueous solutions are more complex because water provides additional H⁺ and OH⁻ ions.

At the cathode

• Hydrogen is produced if the metal is more reactive than hydrogen
• Otherwise the metal is deposited

At the anode

• A halogen is produced if a halide ion is present
• Otherwise oxygen forms from the discharge of hydroxide ions

Reactions at the cathode are always reduction (gain of electrons), and reactions at the anode are always oxidation (loss of electrons).

Writing balanced ionic half-equations with state symbols and correctly classifying them as oxidation or reduction is essential for full marks.