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chemistry igcse acapprinted 25 · PDF 25
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25
Electrolysis of Molten Compounds
Ionic compounds decompose into their constituent elements when molten.
Example: Electrolysis of Lead(II) Bromide (PbBr₂):
Setup: Heat PbBr₂ until molten, then insert graphite electrodes.
At Cathode: Lead (Pb²⁺) ions gain electrons to form solid lead.
Half-equation: Pb²⁺ + 2e⁻ → Pb(s)
At Anode: Bromide (Br⁻) ions lose electrons to form bromine gas.
Half-equation: 2Br⁻ → Br₂(g) + 2e⁻
Overall Reaction: PbBr₂(l) → Pb(s) + Br₂(g)
Electrolysis of Aqueous Ionic Compounds
Water (H₂O) dissociates into H⁺ and OH⁻ ions.
At the Cathode:
More reactive metals (above hydrogen in the reactivity series) remain in solution.
Hydrogen gas (H₂) is produced if metal is more reactive than hydrogen.
Metal deposition occurs if metal is less reactive than hydrogen.
At the Anode:
Halide ions (Cl⁻, Br⁻, I⁻) form halogen gas (Cl₂, Br₂, I₂).
Oxygen gas (O₂) is produced if halide ions are absent (from OH⁻ ions).
Electrolysis of Aqueous Sodium Chloride
Setup: Brine (NaCl solution) is electrolysed with inert electrodes (e.g., graphite or platinum).
At Cathode: Hydrogen ions (H⁺) are reduced to form hydrogen gas.
Half-equation: 2H⁺ + 2e⁻ → H₂(g)
At Anode: Chloride ions (Cl⁻) are oxidized to form chlorine gas.
Half-equation: 2Cl⁻ → Cl₂(g) + 2e⁻
Overall Reaction: 2NaCl(aq) + 2H₂O(l) → Cl₂(g) + H₂(g) + 2NaOH(aq)