Molten ionic compounds conduct electricity because their ions are free to move; the cation is reduced at the cathode and the anion is oxidised at the anode. For molten lead(II) bromide, PbBr2(l) gives Pb(l) at the cathode and Br2(g) at the anode.
Electrolysis of a molten compound is the decomposition of a molten ionic compound into its elements when a direct current is passed through it. It is a Form 5 Redox topic and appears in both the objective paper and the structured questions, so you need the half-equations, the conditions and the observations at the electrodes.
Why a molten compound conducts
An ionic compound is built from a giant lattice of positive and negative ions. When it is solid the ions are locked in fixed positions and cannot move, so a solid ionic compound does not conduct. When the compound is heated until it melts, the lattice breaks down and the ions become mobile. These free-moving ions carry charge to the electrodes, allowing electrolysis to happen.
The balanced equations (molten lead(II) bromide)
Molten lead(II) bromide, PbBr2, is the standard Form 5 example because it melts at a temperature reachable with a Bunsen burner. Carbon (graphite) electrodes are used.
- At the cathode (negative electrode), lead(II) ions are attracted, gain electrons and are reduced: Pb2+(l) + 2e- → Pb(l)
- At the anode (positive electrode), bromide ions are attracted, lose electrons and are oxidised: 2Br−(l) → Br2(g) + 2e-
The overall reaction is:
PbBr2(l) → Pb(l) + Br2(g)
A second common example is molten sodium chloride:
- Cathode: Na+(l) + e- → Na(l)
- Anode: 2Cl−(l) → Cl2(g) + 2e-
- Overall: 2NaCl(l) → 2Na(l) + Cl2(g)
Notice that for a molten compound the choice is simple: there are only the metal cation and the non-metal anion present, so the metal is always deposited at the cathode and the non-metal gas (or vapour) is released at the anode. There is no water and no competing ion, which makes molten electrolysis more predictable than the electrolysis of aqueous solutions.
Conditions required
- The compound must be molten (heated until it melts) so the ions are free to move.
- A direct current (d.c.) supply is connected, because the electrodes must keep a fixed positive and negative identity.
- Inert electrodes, usually carbon, are used so the electrodes themselves do not react.
Observations
- At the cathode a grey, shiny bead of molten lead forms and collects at the bottom (or a soft grey metal for sodium).
- At the anode, reddish-brown fumes of bromine gas are released (for chloride, a greenish-yellow, pungent gas is seen).
- The electrolyte level slowly falls as the compound decomposes.
How it appears in the SPM exam
In Paper 1 (4541/1) you may be asked to identify the product at a named electrode or to complete a half-equation. In Paper 2 (4541/2), a structured question typically gives you a molten compound and asks you to write the half-equation at each electrode, state the observation, and explain why the compound conducts only when molten. Examiners often add a “predict the products” part for a compound you have not seen, testing whether you understand that the cation goes to the cathode and the anion to the anode. Always show the electrons in your half-equations and balance the charge; a half-equation that does not balance for charge loses the mark even when the products are right. Our online one-to-one lessons drill these half-equations until writing and balancing them is automatic.
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