Molten sodium chloride

Using the conventions, predict what happens when molten sodium chloride is electrolysed with carbon electrodes.

Figure to be added

Figure 12.2.3 — molten sodium chloride between two carbon electrodes; Na⁺(l) moves to the negative electrode and Cl^-(l) to the positive; electrons flow in the wire.

At the negative electrode

The only kind of positive ion in the melt, Na⁺(l), is attracted to the negative terminal. There each Na⁺ ion gains an electron and becomes a sodium atom. The Na⁺ ions are reduced, so this is the cathode reaction and the negative electrode is the cathode.

Na⁺ (l) + e → Na (l)

(1)

At the positive electrode

The only negative ion in the melt, Cl^-(l), is attracted to the positive electrode, where the ions give up electrons and become chlorine molecules. The chloride ions are oxidised, so this is the anode reaction and the positive electrode is the anode.

2Cl^- (l) → Cl₂ (g) + 2e

(2)

The overall reaction comes from adding (1) and (2) suitably. Reaction (2) involves two electrons and (1) only one, so (1) is first multiplied by 2; then the 2e on each side cancel.

2Na⁺ (l) + 2e → 2Na (l)

2Na⁺ (l) + 2Cl^- (l) → 2Na (l) + Cl₂ (g)

(1) × 2 = (3); then (2) + (3)

This electrolysis is the reaction in the Downs cell, used in industry to extract sodium metal. You will study that method in more detail later.

Molten salt gives sodium and chlorine

With only Na⁺ and Cl^- present, sodium forms at the cathode and chlorine at the anode.