Electrolysis of Molten Compounds

Electrolysis is the breakdown of an ionic compound, when it is melted (molten), by passing an electric current through it. The ions must be free to move, so the compound is heated until it melts.

Key parts and charge flow

• Cathode (negative electrode): attracts positive ions (cations).
• Anode (positive electrode): attracts negative ions (anions). Remember: PANIC – Positive Anode, Negative Is Cathode.
• Electrolyte: the molten ionic compound.
• Electrons move through the external wires. Ions move in the liquid: cations to the cathode, anions to the anode.

What forms at the electrodes

• At the cathode: cations gain electrons (reduction) to form metals.
• At the anode: anions lose electrons (oxidation) to form non-metals (e.g., halogen gases).

In molten compounds there is no water, so hydrogen and hydroxide are not involved. With inert electrodes (carbon/graphite or platinum), the products come only from the ions in the compound.

Example: molten lead(II) bromide, PbBr₂

Ions present: Pb²⁺ and Br⁻.

Cathode (reduction):
$$\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$$

Anode (oxidation):
$$2\,\text{Br}^- \rightarrow \text{Br}_2 + 2e^-$$

Observations: grey lead metal forms at the cathode; orange-brown bromine gas appears at the anode.

How to predict products (molten binary compounds)

• Split the formula into its ions (e.g., NaCl → Na⁺ and Cl⁻).
• Cathode: metal forms by gaining electrons.
• Anode: non-metal forms by losing electrons (halides give halogen molecules like Cl₂, Br₂).
• Write balanced half-equations showing electrons gained or lost.