Electrolysis of molten compounds

Electrolysis uses electricity to break down an ionic compound that is molten or dissolved in water. The liquid is called the electrolyte. When solid, the ions can't move, but when melted or dissolved they are free to move.

Two electrodes dip into the electrolyte, usually made of inert (unreactive) graphite or platinum.

• Positive ions move to the negative electrode, the cathode.
• Negative ions move to the positive electrode, the anode.

At the electrodes, the ions gain or lose electrons and become elements.

When a molten binary ionic compound (made of just two elements) is electrolysed, the metal forms at the cathode and the non-metal at the anode.

Molten lead bromide (PbBr₂) contains Pb²⁺ and Br⁻ ions:

• At the cathode: Pb²⁺ ions gain electrons to form lead, which collects as a silvery liquid.
• At the anode: Br⁻ ions lose electrons to form bromine, a brown gas.

The heating must be done in a fume cupboard, because bromine is toxic.

Aluminium is more reactive than carbon, so it can't be extracted by heating with carbon. Instead, aluminium oxide (from the ore bauxite) is electrolysed.

Aluminium oxide melts at over 2000 °C, which would need huge amounts of energy. So it is dissolved in molten cryolite, which lowers the melting point and saves energy and money.

• Cathode: aluminium forms.
• Anode: oxygen forms.

Both electrodes are graphite (carbon). At the hot anode, the oxygen reacts with the carbon to make carbon dioxide, so the anodes burn away and must be replaced regularly.

The process uses a lot of electricity, which makes aluminium expensive.

Half equations show the electrons gained or lost at each electrode:

• Cathode: Pb²⁺ + 2e⁻ → Pb — lead ions gain electrons, so they are reduced.
• Anode: 2Br⁻ → Br₂ + 2e⁻ — bromide ions lose electrons, so they are oxidised.

For aluminium oxide: Al³⁺ + 3e⁻ → Al at the cathode, and 2O²⁻ → O₂ + 4e⁻ at the anode.

Remember OIL RIG. Reduction always happens at the cathode and oxidation at the anode.