Electrolysis of molten and aqueous compounds, the movement of ions to the electrodes, predicting the products at the cathode and anode, half equations at the electrodes, and the extraction of reactive metals.

What this dot point is asking

OCR wants you to explain electrolysis of molten and aqueous compounds, describe how the ions move to the electrodes, predict the products at the cathode and anode, write half equations at the electrodes (Higher), and explain the extraction of reactive metals such as aluminium. Electrolysis is an application of redox and a major industrial process.

What electrolysis is

In a solid ionic compound the ions are fixed in the lattice and cannot move, so it cannot be electrolysed; melting or dissolving it frees the ions to carry the current.

Movement of ions and what happens at each electrode

Electrolysis of molten compounds

For a molten ionic compound (such as molten lead bromide or aluminium oxide), the rule is simple:

• The metal is produced at the cathode.
• The non-metal is produced at the anode.

For example, molten lead bromide gives lead at the cathode and bromine at the anode.

Electrolysis of aqueous solutions

Solutions also contain $\text{H}^+$ and $\text{OH}^-$ ions from the water, so there is a competition at each electrode:

For example, electrolysis of copper chloride solution gives copper at the cathode (copper is less reactive than hydrogen) and chlorine at the anode.

Half equations at the electrodes (Higher)

The reactions at each electrode are shown by half equations:

• At the cathode (reduction, electrons gained): for example $\text{Al}^{3+} + 3\text{e}^- \rightarrow \text{Al}$, or $2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2$.
• At the anode (oxidation, electrons lost): for example $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2\text{e}^-$, or $4\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 4\text{e}^-$.

Extracting reactive metals