Describe electrolysis as the decomposition of a molten or aqueous ionic compound by electricity, identify the electrodes and the movement of ions, and predict the products of electrolysing a molten compound

What this dot point is asking

SEAB wants you to describe electrolysis as the decomposition of a molten or aqueous ionic compound by electricity, identify the two electrodes and explain how the ions move, and predict the products when a molten compound is electrolysed. This is the foundation for the next dot points on aqueous electrolysis and its applications, and it draws directly on ionic bonding and redox.

The answer

What electrolysis is

Electrolysis is the breaking down (decomposition) of a molten or aqueous ionic compound by passing an electric current through it. The liquid that is electrolysed is called the electrolyte, and it conducts because it contains ions that are free to move.

An ionic compound can only be electrolysed when its ions are free to move, which happens when it is molten or dissolved in water. In the solid the ions are locked in the lattice and cannot move, so a solid ionic compound does not conduct and cannot be electrolysed.

The electrodes

Two electrodes dip into the electrolyte and are connected to a power supply:

• The cathode is the negative electrode (connected to the negative terminal).
• The anode is the positive electrode (connected to the positive terminal).

Often the electrodes are inert (made of carbon or platinum) so they take part in carrying the current without reacting themselves.

Movement of ions

The ions move toward the electrode of opposite charge, because opposite charges attract:

• Positive ions (cations) move to the cathode (negative electrode).
• Negative ions (anions) move to the anode (positive electrode).

This movement of ions through the electrolyte, together with the flow of electrons through the wires, completes the circuit.

What happens at each electrode

At the electrodes the ions are discharged (turned into neutral atoms or molecules) by gaining or losing electrons:

• At the cathode, positive ions gain electrons (reduction). For molten lead(II) bromide: $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$, so lead forms.
• At the anode, negative ions lose electrons (oxidation). For molten lead(II) bromide: $2\text{Br}^- \rightarrow \text{Br}_2 + 2e^-$, so bromine forms.

So electrolysing a molten ionic compound gives the metal at the cathode and the non-metal at the anode. This is a redox process: reduction at the cathode, oxidation at the anode.

Examples in context

Example 1. Extracting aluminium. Aluminium, too reactive to be reduced by carbon, is extracted by electrolysing molten aluminium oxide. Aluminium ions gain electrons at the cathode to form molten aluminium, while oxygen forms at the anode, a direct industrial use of the molten-electrolysis principle.

Example 2. The conductivity demonstration. A classic experiment shows that solid lead(II) bromide does not conduct, but the bulb lights once it is melted. This demonstrates that free-moving ions are needed for conduction and electrolysis, the central idea of this dot point.

Try this

Q1. Name the negative electrode in electrolysis and state which ions move toward it. [1 mark]

• Cue. The cathode; positive ions (cations) move toward it.

Q2. Explain why molten zinc chloride conducts electricity but solid zinc chloride does not. [2 marks]

• Cue. When molten, the ions are free to move and carry charge; when solid, the ions are fixed in the lattice and cannot move, so it does not conduct.

Q3. Write the half-equation for the discharge of chloride ions at the anode. [1 mark]

• Cue. $2\text{Cl}^- \rightarrow \text{Cl}_2 + 2e^-$.