3.17 - Electrolysis - Molten Compounds

What electrolysis is, and the apparatus

• Electrolysis is the process of splitting an ionic compound using electricity.
• The compound being broken down is the electrolyte: a molten ionic compound or aqueous solution containing ions that are free to move, so it conducts electricity and is decomposed by it.

Why the ions must be free to move

• In a solid, ions are fixed in a lattice and cannot move, so no current flows.
• Freeing the ions requires either:
  • Melting the compound so the ions can move within the liquid.
  • Dissolving it in water so the ions separate and move through the solution.

The apparatus

• Two electrodes (conducting rods) are placed in the electrolyte and connected to a d.c. power supply:
  1. Cathode - The negative electrode, connected to the negative terminal.
  2. Anode - The positive electrode, connected to the positive terminal.
• A direct current (d.c.) is used so each electrode keeps a constant charge and the products form at separate electrodes.

Inert electrodes

• Electrodes are usually made of graphite or platinum.
• These are good conductors of electricity and are inert (unreactive), so they don't take part in the reaction.

Names of the ions

• A cation is a positive ion, which moves to the cathode.
• An anion is a negative ion, which moves to the anode.

Predicting the products of a molten compound

• A molten ionic compound containing two elements breaks down into those two elements:
  1. Metal - Forms positive ions, which are attracted to the negative electrode (cathode) and discharged there.
  2. Non-metal - Forms negative ions, which are attracted to the positive electrode (anode) and discharged there.
• Non-metals that exist as diatomic molecules (such as bromine, chlorine and oxygen) form gas molecules at the anode.

Worked example - Products of molten lead(II) bromide

What is produced at each electrode when molten lead(II) bromide is electrolysed?

The same steps work for any molten compound of two elements. Molten lithium chloride is made of Li⁺ and Cl^- ions, so lithium forms at the negative electrode and chlorine at the positive electrode, seen as bubbles of a pale green gas.

What happens at each electrode

When the current flows, ions move towards the oppositely charged electrodes:

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

Reactions at the electrodes

• At the cathode (negative electrode) - Positive ions gain electrons to become neutral atoms.
• At the anode (positive electrode) - Negative ions lose electrons to become neutral atoms or molecules.
• When an ion loses its charge to become a neutral atom or molecule, it's said to be discharged.

How charge moves through the circuit

• In the wires and power supply, the current is carried by moving electrons.
• In the electrolyte, the current is carried by moving ions.

The half equations at the electrodes

• A half equation shows the reaction and electron transfer at a single electrode.
• The symbol e^- represents an electron (a single negative charge).

Writing a half equation

1. Write the reactant ion on the left and the neutral product on the right.
2. Balance the atoms. Non-metals that form diatomic molecules (e.g. Cl₂, Br₂, O₂) need two ions on the left.
3. Add electrons to balance the charges:
   • At the cathode (negative electrode), ions gain electrons, so write them on the left side.
   • At the anode (positive electrode), ions lose electrons, so write them on the right side.

Worked example - Half equations for molten PbBr₂

Write the half equation at each electrode for the electrolysis of molten lead(II) bromide, which contains Pb²⁺ and Br^- ions.