1.9 ElectrolysisEdexcel IGCSE Chemistry: Revision notes

Covalent compounds are made of neutral molecules with no overall charge and no free-moving charged particles. Since electric current requires charged particles that are free to move, covalent compounds (which have neither free ions nor delocalised electrons) do not conduct electricity.

In a solid ionic compound, ions are held in fixed positions in the giant lattice and cannot move, so it cannot conduct electricity.

When molten (melted) or dissolved in water (aqueous), the ions are free to move throughout the liquid. This allows them to carry electric charge, so the substance can conduct — this process of using an electric current to break down an ionic compound is called electrolysis.

Anion: a negative ion (attracted to the anode). Cation: a positive ion (attracted to the cathode).

For a molten ionic compound with inert electrodes (e.g. graphite or platinum, which do not react), such as molten lead(II) bromide, PbBr₂:

1. The compound is heated until molten, allowing ions to move freely.
2. Electrodes connected to a DC power supply are dipped into the molten compound.
3. Positive lead ions (Pb²⁺) migrate to the negative cathode, gain electrons (reduced) and form molten lead metal.
4. Negative bromide ions (Br⁻) migrate to the positive anode, lose electrons (oxidised) and form bromine gas/vapour.

Only two products are possible from a molten binary ionic compound: the metal at the cathode, the non-metal at the anode.

In aqueous solutions, water itself can also be discharged at the electrodes, competing with the dissolved ions:

• At the cathode: hydrogen gas is produced unless the metal is less reactive than hydrogen (e.g. copper), in which case the metal is deposited instead
• At the anode: oxygen gas is usually produced, unless a concentrated halide solution is used, in which case the halogen (e.g. chlorine) is produced instead

Examples:

• Dilute sulfuric acid: hydrogen at the cathode, oxygen at the anode (electrolysis of water, effectively)
• Sodium chloride solution (brine): hydrogen at the cathode, chlorine at the anode
• Copper(II) sulfate solution: copper metal deposited at the cathode, oxygen at the anode

Half-equations show the reaction occurring at each electrode separately, including electrons gained or lost.

• At the cathode (reduction, gain of electrons): e.g. Pb²⁺ + 2e⁻ → Pb
• At the anode (oxidation, loss of electrons): e.g. 2Br⁻ → Br₂ + 2e⁻

Reduction is gain of electrons (or loss of oxygen); oxidation is loss of electrons (or gain of oxygen) — remembered by the mnemonic OIL RIG (Oxidation Is Loss, Reduction Is Gain).

For aqueous electrolysis producing hydrogen and oxygen from water: 4H⁺ + 4e⁻ → 2H₂ (cathode, reduction); 4OH⁻ → O₂ + 2H₂O + 4e⁻ (anode, oxidation).