Electrolysis explained: what forms at each electrode

In a molten compound, the metal forms at the cathode and the non-metal at the anode.

Molten lead bromide: lead forms at the cathode and bromine at the anode.

• Cathode: $\text{Pb}^{2+} + 2e^- \rightarrow \text{Pb}$ (reduction: gain of electrons)
• Anode: $2\text{Br}^- \rightarrow \text{Br}_2 + 2e^-$ (oxidation: loss of electrons)

Water provides extra H⁺ and OH⁻ ions, so the rules change:

• At the cathode: hydrogen forms if the metal is more reactive than hydrogen; otherwise the metal forms (copper, for example).
• At the anode: oxygen forms, unless halide ions (chloride, bromide, iodide) are present, in which case the halogen forms.

Example: in sodium chloride solution (brine), hydrogen forms at the cathode, chlorine at the anode, and sodium hydroxide is left in solution.

Aluminium is more reactive than carbon, so it's extracted by electrolysis of aluminium oxide dissolved in molten cryolite (which lowers the melting point and saves energy). Aluminium forms at the cathode; oxygen forms at the carbon anode and reacts with it to make carbon dioxide, so the anodes wear away and must be replaced. The process uses a lot of electricity, which makes aluminium expensive to extract.

• PANIC: Positive Anode, Negative Is Cathode.
• OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons).
• Positive ions are cations (attracted to the cathode); negative ions are anions (attracted to the anode).

• Extracting reactive metals such as aluminium.
• Producing chlorine, hydrogen and sodium hydroxide from brine.
• Electroplating: coating objects with a thin layer of metal, such as silver or chromium.
• Purifying copper for electrical wires.

• Forgetting that solids can't be electrolysed: the ions can't move.
• Mixing up which ion goes to which electrode.
• Predicting the metal at the cathode in solution when it's more reactive than hydrogen.

Review ions and bonding in ionic vs covalent bonding.

1. Write the ion and the element it becomes: Cu²⁺ → Cu.
2. Balance the atoms (here, already balanced).
3. Balance the charge with electrons: Cu²⁺ + 2e⁻ → Cu.

At the anode, negative ions lose electrons: 2Cl⁻ → Cl₂ + 2e⁻. Check each side has the same total charge.

1. What forms at each electrode when molten sodium chloride is electrolysed?
2. What forms at each electrode in copper sulfate solution with inert electrodes?
3. What forms at each electrode in potassium bromide solution?

Answers: 1. Sodium at the cathode, chlorine at the anode. 2. Copper at the cathode, oxygen at the anode. 3. Hydrogen at the cathode (potassium is more reactive than hydrogen), bromine at the anode.

• Electrolyte: the molten or dissolved ionic compound.
• Electrode: a conductor that carries current into or out of the electrolyte.
• Cathode: the negative electrode, where reduction happens.
• Anode: the positive electrode, where oxidation happens.
• Inert electrode: one that doesn't react, such as graphite or platinum.
• Half equation: shows what happens at one electrode, with electrons.

Water can be split by electrolysis when a little acid or salt is added so it conducts. Hydrogen forms at the cathode and oxygen at the anode, and twice as much hydrogen as oxygen is collected by volume, matching the formula H₂O. Electrolysis powered by renewable electricity is one way of producing hydrogen as a fuel.