How to explain electrolysis of molten compounds

Electrolysis of molten compounds is the simplest kind of electrolysis, which makes it the perfect place to lock down the ideas before you meet the harder aqueous cases. It sits in the redox and electrochemistry chapter, and once you can explain why a molten compound conducts, what happens at each electrode and how to write the half-equations, you have a method that never changes. This guide builds that explanation step by step.

Why the compound must be molten

An ionic compound such as lead(II) bromide is made of positive and negative ions locked in a giant lattice. In the solid state those ions are held tightly in fixed positions, so they cannot move and the solid does not conduct electricity. When you melt the compound, the lattice breaks apart and the ions become free to move. Those mobile ions are what carry the charge, so a molten ionic compound conducts and can be electrolysed. This single idea, ions free to move, is the reason the compound has to be molten, and stating it clearly is often the first mark.

The two electrodes and what they attract

Electrolysis uses two electrodes, usually made of an inert material such as carbon (graphite) so they do not take part in the reaction.

• The cathode is the negative electrode. It attracts the positive ions (cations), which are the metal ions.
• The anode is the positive electrode. It attracts the negative ions (anions), which are the non-metal ions.

Opposite charges attract, so the naming follows naturally: positive ions go to the negative cathode, negative ions go to the positive anode. Because a molten compound contains only one type of cation and one type of anion, the products are completely predictable, you always get the metal at the cathode and the non-metal at the anode.

What happens at each electrode

At the cathode, the metal ions gain electrons. Gaining electrons is reduction, and the result is the metal, formed as a molten liquid or solid on the electrode.

At the anode, the non-metal ions lose electrons. Losing electrons is oxidation, and the result is the non-metal, often a gas.

A useful memory aid is the pair OIL RIG: Oxidation Is Loss, Reduction Is Gain (of electrons). Reduction happens at the cathode, oxidation at the anode, in every electrolysis.

Worked example: molten lead(II) bromide

Molten lead(II) bromide, PbBr₂, contains lead ions (Pb²⁺) and bromide ions (Br⁻). This is the classic SPM example, covered in the electrolysis of molten lead bromide experiment.

• At the cathode the lead ions are reduced to lead metal, which forms as a silvery bead:

  Pb²⁺ + 2e⁻ → Pb

• At the anode the bromide ions are oxidised to bromine, seen as a brown gas or vapour:

  2Br⁻ → Br₂ + 2e⁻

Notice the numbers of electrons match: two electrons are gained at the cathode and two are lost at the anode, so charge is balanced. Always check that the electrons on each side line up.

A second example: molten sodium chloride

The same logic works for any molten binary ionic compound. Molten sodium chloride, NaCl, contains sodium ions (Na⁺) and chloride ions (Cl⁻):

• At the cathode: Na⁺ + e⁻ → Na, sodium metal forms.
• At the anode: 2Cl⁻ → Cl₂ + 2e⁻, chlorine gas is released.

You can see the reactions for a range of compounds on the electrolysis of molten compounds page. The metal always appears at the cathode and the non-metal at the anode, whatever the compound.

How to write the half-equations for marks

1. Identify the two ions in the molten compound and their charges.
2. At the cathode, write the cation gaining electrons to form the metal.
3. At the anode, write the anion losing electrons to form the non-metal.
4. Balance the electrons so the number lost equals the number gained.
5. Label which electrode is which, and state reduction at the cathode, oxidation at the anode.

Common mistakes to avoid

• Saying the solid conducts, it does not, because its ions cannot move.
• Swapping the electrodes: cathode is negative and attracts metal ions; anode is positive and attracts non-metal ions.
• Forgetting the electrons in a half-equation, or getting their number wrong.
• Writing Br or Cl instead of the diatomic Br₂ or Cl₂ for the gas.

Molten electrolysis is worth mastering first because everything in aqueous electrolysis builds on it. If half-equations and electrode reactions are not yet automatic, that is a topic a teacher can straighten out quickly, our online one-to-one lessons run in English from RM50 an hour, with a paid one-hour trial, and we drill electrode reactions until they are second nature.