When a simple molten ionic compound is electrolysed, the metal forms at the cathode and the non-metal forms at the anode. You can predict both by listing the ions, then sending each one to the electrode of opposite charge.
This follows on from the roles of electrodes and electrolyte. You also need to write ionic formulas correctly, which is practised in formulas and equations.
How do I predict the products?
Use the same three steps every time.
- List the ions in the molten compound. Only these are present.
- Send each ion to the opposite electrode. Cations go to the cathode, anions to the anode.
- Name what each ion becomes once it has gained or lost electrons: a metal atom at the cathode, a non-metal atom or molecule at the anode.
The non-metal at the anode is usually a diatomic molecule: Cl₂, Br₂, I₂ or O₂. Two atoms join after the ions lose electrons.
Worked example
Molten lead(II) bromide, PbBr₂, is electrolysed using inert electrodes. State the products and where they form.
Step 1, ions: Pb²⁺ and Br⁻.
Step 2, movement: Pb²⁺ is positive and moves to the cathode. Br⁻ is negative and moves to the anode.
Step 3, products: at the cathode, each Pb²⁺ gains two electrons and becomes a lead atom, so lead forms. At the anode, bromide ions lose electrons and pair up, so bromine forms.
Step 4, overall: PbBr₂ → Pb + Br₂.
Check the atoms: one Pb and two Br on each side. The equation is balanced.
Hazard idea in words: bromine is a toxic, corrosive vapour, and molten salts are extremely hot. That is why this electrolysis is demonstrated by a teacher with proper controls, and why this page explains only the reasoning and not any set-up.
The mistake to watch for
A common slip is to import hydrogen or oxygen because the student is used to water-based cells.
Mistaken answer: “At the cathode, hydrogen forms, and at the anode, oxygen forms.”
There is no water in a molten compound, so there is no source of hydrogen or of the OH⁻ needed for oxygen.
The correction is to write the list of ions first. If the ions are Pb²⁺ and Br⁻, the only possible products are lead and bromine.
Check yourself
1. Predict the products when molten zinc chloride, ZnCl₂, is electrolysed, and say where each forms.
Show answer
Ions: Zn²⁺ and Cl⁻. Zinc forms at the cathode and chlorine at the anode. Overall: ZnCl₂ → Zn + Cl₂.
2. Predict the products of molten potassium iodide, KI.
Show answer
Ions: K⁺ and I⁻. Potassium at the cathode and iodine at the anode. Overall: 2KI → 2K + I₂ (two KI are needed to give the two iodine atoms of I₂).
3. A student says molten sodium bromide gives hydrogen at the cathode. Explain the error.
Show answer
Molten sodium bromide contains only Na⁺ and Br⁻ ions, with no water. The cathode product is sodium, not hydrogen, and the anode product is bromine.
Where this leads next
The next lesson, half-equations for electron transfer, shows exactly what happens to each ion. The equation balance trainer lets you check overall equations by counting atoms.
If your predictions are sound but your equations slip on balancing, our teachers can work through that with you in online one-to-one Chemistry tuition.