Electrolysis of Molten and Aqueous Solutions
Predicting the products is the heart of electrolysis questions. Molten compounds are simple; aqueous solutions have competition from water — which is where nearly every mark is lost.
1. Molten compounds — the simple case
Only two ions are present, so the METAL forms at the CATHODE and the NON-METAL at the ANODE.
Molten lead(II) bromide, PbBr₂:
| Electrode | Product | Half-equation |
|---|---|---|
| Cathode (−) | lead | Pb²⁺ + 2e⁻ → Pb |
| Anode (+) | bromine | 2Br⁻ → Br₂ + 2e⁻ |
Observations: a silvery molten metal at the cathode; red-brown vapour at the anode.
Molten sodium chloride:
- Cathode: Na⁺ + e⁻ → Na (sodium)
- Anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine — pale green gas)
Molten means NO WATER, so there is no competition — the metal is always produced. Confusion about the products at the cathode for molten sodium chloride was recorded.
2. Aqueous solutions — the competition
Water provides H⁺ and OH⁻ ions, which compete with the compound’s ions.
Always remember H⁺ and OH⁻ are present in ANY aqueous solution. Forgetting them was a specifically recorded error, and it is the whole reason aqueous results differ from molten.
At the cathode — use the reactivity series
If the metal is MORE reactive than hydrogen → HYDROGEN is produced. If the metal is LESS reactive than hydrogen (Cu, Ag) → the METAL is produced.
So for aqueous sodium chloride the cathode gives HYDROGEN, not sodium — because sodium is more reactive than hydrogen and stays in solution as Na⁺.
This was recorded as a misconception twice — a student expected sodium at the cathode. Compare directly with molten NaCl, where sodium is produced. The presence of water changes everything.
Use the reactivity series to decide — tutors flagged this, and forgetting the series was itself recorded.
Cathode half-equations:
- 2H⁺ + 2e⁻ → H₂ (reactive metals present)
- Cu²⁺ + 2e⁻ → Cu (unreactive metal)
At the anode
If a HALIDE (Cl⁻, Br⁻, I⁻) is present and CONCENTRATED → the HALOGEN is produced. Otherwise, or if very DILUTE → OXYGEN is produced from OH⁻.
Anode half-equations:
- 2Cl⁻ → Cl₂ + 2e⁻
- 4OH⁻ → O₂ + 2H₂O + 4e⁻
Concentration matters at the anode. Misunderstanding its role was recorded — concentrated brine gives chlorine; very dilute sodium chloride gives oxygen.
3. The standard examples
| Electrolyte | Cathode (−) | Anode (+) |
|---|---|---|
| Molten PbBr₂ | lead | bromine |
| Molten NaCl | sodium | chlorine |
| Concentrated NaCl (brine) | HYDROGEN | chlorine |
| Dilute NaCl | hydrogen | OXYGEN |
| Copper(II) sulfate (inert electrodes) | COPPER | oxygen |
| Dilute sulfuric acid | hydrogen | oxygen |
| Water (acidified) | hydrogen | oxygen (2 : 1 by volume) |
Electrolysis of concentrated brine gives THREE useful products: hydrogen at the cathode, chlorine at the anode, and sodium hydroxide left in solution.
Copper is produced at the cathode from copper sulfate, because copper is below hydrogen. The blue colour fades as Cu²⁺ ions are removed — that is the observation.
Copper deposits as a red-brown SOLID on the electrode, not as a vapour or precipitate. A recorded error expected blue vapour or a precipitate.
4. Writing half-equations
Cathode (reduction) — electrons on the LEFT Anode (oxidation) — electrons on the RIGHT
Balance atoms AND charges.
Two that are commonly written wrongly:
Aluminium at the cathode: Al³⁺ + 3e⁻ → Al — not “Al³⁺ → Al + 3e⁻”. Electrons are gained, so they go on the left. This exact error was recorded.
Oxygen at the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻, or from oxide ions 2O²⁻ → O₂ + 4e⁻ — not “O₂ → O₂ + 2e⁻”. Also recorded.
Combining half-equations: multiply so the electrons cancel, then add.
5. Observations to quote
| Product | Observation |
|---|---|
| Hydrogen | bubbles/effervescence at the cathode; squeaky pop |
| Oxygen | bubbles at the anode; relights a glowing splint |
| Chlorine | pale green gas; bleaches damp litmus |
| Bromine | red-brown vapour |
| A metal | solid deposit coating the electrode |
Say what you would SEE — bubbles, a colour, a coating — and then the gas test that confirms it.
6. Mistakes that cost marks
Predicting the metal at the cathode for a reactive metal in solution.
Forgetting H⁺ and OH⁻ are present in aqueous solutions.
Ignoring concentration at the anode.
Putting electrons on the wrong side.
Unbalanced charges in half-equations.
Confusing molten with aqueous results for the same compound.
Giving a product without an observation.
Naming “bromide” as the gas instead of bromine.
Frequently asked questions
What forms at the cathode in a molten compound? The metal.
What forms at the anode in a molten compound? The non-metal.
Why are aqueous results different? Water supplies H⁺ and OH⁻ ions, which compete.
What forms at the cathode in aqueous sodium chloride? Hydrogen — sodium is more reactive than hydrogen.
When is the metal produced from a solution? When it is less reactive than hydrogen — copper or silver.
What decides the anode product? Whether a halide is present and how concentrated it is.
What are the three products from concentrated brine? Hydrogen, chlorine and sodium hydroxide.
What is the half-equation for aluminium at the cathode? Al³⁺ + 3e⁻ → Al.
How do I test for chlorine? It bleaches damp litmus paper.
What do I see when copper is produced? A red-brown solid coating the cathode, and the blue colour fading.
Quick revision checklist
- I can predict products for any molten compound
- I remember H⁺ and OH⁻ in aqueous solutions
- I use the reactivity series at the cathode
- I know reactive metals give hydrogen
- I consider concentration at the anode
- I know the standard examples, including brine
- I can write half-equations with electrons on the correct side
- I balance atoms and charges
- I can combine half-equations
- I can give an observation for every product
- I know the gas tests
These notes cover the electrolysis of molten and aqueous compounds in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and data booklet for your own exam series.
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