Electrolysis of Molten Compounds
Section: 4. Electrochemistry | Syllabus: Cambridge IGCSE Chemistry (0620)
Electrolysing a Molten Binary Compound
A binary ionic compound contains just two elements, e.g. NaCl, MgCl₂, PbBr₂. When molten, only the compound's own ions are present - there is no water, so no competition. This makes the products fully predictable:
- Cathode: the metal ion gains electrons → the metal forms
- Anode: the non-metal ion loses electrons → the non-metal forms
Worked Example: Molten Lead(II) Bromide
Molten PbBr₂ is electrolysed using inert graphite electrodes. Ions present: Pb²⁺ and Br⁻.
- Question: name the product at each electrode and describe the observations. [4]
- Answer - cathode: Pb²⁺ + 2e⁻ → Pb; grey molten lead forms [2]
- Answer - anode: 2Br⁻ → Br₂ + 2e⁻; brown bromine vapour forms [2]
Overall: PbBr₂ → Pb + Br₂
In a molten binary compound, the metal always forms at the cathode and the non-metal at the anode
More Examples of Molten Electrolysis
| Molten compound | Cathode product | Anode product |
|---|---|---|
| NaCl | Na (sodium) | Cl₂ (chlorine) |
| MgCl₂ | Mg (magnesium) | Cl₂ (chlorine) |
| PbBr₂ | Pb (lead) | Br₂ (bromine) |
Worked Example: Molten Magnesium Chloride
- Question: molten magnesium chloride is electrolysed using graphite electrodes. Name the product at the negative electrode and at the positive electrode. [2]
- Answer: negative electrode (cathode): magnesium (Mg²⁺ + 2e⁻ → Mg) [1]; positive electrode (anode): chlorine (2Cl⁻ → Cl₂ + 2e⁻) [1]
Extraction of Aluminium: A Special Case
Aluminium oxide (Al₂O₃) has an extremely high melting point (over 2000°C). It is dissolved in molten cryolite to lower the operating temperature to around 900°C, saving energy.
- Ions present: Al³⁺ and O²⁻
- Cathode: Al³⁺ + 3e⁻ → Al - molten aluminium collects at the bottom and is tapped off
- Anode: 2O²⁻ → O₂ + 4e⁻ - oxygen forms, but reacts with the hot carbon (graphite) anode: C + O₂ → CO₂, so the anodes gradually burn away and must be replaced
Overall: 2Al₂O₃ → 4Al + 3O₂
Very reactive metals such as sodium, magnesium, and aluminium are extracted by electrolysis rather than by reduction with carbon, because they are more reactive than carbon and carbon cannot displace them from their compounds.
Common Mistakes
- ❌ Thinking electrolysis of a molten compound involves competition, like in aqueous solution
- ✓ a pure molten binary compound has no water present, so there is no competition - the metal always forms at the cathode and the non-metal always forms at the anode
- ❌ Forgetting why cryolite is used in aluminium extraction
- ✓ cryolite lowers the melting point of the mixture, reducing the energy (and cost) needed to keep the electrolyte molten
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