SOLID ionic compounds have ions FIXED in the lattice — they cannot move → no electrolysis possible.
MOLTEN ionic compounds have ions FREE TO MOVE → electrolysis can occur.
Practical considerations:
Melting ionic compounds often requires very high temperatures.
NaCl melts at 801°C — very high energy input needed.
Special equipment and safety measures required.
This is why electrolysis is expensive for very reactive metal production.
For ALUMINIUM: the compound is DISSOLVED in molten cryolite (not just melted on its own) to lower the operating temperature from ~2050°C to ~950°C — still very high, but more practically manageable.
Observations During Electrolysis of Lead Bromide
Lead bromide is commonly used in school demonstrations:
Before melting: no conductivity — ions fixed in solid.
After melting: circuit completes — ions free to move.
CATHODE observations:
Grey metallic liquid appears at the negative electrode.
Lead forms as liquid (above its melting point at these temperatures).
ANODE observations:
Reddish-brown bromine vapour produced at positive electrode.
Bromine is a brown liquid/red-brown vapour.
OVERALL: PbBr₂(l) → Pb(l) + Br₂(g)
Note: the solid lead bromide MUST be melted before any electrolysis occurs — a clear observation of why ions must be mobile.
⚠️ Common Mistake
In molten ionic compounds, the METAL is always produced at the CATHODE (the negative electrode). This is reduction — metal ions GAIN electrons. The NON-METAL is always produced at the ANODE (positive electrode). This is oxidation — non-metal ions LOSE electrons.
📐 Key Equations
2NaCl(l) → 2Na(l) + Cl₂(g) (electrolysis of molten NaCl)
PbBr₂(l) → Pb(l) + Br₂(g) (electrolysis of molten lead bromide)
Cathode: Na⁺ + e⁻ → Na
Anode: 2Cl⁻ → Cl₂ + 2e⁻
📌 Key Note
Molten ionic compound → only those ions present. Cathode: metal ion + electrons → metal. Anode: non-metal ions lose electrons → non-metal (gas or liquid). Molten NaCl → Na metal + Cl₂ gas. Molten PbBr₂ → Pb metal + Br₂. Solid won't conduct — ions must be mobile.
🎯 Matching Activity — Products at Each Electrode
Match each electrode product to the correct electrolysis of a molten compound. — drag the symbols on the right to match the component names on the left.
Cathode — molten NaCl
Drop here
Anode — molten NaCl
Drop here
Cathode — molten PbBr₂
Drop here
Anode — molten PbBr₂
Drop here
Chlorine gas (yellow-green) — Cl⁻ ions lose electrons
Lead metal (liquid) — Pb²⁺ ions gain 2 electrons each
⭐ Higher Tier Only
Write balanced half equations for cathode and anode. Combine to give overall equation. Explain product prediction from ion identity. Justify why reactive metals above carbon cannot be extracted by carbon reduction — electrolysis of molten compound is the only option.
🧪 Required Practical
🔬 RP4 (Chemistry) — Carry out electrolysis of lead(II) bromide. Observe products at each electrode. Safety: work in fume cupboard — bromine is toxic.
Know the method, variables, equipment and how to analyse results.
🎯 Test Yourself
Question 1 of 2
1. Molten calcium chloride (CaCl₂) is electrolysed. What is produced at the cathode?
2. Why must lead bromide be melted before it can be electrolysed?
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