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.