Electrolysis of Molten Ionic Compounds | Chemistry

📖 In-Depth Theory

Products from Molten Ionic Compounds

When a MOLTEN ionic compound is electrolysed, the ONLY ions present are those from the compound itself.

This makes prediction simple:

At the CATHODE (negative): the POSITIVE METAL ION is discharged → metal is deposited.

At the ANODE (positive): the NEGATIVE NON-METAL ION is discharged → non-metal element (often a gas) is produced.

Example — electrolysis of molten sodium chloride (NaCl):

Ions present: Na⁺ and Cl⁻ only.

Cathode: Na⁺ + e⁻ → Na (sodium metal produced — liquid, very reactive)

Anode: 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas produced — yellow-green, toxic)

Example — electrolysis of molten lead bromide (PbBr₂):

Ions present: Pb²⁺ and Br⁻ only.

Cathode: Pb²⁺ + 2e⁻ → Pb (lead metal deposited)

Anode: 2Br⁻ → Br₂ + 2e⁻ (bromine liquid/vapour produced — brown)

Why Molten Rather Than Solid?

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

Sodium metal (liquid) — Na⁺ ions gain electrons

Bromine liquid/vapour (brown) — Br⁻ ions lose electrons

Lead metal (liquid) — Pb²⁺ ions gain 2 electrons each

Chlorine gas (yellow-green) — Cl⁻ ions lose electrons

🧪 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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🧪 Electrolysis of Molten Ionic Compounds

Products from Molten Ionic Compounds

Why Molten Rather Than Solid?

Observations During Electrolysis of Lead Bromide

🔬 RP4 (Chemistry) — Carry out electrolysis of lead(II) bromide. Observe products at each electrode. Safety: work in fume cupboard — bromine is toxic.

Mr. Badmus AI