Electrolysis of Molten Compounds

This lesson covers the electrolysis of molten ionic compounds for Edexcel GCSE Chemistry (1CH0). You need to be able to predict the products, write half equations, describe observations, and explain the extraction of aluminium.

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Why Molten?

An ionic compound must be molten (or dissolved) for electrolysis to work. In the solid state, the ions are locked in a rigid lattice and cannot move. When the compound is melted, the lattice breaks down and the ions become free to move towards the electrodes.

In a molten ionic compound, the only ions present are those from the compound itself. There are no water molecules or competing ions. This makes predicting the products straightforward:

• The metal is always produced at the cathode.
• The non-metal is always produced at the anode.

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Electrolysis of Molten Lead Bromide

Lead bromide (PbBr₂) is a common example used in Edexcel exams.

Setup

• Lead bromide is heated until it melts (melting point: 373 °C).
• Two graphite (carbon) electrodes are dipped into the molten lead bromide.
• A d.c. power supply is connected.

What Happens

When the electricity is switched on:

1. Pb²⁺ cations migrate to the cathode (negative electrode).
2. Br⁻ anions migrate to the anode (positive electrode).

Products

Electrode	Ion Arriving	Half Equation	Product	Observation
Cathode (−)	Pb²⁺	Pb²⁺(l) + 2e⁻ → Pb(l)	Lead metal	Silvery molten metal collects at the bottom
Anode (+)	Br⁻	2Br⁻(l) → Br₂(g) + 2e⁻	Bromine gas	Brown/orange fumes seen at the anode

Observations in Detail

• At the cathode: A bead of silvery molten lead forms. When cooled, it solidifies into a grey metallic solid.
• At the anode: Brown/orange fumes of bromine gas are produced. Bromine has a pungent, choking smell (the experiment should be done in a fume cupboard).

Overall Equation

Word equation: lead bromide → lead + bromine

Balanced symbol equation: PbBr₂(l) → Pb(l) + Br₂(g)

Exam Tip: When describing the electrolysis of lead bromide, you must state: (1) the product at each electrode, (2) the half equation at each electrode, (3) which process is oxidation and which is reduction, and (4) the observations. This is a classic 6-mark question.

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Electrolysis of Other Molten Compounds

The same principles apply to any molten ionic compound. Here are further examples:

Molten Sodium Chloride (NaCl)

Electrode	Half Equation	Product
Cathode (−)	Na⁺(l) + e⁻ → Na(l)	Sodium metal
Anode (+)	2Cl⁻(l) → Cl₂(g) + 2e⁻	Chlorine gas

Overall: 2NaCl(l) → 2Na(l) + Cl₂(g)

Molten Zinc Chloride (ZnCl₂)

Electrode	Half Equation	Product
Cathode (−)	Zn²⁺(l) + 2e⁻ → Zn(l)	Zinc metal
Anode (+)	2Cl⁻(l) → Cl₂(g) + 2e⁻	Chlorine gas

Overall: ZnCl₂(l) → Zn(l) + Cl₂(g)

Molten Aluminium Oxide (Al₂O₃)

Electrode	Half Equation	Product
Cathode (−)	Al³⁺(l) + 3e⁻ → Al(l)	Aluminium metal
Anode (+)	2O²⁻(l) → O₂(g) + 4e⁻	Oxygen gas

Overall: 2Al₂O₃(l) → 4Al(l) + 3O₂(g)

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Extraction of Aluminium by Electrolysis

Aluminium is too reactive to be extracted by reduction with carbon (it is above carbon in the reactivity series). Therefore, electrolysis must be used.

The Problem

Aluminium oxide (Al₂O₃) has a very high melting point (2072 °C). Melting it would require enormous amounts of energy, making the process extremely expensive.

The Solution: Cryolite

To reduce costs, aluminium oxide is dissolved in molten cryolite (Na₃AlF₆). Cryolite has a much lower melting point (about 1000 °C), so the electrolysis can be carried out at around 950–1000 °C instead of over 2000 °C.

This significantly reduces the electricity costs and makes the extraction economically viable.

The Process

1. Aluminium oxide (alumina) is dissolved in molten cryolite in a large steel container lined with graphite (which acts as the cathode).
2. Large graphite rods are lowered into the molten mixture as anodes.
3. D.C. electricity is passed through.

At the Electrodes

Cathode (lining of the cell): Al³⁺(l) + 3e⁻ → Al(l)

Molten aluminium sinks to the bottom of the cell and is tapped off periodically.

Anode (graphite rods): 2O²⁻(l) → O₂(g) + 4e⁻

Oxygen gas is produced at the anode.

Why Do the Anodes Need Regular Replacement?

At the high operating temperature, the oxygen gas reacts with the graphite (carbon) anodes, oxidising them to form carbon dioxide:

C(s) + O₂(g) → CO₂(g)

This means the graphite anodes gradually burn away and must be replaced regularly. This adds to the running costs.

Costs of Aluminium Extraction

Cost Factor	Explanation
Electricity	Huge amounts of electricity required to maintain the high temperature and drive the electrolysis
Cryolite	Needed to lower the melting point of aluminium oxide
Graphite anodes	Must be replaced regularly because they react with oxygen
Energy to heat	The cell must be kept at approximately 950–1000 °C

Exam Tip: A common question asks why cryolite is used. The answer is: to lower the melting point of aluminium oxide, which reduces the energy required and therefore the cost. Another common question asks why the anodes need replacing: because oxygen produced at the anode reacts with the hot carbon (graphite) anode to form CO₂, wearing it away.

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Predicting Products of Molten Electrolysis

For any molten binary ionic compound:

• Cathode (negative): The metal is produced (cations gain electrons — reduction).
• Anode (positive): The non-metal is produced (anions lose electrons — oxidation).

This is straightforward because in a molten compound there are only two types of ion present.

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Summary

• Molten ionic compounds are electrolysed because the ions must be free to move.
• In a molten compound, the metal forms at the cathode and the non-metal forms at the anode.
• Lead bromide electrolysis: lead at cathode (silvery metal), bromine at anode (brown fumes).
• Aluminium is extracted by electrolysis of aluminium oxide dissolved in cryolite (to lower the melting point).
• The graphite anodes must be replaced because they react with oxygen to form CO₂.
• Half equations: reduction (gain of electrons) at cathode, oxidation (loss of electrons) at anode.

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Worked Examples: Predicting Products of Molten Electrolysis

Worked Example 1 — Molten potassium iodide