Electrolysis of Molten Compounds
Recap Electrolysis of Molten Compounds for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. In a molten ionic compound, the metal forms at the cathode and the non-metal forms at the anode.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
When a molten ionic compound is electrolysed, the products are straightforward: the metal forms at the cathode and the non-metal at the anode.
There are only two types of ion present, so there is no competition. Molten lead bromide gives lead at the cathode and bromine at the anode. Molten aluminium oxide gives aluminium at the cathode and oxygen at the anode.
Aluminium extraction is the key industrial example. Aluminium oxide is mixed with cryolite to lower its melting point, which reduces the energy needed and therefore the cost. The carbon anodes must be replaced regularly because the oxygen produced reacts with them to form carbon dioxide.
Revision notes
Predicting the products
The metal is produced at the cathode and the non-metal at the anode.
With only two ions present there is no competition, so prediction is simple. Molten lead bromide gives lead and bromine.
Aluminium extraction
Aluminium oxide is mixed with cryolite to lower its melting point.
This reduces the amount of energy needed and therefore the cost. Aluminium forms at the cathode and oxygen at the anode.
Why the anodes wear away
The oxygen produced at the anode reacts with the carbon electrodes at the high temperatures used.
This forms carbon dioxide, so the carbon anodes gradually burn away and must be replaced regularly — a significant ongoing cost.
Key points
- The metal forms at the cathode.
- The non-metal forms at the anode.
- Molten compounds have only two ions, so no competition.
- Cryolite lowers the melting point of aluminium oxide.
- This reduces energy use and cost.
- Carbon anodes react with oxygen and must be replaced.
Worked examples
Example 1
Predict the products of electrolysing molten lead bromide. [2 marks]
Model answer
Lead at the cathode (1 mark) and bromine at the anode (1 mark).
Example 2
Explain why cryolite is added during the extraction of aluminium. [2 marks]
Model answer
Cryolite lowers the melting point of the aluminium oxide (1 mark), so less energy is needed to melt it and the process is cheaper (1 mark).
Example 3
Explain why the carbon anodes must be replaced regularly. [2 marks]
Model answer
Oxygen is produced at the anode and reacts with the carbon at the high temperature used (1 mark), forming carbon dioxide so the anodes gradually burn away (1 mark).
Common mistakes
Putting the metal at the anode.
Metals form positive ions, so they are attracted to the negative cathode.
Saying cryolite is a catalyst.
It lowers the melting point; it does not speed up a reaction.
Forgetting why the anodes wear away.
The oxygen produced reacts with the carbon to form carbon dioxide.
Applying molten rules to aqueous solutions.
In solution water is present too, which introduces competition.
Exam tips
- Metal at the cathode, non-metal at the anode — every time for molten compounds.
- Explain cryolite in terms of melting point, energy and cost.
- Learn the anode reaction as the reason for replacement.
- Keep molten and aqueous electrolysis clearly separate.
Key terms
- Molten
- Melted, so ions are free to move.
- Cryolite
- A compound added to lower the melting point of aluminium oxide.
- Inert electrode
- An electrode that does not react, usually graphite or platinum.
- Discharge
- The formation of a neutral atom from an ion at an electrode.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.