Aluminium electrolysis: cryolite and changing carbon anodes
Explain the molten electrolyte, matched electrode reactions and why the anodes need replacement.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Recall bauxite and the electrolysis route.
- Identify anode/cathode signs, mobile ions and balanced ionic half-equations.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which electrode receives aluminium ions for reduction?
What you will learn
- Describe aluminium extraction from purified bauxite-derived aluminium oxide with cryolite.
- Write both electrode half-equations and explain carbon-anode consumption.
Cryolite helps create a workable molten electrolyte
Aluminium oxide obtained from purified bauxite is dissolved in molten cryolite for the stated extraction. Cryolite lowers the melting temperature needed for the mixture compared with melting pure aluminium oxide, reducing the heating demand. It is not the main ore, a replacement for the aluminium-containing oxide, or an aqueous solvent that allows the same industrial metal extraction at room temperature.
The molten mixture provides mobile ions. Aluminium ions move towards the negative cathode and gain electrons: Al³⁺+3e⁻→Al. Oxide ions lose electrons at the positive anode: 2O²⁻→O₂+4e⁻. Liquid aluminium is collected in the industrial cell context. Electrons flow in the external circuit; ion movement carries charge in the molten electrolyte, not a stream of free electrons through its liquid interior.
Oxygen reacts with the carbon electrode
Oxygen formed at the positive carbon anode reacts with the hot carbon, producing carbon dioxide in the stated school explanation. The carbon anode is therefore consumed and needs regular replacement. The cathode aluminium reduction and the subsequent carbon/oxygen reaction are separate events; do not describe carbon as a catalyst that remains unchanged forever.
Match electrons when combining idealised reactions. Four Al³⁺ ions gain twelve electrons, while six O²⁻ ions lose twelve to form three O₂ molecules. If that oxygen all reacts with carbon to CO₂ and no side process is included, the idealised overall accounting is 2Al₂O₃+3C→4Al+3CO₂. This calculation model explains carbon consumption; it does not assert that every real cell has only that gas or perfect efficiency.
Pause and explain
Why are carbon anodes replaced?
Worked example
Write the aluminium cathode and oxide anode half-equations, then explain anode replacement.
Show the worked solution
- At the negative cathode, Al³⁺+3e⁻→Al is reduction.
- At the positive anode, 2O²⁻→O₂+4e⁻ is oxidation.
- The oxygen reacts with hot carbon to form CO₂ in the stated description, consuming the anode so it must be replaced.
Answer Cathode Al³⁺+3e⁻→Al; anode 2O²⁻→O₂+4e⁻; carbon is consumed by oxygen.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the oxide source and cryolite's role, then identify electrode signs and products.
Give me a hint
Keep ore, molten-mixture helper and electrode changes separate.
Compare my reasoning
- Purified bauxite supplies aluminium oxide, which is dissolved in molten cryolite.
- Cryolite lowers the mixture's required melting temperature.
- Negative cathode forms aluminium; positive anode forms oxygen, which reacts with hot carbon.
Bauxite-derived Al₂O₃ in molten cryolite; lower melting temperature; cathode Al and anode oxygen/carbon consumption.
Look for these in your work
- Source and cryolite roles are distinct.
- Signs match reduction and oxidation.
Match the cathode and anode electron counts to form four aluminium atoms and three oxygen molecules, before oxygen reacts with carbon.
Give me a hint
Use the least common multiple of three and four electrons.
Compare my reasoning
- Multiply Al³⁺+3e⁻→Al by four: 4Al³⁺+12e⁻→4Al.
- Multiply 2O²⁻→O₂+4e⁻ by three: 6O²⁻→3O₂+12e⁻.
- Twelve electrons are lost and gained, and cancel in the combined ionic accounting.
4Al³⁺+12e⁻→4Al; 6O²⁻→3O₂+12e⁻; twelve electrons cancel.
Look for these in your work
- Both multipliers match electrons.
- Atom and charge totals are conserved.
Using Al=27 and O=16, an idealised complete conversion starts with 10.2 g pure Al₂O₃ and sufficient carbon. If all released oxygen forms CO₂ with carbon and there are no side losses, find aluminium and consumed-carbon masses.
Give me a hint
Use 2Al₂O₃+3C→4Al+3CO₂ and C=12 for this idealised calculation.
Compare my reasoning
- Al₂O₃ molar mass is 2×27+3×16=102 g/mol, so 10.2 g is 0.100 mol.
- Aluminium amount is 0.200 mol, mass 0.200×27=5.40 g.
- Carbon amount is 0.150 mol, mass 0.150×12=1.80 g under the explicitly idealised CO₂-only accounting.
5.40 g aluminium and 1.80 g consumed carbon under the stated idealised assumptions.
Look for these in your work
- Both coefficient ratios are used.
- The gas and efficiency assumptions are retained.
Common mistakes
- Calling cryolite the main aluminium ore or the aluminium source replacing all oxide.
- Using aqueous electrolysis as the identical industrial route.
- Reversing the electrode signs or electron directions.
- Calling consumed carbon anodes unchanged catalysts.
What can you explain now?
Why does cryolite reduce heating demand in the stated process?
Compare with the explanation
It lowers the melting temperature of the aluminium-oxide-containing mixture.
The mixture can be molten at a lower temperature than pure aluminium oxide; electrolysis still needs an electrical supply.
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Make it stick
Explain cryolite without calling it the ore, write both half-equations and derive an idealised overall balance that shows carbon consumption.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.