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Topic: Electrochemistry / Metals
Level: Ordinary & Higher Level
Exam Focus: Molten Electrolysis
1. Experiment Snapshot
Feature
Details
Apparatus
Electrolysis model, carbon electrodes, power supply, labelled diagram of industrial cell.
Chemicals/Reagents
Aluminium oxide dissolved in molten cryolite.
Main observation
Aluminium forms at the cathode; oxygen forms at the anode and reacts with carbon anodes.
Main measurement
Industrial process observations rather than school bench collection.
2. Aim
To describe and model the extraction of aluminium by electrolysis of molten aluminium oxide dissolved in cryolite.
3. Quick Theory
Aluminium is too reactive to be extracted by carbon reduction. Molten aluminium oxide is electrolysed, with cryolite lowering melting point and saving energy.
Al³⁺ + 3e⁻ → Al
Examiner Tip: Cryolite lowers the melting point of aluminium oxide and reduces energy cost.
4. Apparatus, Chemicals and Safety
Category
Details
Apparatus
Electrolysis model, carbon electrodes, power supply, labelled diagram of industrial cell.
Chemicals
Aluminium oxide dissolved in molten cryolite.
Safety: This is normally studied as an industrial process. Molten electrolytes are extremely hot; school work should use diagrams or models only.
5. Labelled Experiment Diagram
Extraction of Aluminium — Premium Practical Setup
6. Procedure
Study the labelled industrial electrolysis cell.
Identify molten aluminium oxide dissolved in cryolite.
Identify carbon anodes and carbon cathode lining.
Connect the role of electricity to ion movement.
Al³⁺ ions move to the cathode and gain electrons.
O²⁻ ions move to the anode and lose electrons.
Oxygen reacts with carbon anodes to form carbon dioxide.
Molten aluminium collects at the bottom and is tapped off.
Examiner Tip: This is an industrial process, so exam questions focus on diagram labels and reasons, not bench procedure.
7. Observations and Results
Stage / Test
Observation
Conclusion
Cathode
Molten aluminium forms
Al³⁺ ions reduced
Anode
Oxygen forms and reacts with carbon
CO₂ formed; anodes wear away
Cryolite
Melting point lowered
Less energy needed
Conclusion: Aluminium is extracted by electrolysis because it is too reactive for carbon reduction.
8. Calculations / Key Chemical Reasoning
At the cathode, aluminium ions gain three electrons to form aluminium atoms.
Al³⁺ + 3e⁻ → Al
Worked Example: One Al³⁺ ion needs three electrons to become one Al atom.
9. Sources of Error and Improvements
Source of Error
Effect
Improvement
Saying carbon extracts aluminium
Wrong method
State electrolysis
Forgetting cryolite role
Incomplete answer
Mention lower melting point
Anode not replaced
Industrial issue
Carbon anodes must be replaced
Wrong electrode product
Wrong chemistry
Aluminium at cathode
Calling electrolyte solid
No ion movement
It must be molten
10. Student Common Mistakes
Saying aluminium is extracted in a blast furnace.
Forgetting cryolite.
Putting aluminium at anode.
Saying oxygen has no effect on carbon anodes.
Forgetting molten electrolyte is needed.
11. Examiner Tips
Tip: Aluminium is very reactive.
Tip: Electrolysis is used.
Tip: Cryolite lowers melting point.
Tip: Carbon anodes react with oxygen.
12. Examiner Traps
Trap: Aluminium oxide has very high melting point without cryolite.
Trap: Carbon anodes are consumed.
Trap: Molten ions must be free to move.
13. 🎮 Interactive Simulator
Aluminium Extraction Model
Choose conditions and observe industrial outcome.
Observation Window
Molten cell
Choose a condition and run the simulation.
Lab Score: 0/0
14. Past Paper Style Questions
State the aim of this experiment. [2 marks]
Name the main apparatus or reagent used. [2 marks]
Describe the key observation expected. [2 marks]
Give one safety precaution. [1 mark]
Give one source of error and one improvement. [2 marks]
Explain the chemistry behind the result. [3 marks]
15. Mark Scheme Answers
Q1. To describe aluminium extraction by electrolysis [2].
Q2. Molten Al₂O₃, cryolite, carbon electrodes, power supply [2].
Q3. Aluminium forms at cathode; oxygen/CO₂ at anode [2].
Q4. Industrial process uses very high temperature; not school bench [1].
Q5. Forgetting cryolite; state it lowers melting point [2].
Q6. Al³⁺ gains three electrons at cathode to form Al [3].