4.2.2Guiding Spaceship Earth Towards a Sustainable Future
Metal Extraction By Electrolysis — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 8 parts Lesson 8 of 15
Part 1 of 7Why electrolysis
Too reactive for carbon
- Too reactive for carbon Metals more reactive than carbon,
- such as aluminium, cannot be reduced by heating with carbon because carbon cannot take the oxygen away from them.
Molten electrolyte
- Molten electrolyte The ionic compound must be molten (or dissolved) so that its ions are free to move and carry charge.
- A solid ionic compound does not conduct because its ions are held in fixed positions.
Costly
- Costly Large amounts of electrical energy are needed to melt the compound and run the cell,
- so electrolysis plants are often sited near cheap electricity.
Reduction at the cathode
- Reduction at the cathode Metal ions are positive,
- so they are attracted to the negative cathode,
- where they gain electrons and are reduced to metal atoms.
Not suitable for every metal
- Not suitable for every metal Metals below carbon,
- such as iron and lead, are cheaper to obtain by heating with carbon,
- so electrolysis is saved for the reactive metals.
Part 2 of 7Extracting aluminium
1
Aluminium oxide, Al2O3, is purified from the ore bauxite.
2
Its melting point is very high (over 2000 °C), so it is dissolved in molten cryolite instead of being melted on its own.
3
Cryolite lowers the temperature needed to about 950 °C, which saves energy and money.
4
A large direct current is passed between carbon (graphite) electrodes dipping into the molten mixture.
5
Positive aluminium ions move to the cathode, gain electrons and form liquid aluminium, which sinks to the bottom where it is tapped off.
6
Negative oxide ions move to the anode, lose electrons and form oxygen gas.
Part 3 of 7Electrode reactions
| Electrode | Ions attracted | Equation |
|---|---|---|
| Cathode (negative) | Al3+ | Al3+ + 3e− → Al (reduction) |
| Anode (positive) | O2− | 2O2− → O2 + 4e− (oxidation) |
| Overall | 2Al2O3 → 4Al + 3O2 |
Part 4 of 7Anode wear
1Oxygen forms at the hot anode
- Oxygen forms at the hot anode The temperature of the cell is about 950 °C, so the oxygen produced is extremely reactive.
2Carbon reacts
- Carbon reacts Oxygen reacts with the graphite: C + O2 → CO2, so the anode slowly burns away.
3Anodes burn away
- Anodes burn away They must be replaced regularly, which adds to the cost, and carbon dioxide is released into the atmosphere.
Part 5 of 7Balancing the electrons
| Step | Working |
|---|---|
| Charge on ions | Each Al3+ needs 3 electrons; each O2− gives up 2 electrons |
| Make the electrons equal | 4Al3+ need 12e−; 6O2− release 12e− |
| Cathode | 4Al3+ + 12e− → 4Al |
| Anode | 6O2− → 3O2 + 12e− |
| Overall | 2Al2O3 → 4Al + 3O2 |
Part 6 of 7Other metals
Electrolysis in industry
Sodium
- Sodium Obtained by electrolysis of molten sodium chloride, with chlorine gas formed at the anode.
Potassium and calcium
- Potassium and calcium Also extracted from their molten compounds because they are far more reactive than carbon.
Purifying copper
- Purifying copper Impure copper is the anode and pure copper builds up on the cathode while copper sulfate solution carries the ions between them.
Why aluminium is worth it
- Why aluminium is worth it Aluminium is light, strong and resists corrosion, so its high extraction cost is justified.
Energy cost
- Energy cost Because the molten electrolyte must be kept hot and a large current passed through it, electricity is the biggest running cost of any electrolysis plant.
Part 7 of 7Summary of the process
1Ions free to move
- Ions free to move Melting or dissolving the ionic compound frees the ions so that they can carry current through the liquid.
2Ions discharged
- Ions discharged Positive ions gain electrons at the cathode and negative ions lose electrons at the anode, so the compound is broken down.
3Products collected
- Products collected The metal is collected at the cathode and a non-metal such as oxygen or chlorine forms at the anode.
Key terms
| Term | Meaning |
|---|---|
| Electrolysis | Breaking down an ionic compound with electricity. |
| Cryolite | A compound in which aluminium oxide is dissolved to lower the melting point. |
| Bauxite | The ore from which aluminium oxide is obtained. |
| Molten | Melted into a liquid by heating. |
| Cathode | The negative electrode, where metal ions gain electrons. |
| Anode | The positive electrode, where negative ions lose electrons. |
Avoid these mistakes
1
Saying aluminium oxide is dissolved in water; it is dissolved in molten cryolite.
2
Forgetting that the carbon anodes are used up as they burn in the oxygen.
3
Writing that the metal forms at the anode. Metals form at the negative cathode.
4
Claiming electrolysis is cheap; it uses a large amount of electricity.
5
Leaving out that the ions must be free to move for the electrolyte to conduct.
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