2.3.9Chemical Bonding, Application Of Chemical Reactions and Organic Chemistry
Extraction of Aluminium
5 min read 7 parts Lesson 33 of 54 1 quiz
Section 1 of 7Why electrolysis is needed
The ore
- The ore Aluminium is found mainly in bauxite, which is rich in aluminium oxide, Al2O3.
- The bauxite is first purified to leave pure aluminium oxide.
Position in the reactivity series
- Position in the reactivity series Aluminium is more reactive than carbon, so carbon cannot take the oxygen away from aluminium oxide.
- Metals above carbon must be extracted by electrolysis of a molten compound.
Why the oxide must be molten
- Why the oxide must be molten In a solid the ions are locked in place and cannot carry charge.
- When molten, the ions are free to move to the electrodes, so the compound conducts and can be broken down.
Section 2 of 7Using cryolite
1Problem
- Problem Pure aluminium oxide melts at over 2000 °C.
2Solution
- Solution The oxide is dissolved in molten cryolite, another aluminium compound.
3Result
- Result The mixture melts at a much lower temperature, and the cryolite does not interfere with the electrolysis.
4Benefit
- Benefit Far less energy is needed to keep the electrolyte molten, which cuts costs.
Section 3 of 7The electrolysis cell
1
The cell is a steel container lined with graphite (carbon). The lining is connected to the negative terminal and acts as the cathode.
2
Several large graphite blocks are lowered into the molten mixture and connected to the positive terminal. They are the anodes.
3
Aluminium oxide is added to the molten cryolite and a large direct current is passed through.
4
Aluminium ions move to the cathode; oxide ions move to the anodes.
5
Molten aluminium collects at the bottom of the cell and is siphoned off from time to time, while fresh aluminium oxide is added.
Section 4 of 7Electrode reactions
| Electrode | Equation | Type of change |
|---|---|---|
| Cathode (negative) | Al3+ + 3e− → Al | Reduction: ions gain electrons to form aluminium metal |
| Anode (positive) | 2O2− → O2 + 4e− | Oxidation: oxide ions lose electrons to form oxygen gas |
| Anode side reaction | C(s) + O2(g) → CO2(g) | Hot graphite burns in the oxygen produced |
Section 5 of 7Consequences of the process
Aluminium extraction
Anodes wear away
- Anodes wear away Because the graphite anodes react with oxygen to form carbon dioxide, they gradually burn away and must be replaced regularly.
Very high electricity use
- Very high electricity use A huge current is needed, and electricity is the largest cost of making aluminium.
Carbon dioxide released
- Carbon dioxide released The burning anodes release a greenhouse gas, which adds to the environmental impact.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Bauxite | The main ore of aluminium, containing aluminium oxide. |
| Electrolysis | Breaking down an ionic compound, when molten or in solution, using electricity. |
| Cryolite | A compound in which aluminium oxide is dissolved to lower the melting point of the electrolyte. |
| Electrolyte | The molten or dissolved ionic compound that conducts electricity in a cell. |
| Reduction | Gain of electrons, as when aluminium ions form aluminium atoms at the cathode. |
| Oxidation | Loss of electrons, as when oxide ions form oxygen at the anode. |
Section 7 of 7Exam technique
1
Saying aluminium is extracted by heating with carbon: it is above carbon in the reactivity series, so electrolysis is needed.
2
Claiming cryolite is the ore or that it reacts: it is only a solvent that lowers the melting point.
3
Putting the electrons on the wrong side: Al3+ + 3e− → Al is a reduction, so electrons are on the left.
4
Forgetting why the anodes need replacingthe oxygen formed reacts with the carbon to make carbon dioxide.
5
Writing that the oxide is electrolysed as a solid: it must be molten so the ions can move.