4.3.3Chemical Changes
Using Electrolysis to Extract Metals — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 15 of 18
Section 1 of 7Choosing an extraction method
Position in the reactivity series
- Position in the reactivity series The reactivity of the metal compared with carbon decides how it is extracted.
More reactive than carbon
- More reactive than carbon Metals such as potassium, sodium, calcium, magnesium and aluminium hold on to oxygen too strongly for carbon to remove it.
- They are extracted by electrolysis of the molten compound.
Less reactive than carbon
- Less reactive than carbon Zinc, iron and copper are obtained by heating the oxide with carbon, which takes the oxygen away.
- This is cheaper because carbon is cheap and also provides heat.
Unreactive metals
- Unreactive metals Gold and silver can be found as native metal, so no chemical extraction is required.
Section 2 of 7Compare the two methods
| Feature | Reduction with carbon | Electrolysis |
|---|---|---|
| Used for | Metals below carbon | Metals above carbon |
| Energy cost | Lower | Very high (melting plus electricity) |
| Raw materials | Carbon is cheap | Needs a continuous electricity supply |
| Example | Iron in a blast furnace | Aluminium from bauxite |
Section 3 of 7Extracting aluminium
1
Bauxite ore is purified to leave aluminium oxide, Al2O3.
2
Aluminium oxide has a very high melting point, so it is dissolved in molten cryolite.
3
The mixture is electrolysed in a cell with graphite electrodes.
4
Al3+ ions move to the cathode, gain electrons and form molten aluminium that sinks to the bottom and is tapped off.
5
O2− ions move to the anode, lose electrons and form oxygen gas.
Section 4 of 7Why cryolite is used
Molten cryolite
Lower temperature
- Lower temperature Aluminium oxide melts above 2000 °C but the mixture works at about 950 °C.
Energy saving
- Energy saving Less heat is needed, which reduces cost.
Conduction
- Conduction The molten mixture contains free ions so it conducts current.
Section 5 of 7Problem at the anode
1Oxygen forms
- Oxygen forms Oxygen is released at the hot graphite anodes.
2Carbon burns
- Carbon burns Oxygen reacts with carbon in the anodes to form carbon dioxide.
3Anodes wear away
- Anodes wear away The graphite anodes gradually get smaller.
4Replacement
- Replacement The anodes must be replaced regularly, adding to the cost.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Ore | A rock containing enough of a metal compound to make extraction worthwhile. |
| Bauxite | The main ore of aluminium, containing aluminium oxide. |
| Cryolite | A compound in which aluminium oxide is dissolved to lower the melting temperature. |
| Reduction | Removal of oxygen from a compound. |
| Reactivity series | A list of metals ordered by how readily they react. |
Section 7 of 7Exam technique
1
Saying aluminium can be extracted with carbon; it is more reactive than carbon.
2
Stating that cryolite is a raw material that is used up; it just lowers the melting point.
3
Explaining that electrolysis is used because it is cheap — it is used because there is no alternative.
4
Putting aluminium at the anode; the metal forms at the cathode.
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