Occurrence and extraction of metals
Metals are extracted from ores. The method depends on the metal's place in the reactivity series: heating alone, reduction with carbon, or electrolysis. Then the metal is refined.
Steps of extraction
Metals low in the series need only heating. Metals high in the series are extracted by electrolysis of their molten compounds.
Extraction by position in the series
| Metals | Found as | Method | Equations |
|---|---|---|---|
| Low: Hg, Cu (Ag and Au are found free) | Sulphides; gold and silver free | Heating alone in air | Cinnabar (HgS): 2HgS + 3O₂ → 2HgO + 2SO₂, then 2HgO → 2Hg + O₂. Copper: 2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂, then 2Cu₂O + Cu₂S → 6Cu + SO₂ |
| Middle: Zn, Fe, Pb | Sulphides and carbonates | Change to the oxide, then reduce with carbon | Roasting: 2ZnS + 3O₂ → 2ZnO + 2SO₂. Calcination: ZnCO₃ → ZnO + CO₂. Reduction: ZnO + C → Zn + CO |
| High: K, Na, Ca, Mg, Al | Never free; chlorides and oxides | Electrolysis of the molten compound | At the cathode: Na⁺ + e⁻ → Na. At the anode: 2Cl⁻ → Cl₂ + 2e⁻ |
Reduction by a more reactive metal
- Very reactive metals such as sodium, calcium and aluminium can reduce the oxides of less reactive metals.
- 3MnO₂(s) + 4Al(s) → 3Mn(l) + 2Al₂O₃(s) + Heat
- Thermit reaction: Fe₂O₃(s) + 2Al(s) → 2Fe(l) + Al₂O₃(s) + Heat. So much heat is given out that the iron forms as a liquid. It is used to join railway tracks and cracked machine parts.
- Carbon cannot reduce the oxides of sodium, magnesium or aluminium, because these metals hold oxygen more strongly than carbon does. So they are made by electrolysis.
Roasting vs calcination
Most metals come from the earth's crust; sea water also holds salts such as sodium chloride and magnesium chloride. Before extraction, the ore is enriched: the gangue is removed by physical or chemical methods.
How to draw electrolytic refining of copper
- 1Draw a tank of acidified copper sulphate solution. This is the electrolyte.
- 2Dip a thick block of impure copper in it as the anode, joined to the positive terminal of a battery.
- 3Dip a thin strip of pure copper in it as the cathode, joined to the negative terminal.
- 4★When current flows, copper from the anode dissolves, and pure copper is deposited on the cathode.
- 5Soluble impurities go into the solution. Insoluble impurities settle below the anode as anode mud.Label: anode, cathode, electrolyte, anode mud, battery
★ key step
The thermit reaction is used to weld railway tracks, including on Indian Railways lines. The copper in the electric wires of your home is refined by electrolysis to make it very pure.
2 marks: “Why are sodium and aluminium extracted by electrolysis and not by heating their oxides with carbon?” They are very reactive and hold oxygen more strongly than carbon does, so carbon cannot reduce their oxides.
3 marks: “Differentiate between roasting and calcination, with one equation for each.” Roasting: a sulphide ore is heated in excess air, 2ZnS + 3O₂ → 2ZnO + 2SO₂. Calcination: a carbonate ore is heated in limited air, ZnCO₃ → ZnO + CO₂.
5 marks: “Explain the electrolytic refining of copper with a labelled diagram.” Electrolyte: acidified copper sulphate. Anode: impure copper. Cathode: pure copper strip. Copper dissolves from the anode and deposits on the cathode; impurities form anode mud. Draw and label.
Low metals: just heat. Middle metals: oxide, then carbon. Top metals: electricity. And Roast the Sulphide (excess air), Calcine the Carbonate (limited air).