Occurrence of metals: minerals and ores
The earth's crust is the main source of metals; sea water also holds salts like NaCl and MgCl2. Naturally occurring compounds of metals are minerals. A mineral that contains enough metal to extract it profitably is an ore. The unwanted sand and rock mixed with the ore is gangue.
- Low reactivity metals (gold, silver, platinum, some copper) are found free (native).
- Middle metals (Zn, Fe, Pb) occur mainly as oxides, sulphides or carbonates.
- Top metals (K, Na, Ca, Mg, Al) are so reactive that they are never free; they occur as salts.
Enrichment (concentration) of ores
Removing gangue from the ore is enrichment. The method depends on how the ore and gangue differ: washing with water (heavy ore settles), magnetic separation (magnetic iron ores), and froth flotation (sulphide ores stick to oil froth).
Extracting metals low, middle and high in the reactivity series
Low reactivity: heating alone
2HgS + 3O2 → 2HgO + 2SO2; 2HgO → 2Hg + O2. Copper: 2Cu2S + 3O2 → 2Cu2O + 2SO2; 2Cu2O + Cu2S → 6Cu + SO2.
Middle reactivity: roasting or calcination, then reduction
It is easier to get a metal from its oxide, so ores are first changed to oxides.
- Roasting — heating a sulphide ore strongly in excess air: 2ZnS + 3O2 → 2ZnO + 2SO2.
- Calcination — heating a carbonate ore strongly in limited air: ZnCO3 → ZnO + CO2.
- Reduction with carbon (coke): ZnO + C → Zn + CO.
- A more reactive metal can also be the reducing agent. The thermite reaction Fe2O3 + 2Al → 2Fe + Al2O3 releases so much heat that the iron comes out molten; it is used to join railway tracks.
High reactivity: electrolysis
K, Na, Ca, Mg and Al hold oxygen more strongly than carbon, so carbon cannot reduce their oxides. They are obtained by electrolytic reduction of their molten chlorides or oxides. For molten NaCl, sodium collects at the cathode (Na+ + e− → Na) and chlorine at the anode (2Cl− → Cl2 + 2e−).
Refining of metals: electrolytic refining
Metal from reduction is still impure. In electrolytic refining (e.g. copper): the impure metal is the anode, a thin strip of pure metal is the cathode, and a solution of a salt of the same metal (acidified CuSO4) is the electrolyte. On passing current, metal dissolves from the anode into the solution and the same amount of pure metal is deposited on the cathode. Soluble impurities stay in the solution; insoluble ones settle below the anode as anode mud.
Key formulas and definitions
- Roasting (sulphide, excess air): 2ZnS + 3O₂ → 2ZnO + 2SO₂
- Calcination (carbonate, limited air): ZnCO₃ → ZnO + CO₂
- Reduction by carbon: ZnO + C → Zn + CO
- Thermite: Fe₂O₃ + 2Al → 2Fe + Al₂O₃ + heat
- Electrolysis of molten NaCl: cathode Na⁺ + e⁻ → Na; anode 2Cl⁻ → Cl₂ + 2e⁻
- Refining: impure metal = anode, pure metal = cathode, salt solution of same metal = electrolyte
Worked examples
1. Galena (PbS) is a sulphide ore. Name the step used to turn it into an oxide and write the idea of the reaction.
Roasting: heat PbS strongly in excess air. 2PbS + 3O₂ → 2PbO + 2SO₂. The oxide is then reduced with carbon.
2. Why can aluminium not be obtained by heating its oxide with carbon?
Aluminium is above carbon in reactivity and holds oxygen more strongly. Carbon cannot take oxygen away from Al₂O₃, so electrolysis of molten Al₂O₃ is used.
3. In copper refining, what are the anode, cathode and electrolyte?
Anode: impure copper. Cathode: thin strip of pure copper. Electrolyte: acidified copper sulphate solution.
4. Explain how cinnabar gives mercury by heating alone.
HgS is roasted: 2HgS + 3O₂ → 2HgO + 2SO₂. On further heating HgO breaks down: 2HgO → 2Hg + O₂. Mercury is low in the series, so heat is enough.
Common mistakes
- Mixing roasting and calcination: roasting = sulphide in excess air; calcination = carbonate in limited air.
- Saying sodium is made by reducing Na₂O with carbon: very reactive metals need electrolysis.
- Putting pure metal at the anode in refining: pure metal is always the cathode.
- Calling every mineral an ore: only minerals from which the metal can be taken out profitably are ores.