Extraction of MetalsCambridge IGCSE Chemistry: Revision notes
Section 1
How does the reactivity series determine metal extraction methods?
The position of a metal in the reactivity series directly determines how it is extracted from its ore. Metals are arranged by their tendency to lose electrons and form positive ions.
- Highly reactive metals (e.g. aluminium, magnesium) cannot be reduced by carbon or carbon monoxide because they form such stable oxides. These must be extracted by electrolysis.
- Moderately reactive metals (e.g. iron) can be reduced by carbon or carbon monoxide in a furnace.
- Less reactive metals (e.g. copper, silver) can sometimes be extracted by heating alone, as their oxides decompose at achievable temperatures.
The general principle is: a more reactive metal can only be displaced from its ore by an even more reactive substance. This is why we use carbon or electricity rather than relying on chemical displacement for most metals.
Examiners expect you to link extraction method directly to reactivity. Always explain why a particular method is used by referencing the metal's position in the reactivity series.
Section 2
What are the chemical processes in the blast furnace extraction of iron?
Iron is extracted from hematite (iron(III) oxide, Fe₂O₃) using the blast furnace. The process involves multiple chemical stages:
Stage 1: Carbon combustion provides heat and produces CO₂
- Coke (carbon) is burnt in oxygen at high temperature
- Equation: C + O₂ → CO₂
Stage 2: Reduction of CO₂ to CO
- Carbon dioxide reacts with excess coke at high temperature
- Equation: C + CO₂ → 2CO
- This is crucial because carbon monoxide is the actual reducing agent for iron ore
Stage 3: Reduction of iron(III) oxide by CO
- Carbon monoxide removes oxygen from the iron ore
- Equation: Fe₂O₃ + 3CO → 2Fe + 3CO₂
- The molten iron produced collects at the bottom of the furnace
Stage 4: Removal of impurities using limestone
- Limestone (calcium carbonate, CaCO₃) thermally decomposes:
- Equation: CaCO₃ → CaO + CO₂
- Calcium oxide (quicklime) reacts with silicon dioxide impurities to form slag:
- Equation: CaO + SiO₂ → CaSiO₃
- Slag is less dense than molten iron and floats on top; it is tapped off separately and can be used for road building
The overall process is continuous: hot gases move up the furnace whilst ore, coke and limestone move down, maximising efficiency.
When writing equations for blast furnace reactions, remember that examiners always expect the three key equations: combustion of carbon, reduction of CO₂, and reduction of iron(III) oxide. Include state symbols if asked.
To explain why two stages are needed to reduce iron oxide: First, C + O₂ → CO₂ provides heat; then C + CO₂ → 2CO because carbon cannot directly reduce Fe₂O₃ efficiently at blast furnace temperatures. Finally CO acts as the reducing agent in Fe₂O₃ + 3CO → 2Fe + 3CO₂.
Section 3
Why is aluminium extracted by electrolysis rather than by reduction with carbon?
Aluminium is a highly reactive metal and cannot be extracted using carbon reduction. Instead, it is extracted from bauxite (its main ore) by electrolysis.
Why carbon reduction fails:
- Aluminium is more reactive than carbon, so carbon cannot remove oxygen from aluminium oxide
- The oxide Al₂O₃ is far too stable to be broken down by heat alone
The electrolysis process:
- Bauxite is first purified to obtain pure aluminium oxide (Al₂O₃)
- The pure aluminium oxide is dissolved in molten cryolite (sodium aluminium fluoride, Na₃AlF₆) at approximately 900 °C
- Cryolite's critical role: It lowers the melting point of aluminium oxide from 2072 °C to around 900 °C, making the process economically viable. Without cryolite, the energy cost would be prohibitive.
- Electrolysis is then performed using a carbon anode and carbon cathode:
- At the cathode (negative electrode): Aluminium ions are reduced to aluminium metal
- Ionic half-equation: Al³⁺ + 3e⁻ → Al
- At the anode (positive electrode): Oxide ions are oxidised
- Ionic half-equation: 2O²⁻ → O₂ + 4e⁻
- At the cathode (negative electrode): Aluminium ions are reduced to aluminium metal
- Molten aluminium collects at the bottom of the cell and is periodically removed
Why carbon anodes must be regularly replaced:
- At the high temperatures and with oxygen being produced, the carbon anode gradually oxidises and burns away
- The reaction at the anode is: 2C + O₂ → 2CO or C + O₂ → CO₂
- The anode must be replaced frequently to maintain the electrolysis process
- This adds significant cost to aluminium extraction
Examiners frequently ask why cryolite is used—always mention that it lowers the melting point, reducing energy costs and making extraction economically practical. Also be ready to write the half-equations: Al³⁺ + 3e⁻ → Al and 2O²⁻ → O₂ + 4e⁻.
Students often forget that the anode is made of carbon and reacts away during electrolysis. It must be replaced regularly—this is a key cost factor in aluminium production and an important exam point.
Section 4
How do we represent the chemical reactions in iron and aluminium extraction?
Accurate symbol equations are essential for demonstrating understanding of extraction chemistry. Here are the key equations you must know:
Iron extraction from hematite (blast furnace):
| Process | Equation |
|---|---|
| Combustion of coke | C + O₂ → CO₂ |
| Reduction of CO₂ by carbon | C + CO₂ → 2CO |
| Reduction of iron(III) oxide | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| Decomposition of limestone | CaCO₃ → CaO + CO₂ |
| Formation of slag | CaO + SiO₂ → CaSiO₃ |
Aluminium extraction (electrolysis half-equations):
| Electrode | Ionic half-equation |
|---|---|
| Cathode (reduction) | Al³⁺ + 3e⁻ → Al |
| Anode (oxidation) | 2O²⁻ → O₂ + 4e⁻ |
These equations demonstrate:
- Stoichiometry: The correct ratios of reactants and products
- Oxidation and reduction: What is being oxidised (loses electrons) and reduced (gains electrons)
- The role of each substance: Why carbon, limestone, or electricity is necessary
When balancing the half-equations for aluminium electrolysis, check that electrons balance: Al³⁺ gains 3 electrons, and each O²⁻ loses 2 electrons, so 2O²⁻ (total 4e⁻) must balance 4/3 Al³⁺ per 1 O₂ produced overall. Write them separately in exam answers.
For Fe₂O₃ + 3CO → 2Fe + 3CO₂: There are 2 iron atoms on both sides ✓, 3 oxygen atoms from CO on the left (3 × 1) and 3 oxygen atoms in CO₂ on the right (3 × 1) ✓, and 3 carbon atoms on both sides ✓. The equation is balanced and shows iron(III) oxide being reduced by carbon monoxide.
Section 5
What are the key differences between iron and aluminium extraction?
Understanding the contrasts between these two extraction methods reinforces why different processes are used:
| Feature | Iron extraction | Aluminium extraction |
|---|---|---|
| Ore | Hematite (Fe₂O₃) | Bauxite (contains Al₂O₃) |
| Reduction method | Chemical reduction using CO gas | Electrolysis (electricity) |
| Why this method? | Iron is moderately reactive; carbon monoxide can reduce it | Aluminium is highly reactive; too stable to reduce with carbon |
| Temperature | ~1500 °C in blast furnace | ~900 °C (lowered by cryolite) |
| Key additive | Limestone (removes impurities) | Cryolite (dissolves ore, lowers melting point) |
| By-products | Slag (useful in construction) | Oxygen gas (from anode oxidation) |
| Cost factor | Relatively cheap (uses carbon fuel) | Expensive (requires continuous electricity and regular anode replacement) |
| Equations | Chemical equations (C, CO, Fe₂O₃) | Ionic half-equations (Al³⁺, O²⁻) |
Why these differences matter:
- Reactivity determines method: The reactivity series decides whether chemical or electrical reduction is needed
- Economics: Aluminium production is energy-intensive, making recycling particularly important
- Industrial scale: Iron extraction is continuous in blast furnaces; aluminium cells must be carefully managed to maintain electrolysis
Think of reactivity as 'stubbornness': iron is moderately stubborn and agrees to leave its oxide when CO persuades it; aluminium is so stubborn that only electricity can force it out, making the process harder and more expensive.
Must Know
- Reactivity series determines extraction method: Highly reactive metals (Al, Mg) need electrolysis; moderately reactive metals (Fe) can be reduced by carbon/CO; less reactive metals require less energy
- Blast furnace iron extraction: C + O₂ → CO₂; C + CO₂ → 2CO; Fe₂O₃ + 3CO → 2Fe + 3CO₂; limestone thermally decomposes to CaO which reacts with SiO₂ impurities to form slag
- Aluminium extraction requires electrolysis because it is too reactive for carbon reduction; bauxite ore contains Al₂O₃ which must be dissolved in molten cryolite at ~900 °C to make the process economical
- Electrolysis half-equations for aluminium: Cathode: Al³⁺ + 3e⁻ → Al; Anode: 2O²⁻ → O₂ + 4e⁻; carbon anodes oxidise and must be regularly replaced, adding to production costs
- Cryolite's role: Lowers the melting point of Al₂O₃ from 2072 °C to ~900 °C, making electrolysis economically viable
- Iron extraction is cheaper than aluminium because it uses chemical reduction (carbon) rather than electricity, and the by-product slag is valuable for construction
That's the notes covered.
Carry on to the next subtopic.