Extraction of Metals
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
Ease of Extraction Depends on Reactivity
Most metals are found in the Earth's crust combined with other elements, as an ore - a rock from which a metal can be extracted profitably. Only very unreactive metals, like gold, are found native (as the pure element).
The more reactive a metal is, the more tightly it is bonded within its compound, and the harder (and more expensive) it is to extract:
| Position in reactivity series | Examples | How it's obtained |
|---|---|---|
| Above carbon (most reactive) | K, Na, Ca, Mg, Al | electrolysis - expensive, needs large amounts of electrical energy |
| Below carbon | Zn, Fe, Pb, Cu | reduction with carbon - cheaper |
| Very unreactive | Ag, Au | found native - just needs physical separation from rock |
Carbon marks the dividing line between the two extraction methods
Worked Example: Deducing Extraction Method from Data
- Question: metal Q reacts with dilute acid but its oxide is not reduced by carbon; metal P reacts with dilute acid and its oxide is reduced by carbon. Which metal must be extracted by electrolysis rather than by reduction with carbon? [1]
- Answer: Q. Since Q's oxide cannot be reduced by carbon, Q must be above carbon in the reactivity series, so it needs electrolysis. P's oxide can be reduced by carbon, so P is below carbon and can be extracted more cheaply that way.
Extraction of Iron from Hematite
Iron is extracted from its ore, hematite (mainly iron(III) oxide, Fe₂O₃), by reduction with carbon monoxide in a blast furnace. Iron is below carbon in the reactivity series, so this cheaper method works (unlike aluminium, which needs electrolysis).
The Key Reactions (Supplement)
| Step | Equation | Purpose |
|---|---|---|
| 1. Carbon (coke) burns | C + O₂ → CO₂ | provides heat for the furnace |
| 2. CO₂ is reduced to CO | C + CO₂ → 2CO | produces the actual reducing agent, CO |
| 3. Iron(III) oxide is reduced | Fe₂O₃ + 3CO → 2Fe + 3CO₂ | produces the iron |
Molten iron, being the densest material in the furnace, collects at the bottom and is tapped off.
Common Mistakes
- ❌ Thinking carbon itself reduces the iron ore directly
- ✓ the actual reducing agent is carbon monoxide (formed in step 2); carbon's first role is simply to burn and provide heat (step 1)
Extraction of Aluminium from Bauxite
Aluminium's main ore is bauxite, which is mainly aluminium oxide (Al₂O₃). Aluminium is above carbon in the reactivity series, so it cannot be extracted by reduction with carbon - electrolysis is used instead.
Worked Example
- Question: name the main ore of aluminium, and the process by which aluminium is extracted from it. [2]
- Answer: bauxite [1]; electrolysis [1]
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