Extraction of Metals
Section: 9. Metals | Syllabus: Cambridge IGCSE Chemistry (0620)
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
Why Carbon Can't Reduce the Most Reactive Metals
Extracting a metal from its oxide is a reduction reaction (the metal ore loses oxygen). Carbon can only take the oxygen away from a metal oxide if carbon is more reactive than that metal.
2Fe₂O₃ + 3C → 4Fe + 3CO₂ ✓ (iron is below carbon)
Al₂O₃ + C → ❌ does not happen (aluminium is above carbon - more reactive than carbon, so carbon cannot remove its oxygen)
This is exactly why aluminium (Section 9.6, next topics) needs electrolysis instead.
Worked Example: A Metal Extracted Using a More Reactive Metal
Some metals that are too reactive for carbon, but too expensive to extract by electrolysis, are instead extracted by reduction with a more reactive metal. Titanium is obtained this way, using magnesium:
- Question: magnesium reacts with titanium(IV) chloride, TiCl₄, to produce titanium. Complete the symbol equation: …Mg + TiCl₄ → …MgCl₂ + Ti [1]
- Answer: 2Mg + TiCl₄ → 2MgCl₂ + Ti. TiCl₄ has 4 Cl atoms, so 2 MgCl₂ are needed to balance chlorine, which in turn needs 2 Mg on the left.
Magnesium is more reactive than titanium, so it displaces titanium from the chloride - the same displacement principle covered in the reactivity series topic, just applied to extraction.
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.
Interactive revision notes, videos and practice questions load below.