Alloys and Extraction of Metals: Revision Notes

Condensed for the final weeks. For the full explanation, use the Alloys and Extraction of Metals study guide.

Why alloys are harder than pure metals

0620 Core needs only the fact that alloys can be harder and stronger than pure metals. The explanation in terms of structure below is 0620 Extended (syllabus 9.3). 5070 candidates need all of it.

In a pure metal, identical atoms sit in regular layers that can slide over one another — the metal is soft and malleable.

In an alloy, atoms of a different size disrupt the regular layers, so the layers cannot slide as easily. The alloy is therefore harder and stronger.

That explanation, with “different sized atoms” and “layers cannot slide”, follows the syllabus wording.

Alloy	Composition	Use
Brass	Copper + zinc	Instruments, fittings
Bronze	Copper + tin	Bearings, statues
Stainless steel	Iron + chromium + nickel + carbon	Cutlery, chemical plant — resists corrosion
Mild steel	Iron + small % carbon	Car bodies, construction
Duralumin	Aluminium + copper	Aircraft — light and strong

Extraction method depends on reactivity

Position vs carbon	Method	Metals
Above carbon	Electrolysis	K, Na, Ca, Mg, Al
Below carbon	Reduction with carbon	Zn, Fe, Pb
Very unreactive	Found native	Au, Ag

Why aluminium needs electrolysis, not carbon reduction. Aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide back to the metal — it’s not simply that nobody has tried a cheaper method. Iron sits below carbon in reactivity, so carbon reduction works for it, but not for aluminium.

The general rule linking reactivity to ease of extraction: the less reactive a metal, the less energy is needed to reduce its compound back to the metal, so it can be extracted more easily and cheaply — which is also why very unreactive metals like gold occur native (uncombined) rather than needing extraction at all.

Blast furnace — iron from haematite

Raw materials: haematite (Fe₂O₃), coke (C), limestone (CaCO₃), hot air.

0620 Core describes these steps in words; symbol equations are not required. The symbol equations below are 0620 Extended (syllabus 9.6). 5070 candidates need them.

1. C  +  O2   ->  CO2                    (coke burns, releases heat)
2. CO2 +  C   ->  2CO                    (carbon monoxide formed)
3. Fe2O3 + 3CO -> 2Fe + 3CO2             (REDUCTION -- the key step)

Limestone removes acidic impurities:
4. CaCO3      ->  CaO + CO2              (thermal decomposition)
5. CaO + SiO2 ->  CaSiO3  (SLAG)

Molten iron sinks; slag floats on top and is tapped off separately.

Aluminium — electrolysis

0620 Core needs only: the main ore is bauxite, and aluminium is extracted by electrolysis. Cryolite, the anodes and the half-equations below are 0620 Extended (syllabus 9.6). 5070 candidates need all of it.

Ore is bauxite, purified to aluminium oxide (Al₂O₃), dissolved in molten cryolite to lower the melting point and save energy.

Cathode (-):  Al3+ + 3e-  ->  Al          (reduction)
Anode   (+):  2O2-        ->  O2 + 4e-    (oxidation)

The carbon anodes burn away in the oxygen produced and must be replaced regularly — a standard exam point.

Aluminium is expensive because electrolysis uses enormous quantities of electricity.

Uses linked to properties

Metal	Property	Use
Aluminium	Low density, corrosion-resistant (oxide layer), good electrical conductor	Aircraft, drinks cans, overhead cables (low density and good electrical conductivity)
Copper	Excellent conductor, ductile	Wiring, plumbing
Iron/steel	Strong, cheap	Construction, vehicles
Zinc	More reactive than iron	Galvanising

Exam technique for “uses” questions. When a question asks you to explain a use, pair it with the property that explains it — “aluminium is used in aircraft” on its own does not explain anything; add “…because of its low density.” The same logic identifies alloys from structure diagrams: a diagram showing atoms of two different sizes packed together represents an alloy, while identical-sized atoms in regular layers represent a pure metal.

Exam traps

• Explain alloy hardness with different sized atoms disrupting the layers — “it’s mixed” scores nothing.
• Carbon monoxide, not carbon, is the main reducing agent in the blast furnace.
• Cryolite lowers the melting point — it is not a catalyst.
• Aluminium resists corrosion because of a protective oxide layer, not because it is unreactive.
• Slag is calcium silicate and is removed separately from the iron.

Self-test

1. Why is an alloy harder than the pure metal?
2. Write the equation for the reduction of iron(III) oxide in the blast furnace.
3. What is the purpose of limestone?
4. Why must the anodes in aluminium extraction be replaced?
5. Why is aluminium used for overhead power cables despite copper being a better conductor?
6. Why can’t aluminium be extracted from its ore by reduction with carbon, unlike iron?
7. State the general relationship between a metal’s reactivity and how easily it is extracted.
8. Why is “aluminium is used in aircraft” on its own a weak answer to “explain a use of aluminium”?

Answers: 1. Atoms of different sizes disrupt the regular layers, so the layers cannot slide over one another easily. 2. Fe₂O₃ + 3CO → 2Fe + 3CO₂. 3. It decomposes to calcium oxide, which reacts with acidic silicon dioxide impurities to form slag, removing them from the iron. 4. They are carbon and burn away in the oxygen produced at the anode, forming carbon dioxide. 5. Aluminium conducts electricity well and has a much lower density, so cables are lighter and need fewer supporting pylons; it also resists corrosion through its oxide layer. 6. Aluminium is more reactive than carbon, so carbon cannot reduce aluminium oxide; electrolysis is needed instead. 7. The less reactive a metal is, the less energy is needed to extract it from its compound, so it is extracted more easily and cheaply; very unreactive metals such as gold occur native. 8. Because it doesn’t state the property responsible — the full answer needs “…because of its low density”, linking the use to the specific property that explains it.