Class 12 General Principles of Isolation of Elements — Metallurgy
Where ICSE Class 10 introduces metallurgy at a descriptive level — what an ore is, roasting versus calcination, and a handful of named processes — the NCERT Class 12 chapter "General Principles and Processes of Isolation of Elements" goes much further: it uses thermodynamics itself (Ellingham diagrams, Gibbs energy) to explain why a particular reducing agent works at a particular temperature, and covers refining methods this depth of the syllabus actually examines with numericals. This guide covers that Class 12 treatment in full.
Occurrence and concentration of ores
A mineral is a naturally occurring compound of an element; an ore is a mineral from which the metal can be extracted profitably. Before reduction, the ore is concentrated — unwanted impurities (gangue) are removed by a method matched to the ore's physical or chemical properties:
| Method | Principle | Typical ore |
|---|---|---|
| Hydraulic washing (gravity separation) | Density difference between ore and gangue | Oxide ores, e.g. haematite |
| Magnetic separation | Ore or gangue is attracted to a magnet | Magnetite, or ores with magnetic impurities |
| Froth flotation | Sulphide ore particles are wetted by pine oil/collector and float with froth; gangue sinks | Sulphide ores, e.g. galena, zinc blende |
| Leaching | Ore dissolves selectively in a suitable chemical reagent | Bauxite (in hot concentrated NaOH — Bayer's process) |
Worked example 1 — leaching bauxite (Bayer's process)
Bauxite ore (impure Al₂O₃) is concentrated by digesting it with hot, concentrated sodium hydroxide, which dissolves the amphoteric alumina but leaves behind insoluble impurities such as iron oxide.
Al₂O₃(s) + 2NaOH(aq) + 3H₂O(l) → 2Na[Al(OH)₄](aq)
The filtered sodium aluminate solution is then diluted, seeded and treated with CO₂ to
reprecipitate Al(OH)₃, which is calcined to give pure Al₂O₃ — ready for electrolytic
reduction.
Extraction — calcination, roasting and thermodynamic feasibility
Calcination is heating an ore (usually a carbonate or hydroxide) strongly in the absence of air to drive off volatile matter, e.g. CaCO₃ → CaO + CO₂. Roasting is heating an ore (usually a sulphide) strongly in the presence of air/oxygen, e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂. Whether a reduction step will actually proceed is decided by thermodynamics:
An Ellingham diagram plots ΔG° for the formation of a metal oxide against temperature for many metals at once. Two features decide which reducing agent to use at a given temperature: a more negative ΔG° line lies below a less negative one, and the metal whose oxide line is above another element's line can be reduced by that other element (whose oxide-formation line lies below it).
Worked example 2 — calculating ΔG° at a given temperature
For the reaction C(s) + ½O₂(g) → CO(g), ΔH° = −110 kJ/mol and ΔS° = +89 J K⁻¹ mol⁻¹ (entropy increases because a solid converts partly to a gas). Find ΔG° at T = 1000 K, and explain why this reaction becomes more favourable as temperature rises.
ΔG° = ΔH° − TΔS° = −110,000 J − (1000 K × 89 J K⁻¹) = −110,000 − 89,000 = −199,000 J = −199 kJ/mol
Because ΔS° is positive for this reaction, the −TΔS° term becomes more negative as T increases, so ΔG° keeps falling with rising temperature — this is exactly why the C → CO line on an Ellingham diagram slopes downward, making carbon an increasingly effective reducing agent at high temperature, unlike most metal-oxide formation reactions (M + O₂ → MO₂), whose lines slope upward because gaseous O₂ is consumed and ΔS° is negative.
Common reduction methods
- Reduction with carbon/carbon monoxide — the blast furnace process for iron: Fe₂O₃ is reduced in stages by CO and coke to molten iron, with limestone added as a flux to remove silica impurity as slag (CaSiO₃).
- Self-reduction (auto-reduction) — used for metals like copper and lead, where partial roasting followed by heating the roasted and unroasted ore together allows the sulphide to reduce the oxide, e.g. Cu₂S + 2Cu₂O → 6Cu + SO₂.
- Electrolytic reduction — for highly reactive metals (Na, Mg, Ca, Al) that cannot be reduced by carbon; molten Al₂O₃ (dissolved in cryolite to lower its melting point) is electrolysed in the Hall–Héroult process to deposit molten aluminium at the cathode.
Refining the crude metal
| Method | Principle | Example |
|---|---|---|
| Distillation | Low-boiling metals are vaporised and condensed | Zinc, mercury |
| Liquation | Low-melting metal is made to flow away from higher-melting impurities on a sloping hearth | Tin, lead |
| Electrolytic refining | Impure metal anode dissolves; pure metal deposits at the cathode | Copper, silver, aluminium |
| Zone refining | Impurities are more soluble in the molten zone than in the solid, so they migrate to one end | Germanium, silicon (semiconductor-grade) |
| Vapour-phase refining | Metal forms a volatile compound, which is then decomposed to release pure metal | Nickel — Mond's process (via Ni(CO)₄); Titanium/Zirconium — van Arkel method |
Worked example 3 — mass of copper deposited by electrolytic refining
In electrolytic refining, a current of 10 A is passed through a copper sulphate cell for 1 hour. Find the mass of copper deposited at the cathode. (Cu²⁺ + 2e⁻ → Cu; Faraday constant F = 96,500 C/mol; atomic mass of Cu = 63.5)
Charge, Q = I × t = 10 A × 3600 s = 36,000 C
Moles of electrons = Q ÷ F = 36,000 ÷ 96,500 = 0.3731 mol
Moles of Cu deposited = 0.3731 ÷ 2 = 0.1865 mol (2 mol e⁻ deposit 1 mol Cu)
Mass of Cu = 0.1865 × 63.5 = 11.85 g
Common mistakes that cost marks
- Confusing calcination and roasting. Calcination excludes air and suits carbonates/hydroxides; roasting uses air and suits sulphides.
- Misreading an Ellingham diagram. A lower (more negative) line means a more stable oxide, so that element is the better reducing agent for any oxide whose line sits above it — students frequently read this backwards.
- Ignoring that ΔG° must be negative, not just ΔH°. A reaction can have a favourable (negative) ΔH° and still be thermodynamically unfavourable if −TΔS° is positive and large enough to outweigh it.
- Applying carbon reduction to reactive metals. Highly reactive metals such as Na, Mg, Ca and Al are extracted by electrolysis, never by carbon reduction, because their oxides are too thermodynamically stable for carbon to reduce at accessible temperatures.
Where this chapter appears in exams
| Exam | Typical use |
|---|---|
| CBSE Class 12 | Ellingham diagram interpretation, naming reduction/refining methods for a given metal |
| JEE/NEET | Numericals on ΔG°, Faraday's laws applied to electrolytic refining |
| GATE / CSIR-NET | Thermodynamic feasibility of metallurgical reactions, zone refining principles |
Check your Gibbs energy and Faraday's law numericals. The free calculator suite includes dedicated Gibbs Free Energy and Nernst equation tools for this unit's calculations.
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