Isolation of Elements — Part 2: Reduction, refining and uses: visual lesson, CBSE Class 12 Chemistry

Lesson

Reduction and refining

Part 2 — turning an oxide into a metal, and making it pure.

In this lesson you will

Use ΔG = ΔH − TΔS and Ellingham diagrams to explain the choice of reducing agent and temperature
Describe the extraction of iron, copper, zinc and aluminium
Apply ΔG = −nFE to electrolytic extraction
Describe refining methods: distillation, liquation, electrolysis, zone refining, vapour phase refining and chromatography

Table of metals with their reducing agents and refining methods
Metal	Reduced by	Refined by
Iron	Coke and CO (blast furnace)	—
Copper	Self-reduction of Cu₂O by Cu₂S	Electrolysis
Zinc	Coke	Distillation
Aluminium	Electrolysis	—
Silicon, germanium	—	Zone refining

Concept

Thermodynamics of reduction

For reduction to occur, $\Delta G$ must be negative. When a metal oxide is heated with a reducing agent, the overall $\Delta G$ is the sum of $\Delta G$ for the two half-processes. Reduction is feasible if the reducing agent forms its oxide with a more negative $\Delta G$ than the metal does. Raising $T$ helps when the reaction has a positive $\Delta S$ .

\Delta G = \Delta H - T\Delta S

\mathrm{MO} + \mathrm{C} \rightarrow \mathrm{M} + \mathrm{CO}

Table showing how coupling two oxide-formation reactions gives the condition for reduction
Coupled reactions (per mol O₂)	Requirement
2M + O₂ → 2MO (ΔG₁)	—
2C + O₂ → 2CO (ΔG₂)	—
2MO + 2C → 2M + 2CO	ΔG₂ − ΔG₁ < 0, i.e. ΔG₂ more negative than ΔG₁

Diagram

The Ellingham diagram

2C + O₂ → 2CO
C + O₂ → CO₂
2CO + O₂ → 2CO₂
2Fe + O₂ → 2FeO
2Zn + O₂ → 2ZnO
4/3 Al + O₂ → 2/3 Al₂O₃

Approximate, illustrative lines. A metal oxide can be reduced by an element whose line lies below it at that temperature.

An Ellingham diagram plots $\Delta G^\circ$ for forming oxides (per mole of O₂) against $T$ . Most metal oxide lines slope upwards (gas is used up, $\Delta S < 0$ ). The line for $\ce{2C + O2 -> 2CO}$ slopes downwards ( $\Delta S > 0$ ). A metal oxide can be reduced by any element whose line lies below it at that temperature, so carbon can reduce FeO above about 1000 K and ZnO at a higher temperature, but not Al₂O₃.

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