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Oxidation - Reduction

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Oxidation - Reduction - Lesson Summary

Metals are extracted from their ores by the process of reduction.

Non-metals are generally available in combined states in their reduced forms. Hence, non-metals are isolated (or) extracted by oxidising their compounds.

Ex: Chlorine is isolated by oxidising brine solution

Since oxidation cannot be carried out by normal chemical methods, chlorine is extracted by electrolysis.

2NaCl(aq)  + 2H2O(l)      →(Electrolysis)    2NaOH(aq) + H2(g) + Cl2(g)  
             
                   ΔG° = +422 KJ

Minimum potential difference needed
                        E° = -2.2 V
Electrolysis process required an external e.m.f > 2.2 V
 

In the process, chlorine gas liberates at the anode. Hydrogen and sodium hydroxide are the by-products.

The electrolysis of molten sodium chloride also produces chlorine gas at the anode and sodium metal is deposited at the cathode.

Metals are extracted by both oxidation and reduction.

Metals like Gold and silver are extracted by leaching their ores by cyanide ions.

During the leaching process, silver and gold are treated with sodium (or) potassium cyanide in the presence of air to form the respective soluble complexes. In the process, Ag is oxidised to Ag+ and Au oxidised to Au+.

4Ag + 8NaCN + O2(g) + 2H2O(aq) → 4Na[Ag(CN)2](aq) + 4NaOH(aq)

4Au + 8NaCN + O2(g) + 2H2O(aq) → 4Na[Au(CN)2](aq) + 4NaOH(aq)

Followed by the recovery of the metals from their respective complexes by reduction (or ) displacement using zinc (zinc acts as a reducing agent).

2Na[Ag(CN)2] + Zn(s) → Na2[Zn(CN)4](aq) + 2Ag(s)

2Na[Au(CN)2] + Zn(s) → Na2[Zn(CN)4](aq) + 2Au(s)

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