6. Oxidation Reduction

General Principles and Processes of Isolation of Elements (OLD NCERT) | Chemistry

General Principles and Processes of Isolation of Elements (OLD NCERT)

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6.6 Oxidation Reduction

 NEETprep Audio Note:  

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  Besides reductions, some extractions are based on oxidation particularly for non-metals. A very common example of extraction based on oxidation is the extraction of chlorine from brine (chlorine is abundant in sea water as common salt) .

2Cl–(aq) + 2H2O(l) → 2OH–(aq) + H2(g) + Cl2(g)                                       (6.47)

The ∆GƟ for this reaction is + 422 kJ. When it is converted to EƟ (using ∆GƟ = – nEƟF), we get EƟ = – 2.2 V. Naturally, it will require an external emf that is greater than 2.2 V. But the electrolysis requires an excess potential to overcome some other hindering reactions (Unit–3, Section 3.5.1). Thus, Cl2 is obtained by electrolysis giving out H2 and aqueous NaOH as by-products. Electrolysis of molten NaCl is also carried out. But in that case, Na metal is produced and not NaOH.

 NEETprep Audio Note: 
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   As studied earlier, extraction of gold and silver involves leaching the metal with CN–. This is also an oxidation reaction (Ag → Ag+ or Au → Au+). The metal is later recovered by displacement method.

4Au(s) + 8CN–(aq) + 2H2O(aq) + O2(g) → 4[Au(CN)2]–(aq) + 4OH–(aq)      (6.48)

2[Au(CN)2]–(aq) + Zn(s) → 2Au(s) + [Zn(CN)4]2– (aq)                                  (6.49)

In this reaction zinc acts as a reducing agent.