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Explain extraction of elements by oxidation. |
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Answer» Solution :Besides reductions, some extractions are based on oxidation particularly for non-metals. (i) Extraction of chlorine from brine : The extraction is based on oxidation. `2Cl_((aq))^(-) + 2H_2O_((l)) to 2OH_((aq))^(-) + H_(2(g)) + Cl_(2(g))` The change in Gibb.s free energy for the reaction is `+ 422 kJ`. When it is converted to `E^@` using equation `DeltaG^@ = -nFE^@`, we get `E^@ = -2.2 V`. Thus external emf higher than 2.2 V needs to supplied to carry out the process. The electrolysis HOWEVER, acquires an excess potential to OVERCOME some other hindering reactions. Thus, `Cl_2`, is obtained by electrolysis giving out `H_2` 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. (ii) Gold CYANIDATION process : Extraction of gold or silver involves leaching with `""^(-)CN`. This is also an oxidationreaction `(Ag to Ag^(+), Au to Au^(+))`. The metal is later recovered by displacementmethod. `4Au_((s)) + 8CN_((aq))^(-) + 2H_2O_((aq)) + O_(2(g)) to 4[Au(CN_2)]_(aq)^(-) + 4OH_((aq))^(-))` `2[Au(CN)_(2)]_(aq)^(-) + Zn_((s)) to 2Au_((s)) + [Zn(CN)_(4)]_((aq)^(2-)` Zinc acts as a reducing agent. |
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