Explain the extraction of elements by oxidation.

(N/A) Besides reduction,some extractions are based on oxidation,particularly for non-metals.
$(i)$ Extraction of chlorine from brine: The extraction is based on oxidation.
$2 Cl_{(aq)}^{-} + 2 H_2O_{(l)} \rightarrow 2 OH_{(aq)}^{-} + H_{2(g)} + Cl_{2(g)}$
The change in Gibbs free energy for the reaction is $+422 \ kJ$. When it is converted to $E^{0}$ using the equation $\Delta G^{0} = -nFE^{0}$,we get $E^{0} = -2.2 \ V$. Thus,an external $emf$ higher than $2.2 \ V$ needs to be supplied to carry out the process.
The electrolysis,however,requires 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 oxidation reaction ($Ag \rightarrow Ag^{+}$,$Au \rightarrow Au^{+}$).
The metal is later recovered by the displacement method.
$4 Au_{(s)} + 8 CN_{(aq)}^{-} + 2 H_2O_{(l)} + O_{2(g)} \rightarrow 4[Au(CN)_2]^{-}_{(aq)} + 4 OH_{(aq)}^{-}$
$2[Au(CN)_2]^{-}_{(aq)} + Zn_{(s)} \rightarrow 2 Au_{(s)} + [Zn(CN)_4]^{2-}_{(aq)}$
$Zinc$ acts as a reducing agent.