The extraction of `Au` by leaching with `NaCN` both oxidation and reduction. Justify giving equations.

Explanation

During leaching process, Au is first oxidised by `O_2 ` of the air to `Au^(+)` which then combines with `CN^(-)` ions to form the soluble complex, dicyanidoaurate (I).
`4Au(S) + 8NaCN (aq) + O_2 (g) + 2H_2 O (l) to underset("Soluble complex")underset("Sodium dicyanidoaurate(I)")(4Na[Au(CN)_2 ](aq) + 4NaOH (aq)`
Gold is then extracted from this complex by displacement method by using a more electropositive zinc metal. in this method, zinc acts as a reducing agent and it reduces `Au^(+)` to Au. Zinc itself gets oxidised to `Zn^(2+)` ions which combine with `CN^-` ions to form soluble complex, sodium tetracyanidozincate (II).
`2Na[Au(CN)_2](aq) + Zn(s) to 2Au(s) + underset("(Soluble complex)")underset("Zincate (II)")underset("Sodium tetracyanido")(Na_2 [Zn(CN)_4]`
Thus , extraction of gold, by leaching with NaCN involves both oxidation and reduction.