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

Explanation

During the leaching process, Au is first oxidised to ` Au^(+) ` by `O_ 2 ` of the air which then combines with ` CN^(-)` ions to form the soluble complex, sodium dicyanoauraste (I)
` underset ("(Impure)") underset ("Gold" ) (4Au (s)) + 8 NaCN(aq) + 2 H _ 2 O (l) + O _ 2 (g) to underset("(Soluble complex)")underset ("Sod. dicyanoaurate (I)")(4Na [Au (CN)_2])(aq) + 4 NaOH (aq) `
Gold is then extracted from this complex, by displacement method using a more electropositive zinc metal. In this reaction, Zn acts as a reducing agent. It reduces ` Au^(+) ` to Au while it itself get oxidized to ` Zn^( 2+ ) ` which combines with `CN^(-) ` ions to form soluble complex, sodium tetracyanozincate (II).
` 2 Na [Au (CN)_2] (aq) + Zn (s) to underset ("(Pure)")underset ("Gold") (2Au(s)) + underset ("Sod. tetracyanozincate (II)")(Na_2[Zn (CN)_4](aq) `
Thus, extraction of Au by leaching with NaCN involves both oxidation and reduction.