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

Explanation

During leaching process, `Au` is first oxidised to `Au^(olpus)` by `O_(2)` of the air, which then combines with `CN^(Ө)` ions to form the soluble complex, sodium dicyaniodaurate `(I)`
`underset("Gold(impure)") (4Au_((s))) + 8NaCN_((aq)) + 2H_(2) O_((1)) + O_(2(g)) rarr underset(("Soluble complex"))underset("Sodium dicyaniodaurate(I)")(4Na[Au(CN)_(2)]_((aq)))+ 4NaOH_((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^(oplus)` to `Au` while it itself gets oxidised to `Zn^(2+)`, which combines with `CN^(Ө)` ions to form a soluble complex, sodium tetracyanidozincate `(II)`
`2Na[Au(CN)_(2)]_((aq)) + Zn_((s)) rarr underset("Gold (pure)") (2 Au_((s))) + underset("Sodium tetracyanidozincate (II)") (Na_(2)[Zn(CN)_(4)]_((aq)))`
Thus, extraction of `Au` by leaching with `NaCN` involves both oxidation and reduction.