Question
The extraction of Au by leaching with $NaCN$ involves both oxidation and reduction. Justify giving equations.
✓
Answer
$4\text{Au}(\text{s})+8\text{CN}^{-}(\text{aq})+2\text{H}_2\text{O}(\text{aq})+\text{O}_2(\text{g})\\\xrightarrow{ \ \ \ \ \ \ \ \ \ \ }4[\text{Au}(\text{CN})_2]^{-}(\text{aq})+4\text{OH}^{-}(\text{aq})$
$2[\text{Au}(\text{CN})_2]^{-}(\text{aq})+\text{Zn}(\text{s})\\\xrightarrow{ \ \ \ \ \ \ \ }2\text{Au}(\text{s})+[\text{Zn}(\text{CN})_4]^{2-}(\text{aq})$
In the first reaction, $Au$ changes into $Au^+$, i.e., its oxidation takes place. In the second case, $Au^+$ changes to $Au,$ i.e., reduction takes place.
$2[\text{Au}(\text{CN})_2]^{-}(\text{aq})+\text{Zn}(\text{s})\\\xrightarrow{ \ \ \ \ \ \ \ }2\text{Au}(\text{s})+[\text{Zn}(\text{CN})_4]^{2-}(\text{aq})$
In the first reaction, $Au$ changes into $Au^+$, i.e., its oxidation takes place. In the second case, $Au^+$ changes to $Au,$ i.e., reduction takes place.
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