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Write the chemical reactions involved in the extraction of gold cyanide process. Also give the role of zinc in the extraction.
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(i) $4 \mathrm{Au}(\mathrm{s})+8 \mathrm{CN}^{-}(\mathrm{aq})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{aq})+\mathrm{O}_2(\mathrm{~g}) \longrightarrow$
$4\left[\mathrm{Au}(\mathrm{CN})_2\right]^{-}(\mathrm{aq})+4 \mathrm{OH}^{-}(\mathrm{aq})$
(ii) $2\left[\mathrm{Au}(\mathrm{CN})_2\right]^{-}(\mathrm{aq})+\mathrm{Zn}(\mathrm{s}) \longrightarrow$
$$
2 \mathrm{Au}(\mathrm{s})+\left[\mathrm{Zn}(\mathrm{CN})_4\right]^{2-}(\mathrm{aq})
$$
In this reaction, Zinc acts as a reducing agent.
$4\left[\mathrm{Au}(\mathrm{CN})_2\right]^{-}(\mathrm{aq})+4 \mathrm{OH}^{-}(\mathrm{aq})$
(ii) $2\left[\mathrm{Au}(\mathrm{CN})_2\right]^{-}(\mathrm{aq})+\mathrm{Zn}(\mathrm{s}) \longrightarrow$
$$
2 \mathrm{Au}(\mathrm{s})+\left[\mathrm{Zn}(\mathrm{CN})_4\right]^{2-}(\mathrm{aq})
$$
In this reaction, Zinc acts as a reducing agent.
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