JEE Challenger
More from General Principles and Processes of Isolation of Elements
ChemEasyAdv 2008 • Paper 1

Leaching of Native Silver with Dilute Sodium Cyanide

Native silver metal forms a water soluble complex with a dilute aqueous solution of NaCN\mathrm{NaCN} in the presence of

Options

A

nitrogen

B

oxygen

Correct
C

carbon dioxide

D

argon

Topics & Concepts

General Principles and Processes of Isolation of ElementsMetallurgyCoordination Compounds

Step-by-Step Solution

In the extraction of silver by hydrometallurgy (the MacArthur-Forrest process), native silver metal is leached using a dilute aqueous solution of sodium cyanide (NaCN\mathrm{NaCN}).

Since native silver is in the metallic state (oxidation state 00), it must be oxidized to the +1+1 oxidation state to form the soluble dicyanoargentate(I) complex, [Ag(CN)2]−[\mathrm{Ag(CN)}_2]^-. Atmospheric oxygen (O2\mathrm{O}_2) acts as the oxidizing agent in this process.

The balanced chemical equation for the reaction is:
4Ag(s)+8NaCN(aq)+2H2O(l)+O2(g)⟶4Na[Ag(CN)2](aq)+4NaOH(aq)4\mathrm{Ag}(\mathrm{s}) + 8\mathrm{NaCN}(\mathrm{aq}) + 2\mathrm{H}_2\mathrm{O}(\mathrm{l}) + \mathrm{O}_2(\mathrm{g}) \longrightarrow 4\mathrm{Na}[\mathrm{Ag}(\mathrm{CN})_2](\mathrm{aq}) + 4\mathrm{NaOH}(\mathrm{aq})

In ionic form:
4Ag(s)+8CN−(aq)+2H2O(l)+O2(g)⟶4[Ag(CN)2]−(aq)+4OH−(aq)4\mathrm{Ag}(\mathrm{s}) + 8\mathrm{CN}^-(\mathrm{aq}) + 2\mathrm{H}_2\mathrm{O}(\mathrm{l}) + \mathrm{O}_2(\mathrm{g}) \longrightarrow 4[\mathrm{Ag}(\mathrm{CN})_2]^-(\mathrm{aq}) + 4\mathrm{OH}^-(\mathrm{aq})

Thus, the reaction requires the presence of oxygen.

Therefore, the correct option is (B).

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