Question

\[M + 8C{N^ - } + 2{H_2}O + {O_2} \to 4{\left[ {M{{\left( {CN} \right)}_2}} \right]^ - } + 4O{H^ - }\] metal $M$ is:
(A) $Cu$
(B) $Hg$
(C) $Ag$
(D) $Au$

Answer 1

Hint:
Cyanide process, also called Macarthur Forrest process, is a method of extracting silver and gold from their ores by dissolving them in a dilute solution of sodium cyanide or potassium cyanide.

Complete step by step answer:
Least reactive metals like silver and gold are obtained from the cyanide process. In this process, the impure metal is reacted with $NaCN$ (solution) and the air is passed. Metal is converted into a soluble complex. According to the equation reaction given in the question after balancing.
\[4M + 8C{N^ - } + 2{H_2}O + {O_2} \to 4{\left[ {M{{\left( {CN} \right)}_2}} \right]^ - } + 4O{H^ - }\] (soluble complex)
So, we know that only $Au$ and $Ag$ react via cyanide process so the options (A) $Cu$ and (B) $Hg$ are the wrong option, so they get rejected.
We are left only with options (C) $Ag$ and (D) $Au$.
\[4Ag + 8C{N^ - } + 2{H_2}O + {O_2} \to 4{\left[ {Ag{{\left( {CN} \right)}_2}} \right]^ - } + 4O{H^ - }\] (soluble)
This process is used in extraction of silver from argentite $\left( {A{g_2}S} \right)$.
Also, we can get a similar reaction with $Au$.
$4Au + 8C{N^ - } + 2{H_2}O + {O_2} \to 4{\left[ {Au{{\left( {CN} \right)}_2}} \right]^ - } + 4O{H^ - }$ (soluble)
So, the answer for the question can be (C) $Ag$ or (D) $Au$.
Both are considered to be the correct answer for this particular question.

Additional information:
The extraction of silver involves in cyanide process are –
(i) \[A{g_2}S + 2C{N^ - } + {O_2} \to 23{\left[ {Ag{{\left( {CN} \right)}_2}} \right]^ - } + S{O_2}\]
(ii) ${\left[ {Ag{{\left( {CN} \right)}_2}} \right]^ - } + Zn \to {\left[ {Zn{{\left( {CN} \right)}_4}} \right]^{2 - }} + Ag$
Thus, ${O_2}$ acts as oxidizing agent (oxidize sulphide to \[S{O_2}\]) and $Zn$ acts as reducing agent (reduces $A{g^ + }$ to $Ag$).


Note:
 Cyanide has an impact on the body that it prevents the cells of the body from using oxygen. After this happens, the cells die. It is harmful for the heart and brain.