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Question: \[M + 8C{N^ - } + 2{H_2}O + {O_2} \to 4{\left[ {M{{\left( {CN} \right)}_2}} \right]^ - } + 4O{H^ - }...

M+8CN−+2H2O+O2→4[M(CN)2]−+4OH−M + 8C{N^ - } + 2{H_2}O + {O_2} \to 4{\left[ {M{{\left( {CN} \right)}_2}} \right]^ - } + 4O{H^ - } metal MM is:
(A) CuCu
(B) HgHg
(C) AgAg
(D) AuAu

Explanation

Solution

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 NaCNNaCN (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+8CN−+2H2O+O2→4[M(CN)2]−+4OH−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 AuAu and AgAg react via cyanide process so the options (A) CuCu and (B) HgHg are the wrong option, so they get rejected.
We are left only with options (C) AgAg and (D) AuAu.
4Ag+8CN−+2H2O+O2→4[Ag(CN)2]−+4OH−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 (Ag2S)\left( {A{g_2}S} \right).
Also, we can get a similar reaction with AuAu.
4Au+8CN−+2H2O+O2→4[Au(CN)2]−+4OH−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) AgAg or (D) AuAu.
Both are considered to be the correct answer for this particular question.

Additional information:
The extraction of silver involves in cyanide process are –
(i) Ag2S+2CN−+O2→23[Ag(CN)2]−+SO2A{g_2}S + 2C{N^ - } + {O_2} \to 23{\left[ {Ag{{\left( {CN} \right)}_2}} \right]^ - } + S{O_2}
(ii) [Ag(CN)2]−+Zn→[Zn(CN)4]2−+Ag{\left[ {Ag{{\left( {CN} \right)}_2}} \right]^ - } + Zn \to {\left[ {Zn{{\left( {CN} \right)}_4}} \right]^{2 - }} + Ag
Thus, O2{O_2} acts as oxidizing agent (oxidize sulphide to SO2S{O_2}) and ZnZn acts as reducing agent (reduces Ag+A{g^ + } to AgAg).

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.