Question

$CuS{O}_4$ when reacts with $KCN$ forms $CuCN$ which is insoluble in water. It is soluble in excess of $KCN$ due to the formation of the complex :-

• A. $K_2[Cu(CN{)}_4]$
• B. $K_3[Cu(CN{)}_4]$
• C. $Cu(CN{)}_2$
• D. $Cu[KCu(CN{)}_4]$

Answer 1

Answer: (B)

$CuS{O}_4$ reacts with $KCN$ to give a white precipitate of cuprous cyanide and cyanogen gas.
The cuprous cyanide dissolves in excess of $KCN$ forming $K_3[Cu(CN{)}_4]$
$\underset{\text{Unstable }}{CuS{O}_4}+2KCN\to K_{2}S{O}_4+Cu(CN{)}_2$
$\underset{\text{Insoluble cyanogen }}{2Cu(CN{)}_2}\to 2CuCN+CN-CN$
$\underset{\text{Soluble }}{\mathrm{CuCN}}+3KCN\to K_3[Cu(CN{)}_4]$