Question

A coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate? when $H_{2}S$(g) is passed through this solution?

• A. ppt formed
• B. no ppt formed
• C. ppt disappears for a while
• D. none

Answer 1

Answer: (B)

no ppt formed

When excess of aqueous KCN is added to an aqueous solution of copper sulphate, the coordination entity $\left[Cu\right(CN\left)4\right]3^{-}$ is formed. which on reaction with $H_{2}S$ does not give any ppt.
The reaction taking place is as follows:

$CuS{O}_4+KCN\to Cu(CN{)}_2$
$2Cu(CN{)}_2\to 2CuCN+NC-CN$ (Cyanogen)
$CuCN+KCN\to K_3\left[Cu\right(CN{)}_4]$
$K_3\left[Cu\right(CN{)}_4]\to 3K^{\oplus }+[Cu(CN{)}_4{]}^{-3}\stackrel{H_{2}S}{\to }$ no ppt