Question

What is the coordination entity formed when excess of aqueous KCN is added to an aqueous solution of copper sulphate? Why is it that no precipitate of copper sulphide is obtained when $H_{2}S\left(g\right)$is passed through this solution?

Answer 1

When excess of $KCN\left(aq\right)$ is mixed with $CuS{O}_4\left(aq\right),$ a complex named potassiumtetracyanocuprate (II) is formed. Since, $C{N}^{-}$ ions are strong ligands, the complex is quite stable. It is evident from the stability constant value $(K=2.0\times {10}^{27}).$

$\begin{array}{c}\underset{\left(Excess\right)}{4KCN\left(aq\right)}+CuS{O}_4\left(aq\right)\to \underset{\left(Soluble\right)}{K_2\left[Cu\right(CN{)}_4\left]\right(aq)}+K_{2}S{O}_4\left(aq\right)\\ {\downarrow }^{+H_{2}S}\\ Nocleavageandno\\ ProductionofC{u}^{2+}ions\end{array}$