Question

In the separation of $C{u}^{2+}$ and $C{d}^{2+}$ in 2nd group of qualitative analysis of eations, tetrammine copper(ll) sulphate and tetraammine cadmium(ll) sulphate react with $KCN$ to form the corresponding cyanide complexes; which one of the following pairs of the complexes and their relative stabilities enables the separation of $C{u}^{2+}$ and $C{d}^{2+}$?

• A. $K_3\left[Cu{\left(CN\right)}_4\right]$: less stable and $K_2\left[Cd{\left(CN\right)}_4\right]$: more stable
• B. $K_3\left[Cu{\left(CN\right)}_4\right]$: more stable and $K_2\left[Cd{\left(CN\right)}_4\right]$: less stable
• C. $K_2\left[Cu{\left(CN\right)}_4\right]$: less stable and $K_2\left[Cd{\left(CN\right)}_4\right]$: more stable
• D. $K_2\left[Cu{\left(CN\right)}_4\right]$: more stable and $K_2\left[Cd{\left(CN\right)}_4\right]$: less stable

Answer 1

Answer: (B)

$K_3\left[Cu{\left(CN\right)}_4\right]$: more stable and $K_2\left[Cd{\left(CN\right)}_4\right]$: less stable

$K_3\left[Cu{\left(CN\right)}_4\right]$ is more stable and $K_2\left[Cd{\left(CN\right)}_4\right]$ is less stable. Second ionisation of pot. cadmicyanide is higher than that of pot. cuprocyanide. Hence, solution contains sufficient $C{d}^{2+}$ ions and not $C{u}^{2+}$ ions. When $H_{2}S$ gas is passed, only CdS is precipitated and not CuS.