Question

In the separation of ${Cu}^{2+}$ and ${Cd}^{2+}$ in 2nd group of qualitative analysis of cations tetrammine copper(II) sulphate and tetraammine cadmium(II) sulphate react with KCN to form the corresponding cyanide complexes, which one of the following pairs of the complexes and their relative stablities enables the separation of ${Cu}^{2+}$ and ${Cd}^{2+}$?

• 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
$\left[Cu\right(N{H}_3){]}_{4}S{O}_4+10KCN+8H_{2}O\to 2K_3[Cu\left(CN{)}_4\right]+2K_{2}S{O}_4+8N{H}_{4}OH+(CN{)}_2$
$\left[Cd\right(N{H}_3){]}_{4}S{O}_4+4KCN+4H_{2}O\to K_2[Cd\left(CN{)}_4\right]+K_{2}S{O}_4+4N{H}_{4}OH$
$K_3\left[{Cu\left(CN\right)}_4\right]$ more stable and $K_2\left[{Cd\left(CN\right)}_4\right]$ is less stable. This is because the second ionization of potassium cadmicyanide is higher than that of potassium cuprocyanide .