Question

Assertion: In a mixture of Cd(II) and Cu(II), $C{d}^{2+}$ gets precipitated in presence of KCN by $H_{2}S$.
Reason: The stability constant of $[Cu(CN{)}_4{]}^{3-}$ is greater than $[Cd(CN{)}_4{]}^{2-}$.

• A. If both assertion and reason are true and reason is the correct explanation of assertion.
• B. If both assertion and reason are true but reason is not the correct explanation of assertion.
• C. If assertion is true but reason is false.
• D. If both assertion and reason are false.

Answer 1

Answer: (A)

$C{d}^{2+}+2C{N}^{-}⟶Cd(CN{)}_2\downarrow$

$Cd(CN{)}_2\downarrow +2C{N}^{-}⟶{\left[Cd(CN{)}_4\right]}^{2-}$

${\left[Cd(CN{)}_4\right]}^{2-}$ is colourless compound and not too stable. When hydrogen sulphide gas is added, cadmium sulphide is precipitated.

${\left[Cd(CN{)}_4\right]}^{2-}+H_{2}S⟶CdS\downarrow +2H^{+}+4C{N}^{-}$

But in case of $C{u}^{2+}$

$C{u}^{2+}+2C{N}^{-}⟶Cu(CN{)}_2\downarrow$

$2Cu(CN{)}_2\downarrow \underset{\text{decomposes}}{\overset{\text{Quickly}}{\to }}2CuCN\downarrow +(CN{)}_2\uparrow$

$CuCN\downarrow +3C{N}^{-}⟶{\left[Cu(CN{)}_4\right]}^{3-}$

This complex is so stable (i.e., $\left[C{u}^{+}\right]$ is too low) that $H_{2}S$ cannot precipitate $Cu(I)$ sulphide $\left(C{u}_{2}S\right)$