Question

Assertion: In a mixture of Cd(II) and Cu(II), $Cd^{2+}$ gets precipitated in presence of KCN by $H_2S$.
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)

$Cd ^{2+}+2 CN ^{-} \longrightarrow Cd ( CN )_{2} \downarrow$

$Cd ( CN )_{2} \downarrow+2 CN ^{-} \longrightarrow \left[ Cd ( CN )_{4}\right]^{2-}$

$\left[ Cd ( CN )_{4}\right]^{2-}$ is colourless compound and not too stable. When hydrogen sulphide gas is added, cadmium sulphide is precipitated.

$\left[ Cd ( CN )_{4}\right]^{2-}+ H _{2} S \longrightarrow CdS \downarrow+2 H ^{+}+4 CN ^{-}$

But in case of $Cu ^{2+}$

$Cu ^{2+}+2 CN ^{-} \longrightarrow Cu ( CN )_{2} \downarrow$

$2 Cu ( CN )_{2} \downarrow \xrightarrow[\text{decomposes}]{\text{Quickly}}2 CuCN \downarrow+( CN )_{2} \uparrow$

$CuCN \downarrow+3 CN ^{-} \longrightarrow \left[ Cu ( CN )_{4}\right]^{3-}$

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