Q49 / 180 · AIIMS 2013 · Chemistry
Assertion : In a mixture of $\mathrm{Cd}(\mathrm{II})$ and $\mathrm{Cu}(\mathrm{II})$, $\mathrm{Cd}^{2+}$ gets precipitated in presence of $\mathrm{KCN}$ by $\mathrm{H}_2 \mathrm{~S}$.
Reason : The stability constant of $\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{3-}$ is greater than $\left[\mathrm{Cd}(\mathrm{CN})_4\right]^{2-}$.
- Correct answerAIf both assertion and reason are true and reason is the correct explanation of assertion.
- BIf both assertion and reason are true but reason is not the correct explanation of assertion.
- CIf assertion is true but reason is false.
- DIf both assertion and reason are false.
Solution
$\mathrm{Cd}^{2+}+2 \mathrm{CN}^{-} \longrightarrow \mathrm{Cd}(\mathrm{CN})_2 \downarrow$
$\mathrm{Cd}(\mathrm{CN})_2 \downarrow+2 \mathrm{CN}^{-} \longrightarrow\left[\mathrm{Cd}(\mathrm{CN})_4\right]^{2-}$
$\left[\mathrm{Cd}(\mathrm{CN})_4\right]^{2-}$ is colourless compound and not too stable. When hydrogen sulphide gas is added, cadmium sulphide is precipitated.
$\left[\mathrm{Cd}(\mathrm{CN})_4\right]^{2-}+\mathrm{H}_2 \mathrm{~S} \longrightarrow \mathrm{CdS} \downarrow+2 \mathrm{H}^{+}+4 \mathrm{CN}^{-}$
But in case of $\mathrm{Cu}^{2+}$,
$\mathrm{Cu}^{2+}+2 \mathrm{CN}^{-} \longrightarrow \mathrm{Cu}(\mathrm{CN})_2 \downarrow$
$2 \mathrm{Cu}(\mathrm{CN})_2 \downarrow \frac{\text { Quickly }}{\text { decomposes }} 2 \mathrm{CuCN} \downarrow+(\mathrm{CN})_2 \uparrow$
$\mathrm{CuCN} \downarrow+3 \mathrm{CN}^{-} \longrightarrow\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{3-}$
This complex is so stable (i.e., $\left[\mathrm{Cu}^{+}\right]$is too low) that $\mathrm{H}_2 \mathrm{~S}$ cannot precipitate $\mathrm{Cu}(\mathrm{I})$ sulphide $\left(\mathrm{Cu}_2 \mathrm{~S}\right)$
Hence, the correct answer is option: 'A'.