Assertion : In a mixture of $\mathrm{Cd}(\mathrm{II})$ and $\mathrm{Cu}(\mathrm{II})$, $\mathrm{Cd}^{2+}$…
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 $\left[\mathrm{Cd}(\mathrm{CN})_4\right]^{2-}$.
If both assertion and reason are true and reason is the correct explanation of assertion.
If both assertion and reason are true but reason is not the correct explanation of assertion.
If assertion is true but reason is false.
If 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 $\left(\mathrm{Cu}_2 \mathrm{~S}\right)$