Question

Assertion :${Cu}^{2+}$ and ${Cd}^{2+}$ solution is complexed by addition of $KCNS$. Reason: On passing $H_{2}S$ gas stable complex $[{Cu\left(CN\right)}_4{]}^{3-}$ is not affected.

Read the above assertion and reason and choose the corret option regarding it.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Both Assertion and Reason are incorrect

Answer 1

The correct option is D Both Assertion and Reason are incorrect

${Cu}^{2+}$ and ${Cd}^{2+}$ solution is complexed by addition of $KCNS$.
On passing $H_{2}S$ gas stable complex $[{Cu\left(CN\right)}_4{]}^{3-}$ is not affected.
$KCN$ is used in the separation of ${Cu}^{2+}$ and ${Cd}^{2+}$
$\left[Cu\right(N{H}_3){]}_{4}S{O}_4+10KCN+8H_{2}O\to 2K_3[Cu\left(CN{)}_4\right]+2K_{2}S{O}_4+8N{H}_{4}OH+(CN{)}_2$
$\left[Cd\right(N{H}_3){]}_{4}S{O}_4+4KCN+4H_{2}O\to K_2[Cd\left(CN{)}_4\right]+K_{2}S{O}_4+4N{H}_{4}OH$
In presence of KCN , only Cd is precipited as sulphide on passing H₂S.
This is because the second ionization of potassium cadmicyanide is higher than that of potassium cuprocyanide (or similar complexes of Co and Ni)so that suffiient Cd (II) ions are present in solution. Hence, when hydrogen sulphide is passed, only CdS is precipitated.
Both Assertion and Reason are incorrect