Question

When excess of aqueous KCN solutions is added to an aqueous solutions of copper sulphate, the complex $\lbrack Cu(CN)_{4}\rbrack^{2 -}$is formed. On passing $H_{2}S$gas through this solutions no precipitate of CuS is formed because

• A. Sulphide ions cannot replace $CN^{-}$ions
• B. $\lbrack Cu(CN)_{4}\rbrack^{2 -}$does not give $Cu^{2 +}$ion in the solutions
• C. Sulphide ions from $H_{2}S$do not form complexes
• D. Sulphide ions cannot replace sulphate ions from copper sulphate solutions.

Answer 1

Answer: (B)

$\lbrack Cu(CN)_{4}\rbrack^{2 -}$does not give $Cu^{2 +}$ion in the solutions

Answer 2

When excess of aqueous KCN solutions is added to aqueous $CuSO_{4}$Solutions, highly stable $\lbrack Cu(CN)_{4}\rbrack^{2 -}$is formed which does not give $Cu^{2 +}$ion in the solution, hence copper sulphide is not formed.

$\lbrack Cu(H_{2}O)_{4}\rbrack^{2 +} + 4CN^{-} \rightarrow \lbrack Cu(CN)_{4}\rbrack^{2 -} + 4H_{2}O$