Question

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

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

Answer 1

Answer: (B)

Copper sulphate reacts with excess potassium cyanide to form $\left[ Cu ( CN )_{4}\right]^{2-}$ ion.
$CuSO _{4}+4 KCN \rightarrow\left[ Cu ( CN )_{4}\right]^{2-}+4 K ^{+}+ SO _{4}^{2-}$
$\left[ Cu ( CN )_{4}\right]^{2-}$ is a complex ion.
Hence, it does not dissociates into $Cu ^{2+}$ ions and $CN ^{-}$ions.
In absence of $Cu ^{2+}$ ions, no precipitate of CuS is obtained.