$\mathrm{CuSO}_{4}$ decolourises on addition of KCN, the product is

(A) $[\mathrm{Cu}(\mathrm{CN})_{4}]^{2-}$

(B) $\mathrm{Cu}^{2+}$ get reduced to form $[\mathrm{Cu}(\mathrm{CN})_{4}]^{3-}$

(C) $\mathrm{Cu}(\mathrm{CN})_{2}$

JEE Advanced · 2006 · 09 Apr 2006 · d and f Block Elements

CuSO_{4}

decolourises on addition of KCN, the product is

Answer: (D)

When

CuSO_{4}

reacts with KCN,

Cu^{2 +}

is first reduced to

Cu^{+}

by cyanide ions. The reaction proceeds:

Cu^{2 +} + 2 CN^{-} \to Cu (CN)_{2}

, followed by decomposition:

2 Cu (CN)_{2} \to 2 CuCN + (CN)_{2}

. The final product is CuCN, which is white/colorless, explaining the decolorization.

JEE Advanced · 2006 · 09 Apr 2006 · Single correct option