Copper sulphate solution reacts with `KCN` to give
(a) `Cu(CN)_(2)`
(b) `CuCN`
(c ) `K_(2)[Cu(CN)_(4)]`
(d) `K_(3)[Cu(CN)_(4)]` .

Options

A. `Cu (CN)_(2)`
B. CuCN
C. `[Cu (CN)_(4)]^(3-)`
D. `[Cu (CN)_(4)]^(2-)`

Answer

C

Explanation

First cupric cyanide is formed which decomposes to give cuprous cyanide and cyanogen gas.
`[CuSO_(4) + 2KCN rarr Cu (CN)_(2) + K_(2)SO_(4)]xx2`
`2Cu(CN)_(2) rarr Cu_(2) (CN)_(2) + (CN)_(2)`
Cuprous cyanide dissolves in excess of potassium cyanide to form a complex, potassium cuprocyanide, `[K_(3)CU(CN)_(4)]` :
`Cu_(2)(CN)_(2) + 6KCN rarr 2K_(3) [Cu (CN)_(4)]`
Thus, the net reaction is
`2CuSO_(4) + 10KCN rarr 2K_(3) [Cu (CN)_(4)] + 2K_(2)SO_(4) + (CN)_(2)` . Thus, `CuSO_(4)` when reacts with KCN forms CuCN, which is insoluble in water. It is soluble in excess KCN due to formation of `K_(3) [Cu (CN)_(4)]` .