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  4. CuSO4 when reacts with KCN forms CuCN which is insoluble in water. It is soluble in excess of KCN due to the formation of the complex :-

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Q. $CuSO_4 $ when reacts with $KCN$ forms $CuCN$ which is insoluble in water. It is soluble in excess of $KCN$ due to the formation of the complex :-

AIPMTAIPMT 2002Coordination Compounds

Solution:

$CuSO_4 $ reacts with $KCN$ to give a white precipitate of cuprous cyanide and cyanogen gas.
The cuprous cyanide dissolves in excess of $KCN$ forming $K_3[Cu(CN)_4] $
$\underset{\text{Unstable }}{CuSO_4}+2KCN \rightarrow K_2SO_4+Cu(CN)_2 $
$\underset{\text{Insoluble cyanogen }}{2Cu(CN)_2} \rightarrow 2CuCN+CN-CN$
$\underset{\text{Soluble }}{CuCN}+3KCN \rightarrow K_3[Cu(CN)_4]$