Question

$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 :-

• A. $K_2[Cu(CN)_4] $
• B. $K_3[Cu(CN)_4] $
• C. $Cu(CN)_2 $
• D. $Cu[KCu(CN)_4] $

Answer 1

The Correct Option is B

 To solve this problem, we need to understand the chemical reactions taking place when copper sulfate (\( \text{CuSO}_4 \)) reacts with potassium cyanide (KCN).

1. Initially, when \(\text{CuSO}_4\) reacts with KCN, copper(I) cyanide (\(\text{CuCN}\)) is formed which precipitates out as it is insoluble in water:

\(\text{CuSO}_4 + 2\text{KCN} \rightarrow \text{CuCN} + \text{K}_2\text{SO}_4\\)

1. \(\text{CuCN}\) is then soluble in excess KCN due to the formation of a complex cyanide:

In excess KCN, \(\text{CuCN}\) further reacts to form a complex ion. The reaction is:

\(\text{CuCN} + 3\text{KCN} \rightarrow \text{K}_3[\text{Cu}(\text{CN})_4]\\)

1. The complex formed is \(\text{K}_3[\text{Cu}(\text{CN})_4]\), which is soluble in water due to the strong complexation of copper with cyanide ions.

Therefore, among the given options, the correct complex that is formed is \(K_3[Cu(CN)_4]\).

1. Let's briefly analyze incorrect options:
   • \(K_2[Cu(CN)_4]\): This complex does not match the stoichiometry required for copper's coordination number.
   • \(Cu(CN)_2\): Incorrect, as it does not form a soluble complex with excess KCN.
   • \(Cu[KCu(CN)_4]\): Not a conventional or known complex formation with KCN.

In summary, the reaction proceeds to form a soluble complex \(\text{K}_3[\text{Cu}(\text{CN})_4]\) with excess KCN, making it the correct answer choice.