Question

Copper sulphate dissolves in excess of KCN to give

• A. $CuCN$
• B. $[Cu(CN)_4]^{3-}$
• C. $[Cu(CN)_4]^{2-}$
• D. $Cu(CN)_2 $

Answer 1

The Correct Option is B

The question asks about the product formed when copper sulfate (\( \text{CuSO}_4 \)) dissolves in excess of potassium cyanide (\( \text{KCN} \)). Let's understand the chemical reaction involved here to determine the correct product among the given options:

1. **Dissolution of Copper Sulphate in Potassium Cyanide:** When copper sulfate reacts with potassium cyanide, the \( \text{KCN} \) provides cyanide ions (\( \text{CN}^- \)).

2. **Formation of Initial Complex:** Initially, copper cyanide (\( \text{CuCN} \)) is formed:

\[\text{Cu}^{2+} + 2\text{CN}^- \rightarrow \text{CuCN} + \text{CN}^-\]

3. **Further Reaction in Excess Cyanide:** The formed \( \text{CuCN} \) further reacts with additional cyanide ions present in the excess of \( \text{KCN} \). Copper forms a stable complex ion, leading to the formation of tetracyanocuprate(II) complex:

\[\text{CuCN} + 3\text{CN}^- \rightarrow [\text{Cu(CN)}_4]^{3-}\]

4. **Correct Interpretation:** The complex \([\text{Cu(CN)}_4]^{3-}\) is a stable complex and is the final product when copper sulfate dissolves in excess potassium cyanide.

The correct answer is the complex ion [Cu(CN)_4]^{3-}. It is the only stable complex that forms in the presence of excess cyanide ions. Let's justify why other options are incorrect:

• CuCN and Cu(CN)_2 are intermediate and not stable in excess cyanide.
• [Cu(CN)_4]^{2-} does not form because, in the environment of excess cyanide, the cuprous ion (\( \text{Cu}^+ \)) combines with four cyanide ions to form [Cu(CN)_4]^{3-}.

Thus, the correct answer is [Cu(CN)_4]^{3-}.