Question

v\(\mathrm{CuSO}_{4}\) decolourize on addition of \(\mathrm{KCN}\), the product is (a) \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{2-}\) (b) \(\mathrm{Cu}^{2+}\) gets reduced to form \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\) (c) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (d) \(\mathrm{CuCN}\)

Short answer

The product is \\\([ \mathrm{Cu}(\mathrm{CN})_{4} ]^{3-}\\\) due to reduction by cyanide.

Step-by-step solution

Understand the Reaction

When \(\mathrm{CuSO}_{4}\) is added to \( ext{KCN}\), a reaction occurs where copper ions, \( ext{Cu}^{2+}\), react with cyanide ions, \( ext{CN}^{-} \), to form a product. Cyanide is a very strong ligand and has the ability to form complex ions with metal ions.

Reaction Analysis

In the reaction between \( ext{Cu}^{2+}\) and \( ext{CN}^{-} \), initially \( ext{Cu}^{2+} \) can form a white precipitate of \( ext{CuCN}\), which can dissolve in excess \( ext{CN}^{-} \) to form a complex ion. There is also a possibility of reduction of \( ext{Cu}^{2+} \) due to cyanide's ability to act as a reducing agent.

Forming a Complex Ion

With an excess of cyanide ions \( ( ext{CN}^{-}) \), the \( ext{CuCN} \) dissolves and forms a complex ion \([ ext{Cu}( ext{CN})_4 ]^{3-}\), due to the reduction of \( ext{Cu}^{2+} \) to \( ext{Cu}^{+} \) in the presence of cyanide.

Identify the Product

Given that reduction occurs and a complex ion is formed, the color change, or decolorization, implies that the blue color from \( ext{CuSO}_4 \) is lost. Thus the resulting product is \([ ext{Cu}( ext{CN})_4 ]^{3-}\).

Key concepts

Coordination Chemistry

Coordination chemistry delves into the fascinating field of complex formation between metal ions and ligands. A ligand is a molecule or ion that binds to a central metal atom to form a coordination complex. Copper, a transition metal, exhibits diverse coordination behavior due to its d-orbitals, which allow it to form stable complexes with various ligands.
Transition metals like copper can coordinate with multiple ligands, forming structures known as coordination compounds or complexes. Cyanide ions, noted for their ability to form strong covalent bonds with metal ions, serve as exemplary ligands. In coordination chemistry, these cyanide ions bind to copper ions to form complex structures such as \([\mathrm{Cu}(\mathrm{CN})_4 ]^{3-}\).
Such complexes are crucial because they influence the chemical properties and stability of the metal ions involved. This interaction can lead to intriguing effects, such as changes in color, as seen when the blue color of \(\text{CuSO}_4\) disappears upon forming the copper-cyanide complex.

Ligand Exchange Reactions

Ligand exchange reactions occur when one ligand in a metal complex is replaced by another ligand. This dynamic process is significant in coordination chemistry and can lead to changes in the chemical properties of the complex.
In the case of copper and cyanide, the initially formed \(\text{CuCN}\) precipitate can dissolve when more \(\text{CN}^{-}\) ions are present. This is because the cyanide ions replace water molecules or other ligands that may have surrounded the copper ions initially. The phenomenon leads to the formation of a distinct complex, \([\text{Cu}(\text{CN})_4]^{3-}\).
Ligand exchange is not just a matter of swapping one ligand for another. It often involves changes in oxidation states, formation of new bonds, and structural reconfigurations. These reactions are crucial for numerous biological processes and industrial applications, providing versatile pathways for chemical transformations.

Reduction of Copper Ions

The reduction of copper ions involves the gain of electrons, transforming copper from a higher oxidation state to a lower one. This process is intimately connected to the formation of copper-cyanide complexes.
In the copper sulfate (\(\text{CuSO}_4\)) and cyanide (\(\text{KCN}\)) reaction, the initial blue copper(II) ions (\(\text{Cu}^{2+}\)) undergo reduction to copper(I) (\(\text{Cu}^{+}\)) ions. Cyanide acts as a reducing agent, donating electrons to the copper ions. As a result, the \([\text{Cu}(\text{CN})_4]^{3-}\) complex is formed.
This reduction not only shifts the oxidation state but also affects the structure and electronic configuration of the copper ion. The electrons provided by cyanide enable the copper ion to bond more effectively with the cyanide ligands, resulting in the decolorization observed during the reaction as the blue color dissipates.