Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{CuSO}_{4}\) 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) \(\mathrm{CuCN}\)

Step-by-Step Solution

Answer

(b) \( \mathrm{Cu}^{2+} \) gets reduced to form \( \left[\mathrm{Cu}( ext{CN})_4\right]^{3-} \).

Step 1: Understanding the Problem

The exercise involves understanding the reaction between copper sulfate (\( \mathrm{CuSO}_{4} \)) and potassium cyanide (\( \mathrm{KCN} \)). When \( \mathrm{CuSO}_{4} \) is added to \( \mathrm{KCN} \), a complex is formed causing the solution to decolourize. We are to determine the formula of the resultant copper complex.

Step 2: Chemical Reaction Overview

When \( \mathrm{CuSO}_{4} \) is added to \( \mathrm{KCN} \), the \( \mathrm{Cu}^{2+} \) ions initially form \( \mathrm{Cu(CN)}_2 \), which is an insoluble white precipitate.

Step 3: Further Reaction and Complex Formation

\( \mathrm{Cu(CN)}_2 \) is unstable and breaks down to form \( \mathrm{CuCN} \) and \( \mathrm{(CN)}_2 \). The \( \mathrm{CuCN} \) further reacts with excess \( \mathrm{CN}^{-} \) ions to form the soluble complex \( \left[\mathrm{Cu}( ext{CN})_4\right]^{3-} \).

Step 4: Decolourization Explanation

The decolourization occurs because the resultant complex \( \left[\mathrm{Cu(CN)}_4\right]^{3-} \) is a colorless species in solution, thus removing the characteristic blue color of \( \mathrm{CuSO}_{4} \).

Key Concepts

Copper Complexes

Copper complexes are fascinating compounds that involve a central copper ion surrounded by other molecules or ions, known as ligands. In the example given, the key copper complex involved is the reaction between copper sulfate and potassium cyanide.
The copper in copper sulfate (\( \mathrm{CuSO}_{4} \)) begins in the +2 oxidation state. As the reaction progresses, the copper ions form an intermediary compound, \( \mathrm{Cu(CN)}_2 \), before finally establishing \( \left[\mathrm{Cu}(\mathrm{CN})_4\right]^{3-} \). This complex ion involves a copper ion bonded to four cyanide ions through coordination bonds.
The coordination sphere around copper creates a stable environment that allows for interesting chemical behavior, such as the decolourization observed in the reaction. This transformation highlights how the nature of the ligands affects copper’s properties as it transforms to the colorless complex.

Coordination Compounds

Coordination compounds are chemical species featuring a central metal atom connected to surrounding ligands. Ligands are ions or molecules that donate electron pairs to the metal, forming coordinate covalent bonds.
In this reaction, the ligand involves the cyanide ions \(\mathrm{CN}^{-}\). Coordination compounds like \(\left[\mathrm{Cu}(\mathrm{CN})_4\right]^{3-}\) are established when these ligands create a complex with the copper ion. This complex formation changes the metal ion’s properties, allowing it to interact with light differently, leading to changes in color.
Coordination compounds have unique characteristics due to the geometry of the ligands surrounding the metal. This geometry impacts various properties, including color, reactivity, and solubility. The formation of coordination compounds is a crucial aspect of inorganic chemistry, demonstrating how chemical structures determine physical behaviors like color.

Color Changes in Reactions

Color changes in reactions are pivotal indicators of chemical changes. When \( \mathrm{CuSO}_{4} \) and \( \mathrm{KCN} \) interact, the noticeable shift from blue to colorless signifies the creation of a new and different copper complex.
Initially, copper sulfate in water appears blue due to the absorption of light in the visible range, which promotes electrons to higher energy states. When \( \left[\mathrm{Cu}(\mathrm{CN})_4\right]^{3-} \) forms, the absorption pattern shifts, and no light is absorbed in the visible range, rendering the solution colorless.
Such color changes not only give visual cues about the extent and nature of chemical reactions but also provide insights into molecular transformations at the atomic level. This teaches an essential lesson in observational chemistry, where color provides qualitative data on solution state and the identity of the complexes formed.