Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The product is CuCN.

Step 1: Understanding the Reaction

When \(Cu^{2+}\) from \(CuSO_4\) reacts with \(KCN\), a complex reaction occurs that leads to decolourization. This is because \(Cu^{2+}\) can form complexes with cyanide ions.

Step 2: Determining Reduction and Complex Formation

In the presence of excess \(CN^-\), the \(Cu^{2+}\) ion from \(CuSO_4\) is reduced to \(Cu^+\). The reduction changes the oxidation state of copper from +2 to +1, and further forms a complex with cyanide ions.

Step 3: Identifying the Complex Ion Formed

The reduced copper ion \(Cu^+\) forms a stable complex with four cyanide ions, resulting in the complex ion \([Cu(CN)_4]^{3-}\). However, taking into account stoichiometry and charge calculations, the most plausible immediate result due to decolourization is \(CuCN\).

Step 4: Selecting the Correct Product

Based on chemical reactions and previous steps, the addition of \(KCN\) to \(CuSO_4\) in excess will produce \(CuCN\). Thus, among the given options, \(CuCN\) is the initial product responsible for the decolourization.

Key Concepts

Copper-Cyanide Complex

Copper compounds are known for their vibrant colors. However, when you mix copper sulfate (\(\mathrm{CuSO}_4\)) with potassium cyanide (\(\mathrm{KCN}\)), a fascinating reaction takes place that decolorizes the solution. This happens because of the formation of a copper-cyanide complex.
When copper sulfate interacts with potassium cyanide, the copper ions (\(\mathrm{Cu}^{2+}\)) are initially converted into copper(I) cyanide (\(\mathrm{CuCN}\)), which is less colorful.
Copper(I) cyanide is known to form a white precipitate, making the overall solution appear less colorful or colorless.

• The transformation occurs when copper ions try to find a stable form by bonding with cyanide ions. This leads to the formation of the less colorful \(\mathrm{CuCN}\).
• In excess cyanide conditions, further complex ions can form, such as \([\mathrm{Cu}(\mathrm{CN})_4]^{3-}\), but the primary step in the decolorization is the formation of \(\mathrm{CuCN}\).

This interesting reaction is pivotal in understanding how copper changes its characteristics based on the complex it forms.

Redox Reaction

A redox reaction is central to the process that occurs when \(\mathrm{CuSO}_4\) is added to \(\mathrm{KCN}\). In a redox reaction, one element undergoes oxidation while another undergoes reduction. In our scenario, copper plays the role of the element that is reduced.
Initially, copper is in a +2 oxidation state in \(\mathrm{CuSO}_4\).

• The addition of cyanide ions (\(\mathrm{CN}^- \)) leads to the reduction of copper's oxidation state from +2 to +1.
• This is due to the transfer of electrons from cyanide to copper, enabling the conversion from \(\mathrm{Cu}^{2+}\) to \(\mathrm{Cu}^{+}\), resulting in the formation of \(\mathrm{CuCN}\).

Understanding redox reactions helps highlight the changes in electron configurations that metals like copper undergo during chemical reactions.
Identifying such changes is crucial for predicting the final products of reactions similar to this one.

Complex Ion Formation

Complex ions are formed when a central metal ion bonds with surrounding non-metal ions or molecules, known as ligands. Cyanide ions are strong ligands that can interact with metals like copper to form complex ions.
When \(\mathrm{Cu}^{2+}\) is reduced to \(\mathrm{Cu}^{+}\), it can attract more cyanide ions to form a complex structure.

• Despite \(\mathrm{CuCN}\) being the initial product, in the presence of excess cyanide ions, \([\mathrm{Cu}(\mathrm{CN})_4]^{3-}\) may form as a complex ion.
• This transformation is characterized by the metal ion being surrounded by multiple cyanide ions, resulting in a more stabilized configuration.

The formation of copper-cyanide complexes is not only essential in chemistry but also crucial in industries such as mining, where these compounds play a role in ore processing techniques.
By understanding complex ion formation, students can grasp more advanced concepts in coordination chemistry and the reactions of transition metals.