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

Question

\(\mathrm{CuSO}_{4}\) decolourize on addition of 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

The product is \( \left[ \mathrm{Cu}(\mathrm{CN})_4 \right]^{3-} \), so option (b) is correct.

Step 1: Identify Reaction Components

Copper(II) sulfate, represented by \( \mathrm{CuSO}_4 \), is a blue-colored salt that decolourizes when potassium cyanide \( (\mathrm{KCN}) \) is added to it. Potassium cyanide acts as both a complexing agent and a reducing agent.

Step 2: Complex Formation

When \( \mathrm{KCN} \) is added to \( \mathrm{CuSO}_4 \), \( \mathrm{KCN} \) can form complexes with \( \mathrm{Cu}^{2+} \). A common complex that forms is \( \left[ \mathrm{Cu}(\mathrm{CN})_4 \right]^{3-} \).

Step 3: Reaction Mechanism and Charge Balance

In the reaction, \( \mathrm{KCN} \) not only complexes with copper but also reduces \( \mathrm{Cu}^{2+} \) ions to \( \mathrm{Cu}^+ \) ions. This reduction aids in forming the stable complex \( \left[ \mathrm{Cu}(\mathrm{CN})_4 \right]^{3-} \), explaining the charge on the complex.

Step 4: Select the Correct Product

Based on the formation of the complex and the reduction step, we determine that the product of the reaction when \( \mathrm{CuSO}_4 \) is decolourized by \( \mathrm{KCN} \) is \( \left[ \mathrm{Cu}(\mathrm{CN})_4 \right]^{3-} \). This is answer (b).

Key Concepts

Complex Formation

Complex formation is a fascinating aspect of coordination chemistry. It occurs when a central metal ion binds with molecules or ions, known as ligands, to create a structured entity called a complex. When potassium cyanide (KCN) is mixed with copper(II) sulfate (CuSO_4), the cyanide ions (CN^-) act as ligands. These negatively charged ions are strongly attracted to the positively charged metal ions like Cu^2+.

• The complex is formed through coordinate covalent bonds, where the ligand donates electron pairs to the central metal.
• In the given reaction, the stable complex \( [Cu(CN)_4]^{3-}\) is formed as four cyanide ions coordinate with one copper ion.

This process greatly stabilizes the copper in a new environment, altering the properties of the original salt.

Reduction Reactions

Reduction reactions are fundamental to changes in oxidation states among elements, especially metal ions. Here, when KCN is added to CuSO_4, the cyanide acts as a reducing agent. It effectively lowers the oxidation state of copper from Cu^2+ to Cu^+.

• Such reduction involves the gain of electrons by Cu^2+  ions, converting them into Cu^+  ions.
• This electron gain aids in the formation of the less common copper(I) complexes rather than the usual copper(II) forms.

The overall decolourization of the copper sulfate solution is due to this reduction, as the distinct blue color of Cu^2+  is diminished, leading to the stable complex formation.

Copper Cyanide Complexes

Copper cyanide complexes demonstrate interesting chemistry due to their unique properties and stability in solution. The \([Cu(CN)_4]^{3-}\) complex is a result of comprehensive interaction between copper ions and cyanide ligands.

• The complex has a tetrahedral structure, which allows for maximum stability due to bond angles and electron arrangement.
• These complexes are commonly found in industrial applications and metal processing due to their robust nature.

Cyanide complexes with copper can have various applications, including in electroplating and as precursors in chemical syntheses. Understanding this complex formation and its stability is crucial for practical and industrial chemists dealing with copper and its compounds.