Problem 77 - 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 \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\) (c) \(\mathrm{Cu}(\mathrm{CN})_{2}\) (d) \(\mathrm{CuCN}\)

Step-by-Step Solution

Answer

The correct product is (b) \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\).

Step 1: Understand the Reaction

When copper(II) sulfate \((\mathrm{CuSO}_{4})\) is mixed with potassium cyanide \(\mathrm{KCN}\), a reaction occurs where the copper ions \(\mathrm{Cu}^{2+}\) can get complexed or reduced by cyanide ions \(\mathrm{CN}^-\). The key to this problem is understanding that cyanide is a strong reducing agent and can complex with metal ions.

Step 2: Copper Cyanide Complexes

Cyanide can form complexes with copper ions. Due to the presence of \(\mathrm{CN}^{-}\), it can reduce \(\mathrm{Cu}^{2+}\) to different oxidation states and form complexes such as \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\) in case of further reduction.

Step 3: Reduction Potential of \(\mathrm{Cu}^{2+}\)

Given cyanide's ability to act as a reducing agent, \(\mathrm{Cu}^{2+}\) is reduced by \(\mathrm{CN}^{-}\) ions. After the addition of \(\mathrm{KCN}\), \(\mathrm{Cu}^{2+}\) can potentially undergo complexation and reduction forming \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\).

Step 4: Decolorization Explained

The solution potentially becomes colorless because the copper complex formed is generally a colorless species. The transformation indicates complex formation, particularly indicating the presence of \(\mathrm{Cu}\) ions in a lower oxidation state post reduction, leading us to \(\left[\mathrm{Cu}(\mathrm{CN})_{4}\right]^{3-}\).

Key Concepts

Copper Cyanide Complexes

Copper cyanide complexes are interesting and important in chemistry, especially when discussing reactions with cyanide. When copper sulfate (\( \mathrm{CuSO}_{4} \)) mixes with potassium cyanide (\( \mathrm{KCN} \)), we see how copper, normally existing as (\( \mathrm{Cu}^{2+} \)) ions, can form complexes with cyanide ions (\( \mathrm{CN}^- \)).
This occurs because cyanide ions are good at binding tightly to metal ions like copper, creating stable, water-soluble complexes. This process can lead to the formation of various copper-cyanide complexes, such as (\( \left[ \mathrm{Cu}(\mathrm{CN})_{4} \right]^{3-} \)).
The ability of cyanide to form these complex structures depends on the coordination capability of the cyanide ions, where they surround the copper ion, stabilizing it in a lower oxidation state and affecting its chemical properties, such as color.

Reduction Potential

Reduction potential is a crucial concept when studying complex formation involving transition metals like copper. It aids in understanding how cyanide acts as a reducing agent.
Cyanide ions (\( \mathrm{CN}^- \)) have the capability to alter the oxidation state of copper ions, reducing (\( \mathrm{Cu}^{2+} \)) to copper in a lower oxidation state, such as (\( \left[ \mathrm{Cu}(\mathrm{CN})_{4} \right]^{3-} \)).
This reduction occurs because cyanide ions possess a significant ability to donate electrons. This process lowers the oxidation state of copper, enabling the formation of different copper-cyanide complexes. Understanding this reduction potential helps explain why reactions involving cyanide often result in new compounds and color changes.

Chemical Decolorization

Chemical decolorization is an observable phenomenon that can occur during complexation reactions. In the case of mixing (\( \mathrm{CuSO}_{4} \)) with (\( \mathrm{KCN} \)), the resulting solution can become colorless. This is because the copper ion, which normally imparts a blue color to copper sulfate solutions, changes its state upon forming a complex with cyanide.
The color change to clear or colorless indicates that copper is no longer in the form (\( \mathrm{Cu}^{2+} \)) but rather in a reduced and complexed form, such as (\( \left[ \mathrm{Cu}(\mathrm{CN})_{4} \right]^{3-} \)).
Understanding decolorization is important, as it illustrates a visible confirmation of chemical reaction progression, suggesting the complexation or reduction of the metal ion in its new state.