Question

The species that undergoes disproportionation in an alkaline medium are 1\. \(\mathrm{Cl}\), 2\. \(\mathrm{MnO}_{4}^{2-}\) 3\. \(\mathrm{NO}_{2}\) 4\. \(\mathrm{ClO}_{4}^{1}\) (a) 1,2 (b) 1,3 (c) \(1,3,4\) (d) \(1,2,3,4\)

Short answer

The correct species are 1, 3, so the answer is (b).

Step-by-step solution

Understanding Disproportionation

Disproportionation is a redox reaction where a single substance is both oxidized and reduced, forming two different products. In the context of the given options, we need to find which species can undergo such reactions in an alkaline medium.

Analyzing Option 1: \( \mathrm{Cl} \)

In alkaline medium, \( \mathrm{Cl_2} \) can disproportionate into \( \mathrm{Cl^-} \) and \( \mathrm{ClO^-} \). Hence, \( \mathrm{Cl} \) is capable of undergoing disproportionation.

Analyzing Option 2: \( \mathrm{MnO}_{4}^{2-} \)

\( \mathrm{MnO_4^{2-}} \) can undergo disproportionation in an alkaline medium to form \( \mathrm{MnO_2} \) and \( \mathrm{MnO_4^-} \). Therefore, this option also undergoes disproportionation.

Analyzing Option 3: \( \mathrm{NO}_{2} \)

In an alkaline medium, \( \mathrm{NO_2} \) disproportionates to form \( \mathrm{NO_3^-} \) and \( \mathrm{NO^-} \). Thus, \( \mathrm{NO_2} \) undergoes disproportionation.

Analyzing Option 4: \( \mathrm{ClO}_4^{1-} \)

\( \mathrm{ClO_4^{-}} \) is a stable ion that typically does not undergo disproportionation due to the high oxidation state and the stability of the perchlorate ion in alkaline medium. So, it does not undergo disproportionation.

Determine the Correct Options

From the analysis, the species that undergo disproportionation in alkaline medium are \( \mathrm{Cl} \), \( \mathrm{MnO_4^{2-}} \), and \( \mathrm{NO_2} \). Therefore, the correct options are 1, 2, and 3, corresponding to choice (b).

Key concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, involve the transfer of electrons between two species. These reactions are fundamental in chemistry because they help to illustrate the movement of electrons from one reactant to another. When we talk about redox reactions, two main processes take place:

• Oxidation: This is when a molecule, atom, or ion loses electrons, which results in an increase in its oxidation state.
• Reduction: This occurs when there is a gain of electrons, leading to a decrease in the oxidation state of the molecule, atom, or ion.

A disproportionation reaction is a special type of redox reaction where a single element in a compound is simultaneously oxidized and reduced. This means that the element undergoes both processes at once. It's like one part of the molecule is giving up electrons while another part is taking them in. In the exercise context, for example, \( \mathrm{Cl_2} \) disproportionates to \( \mathrm{Cl^-} \) and \( \mathrm{ClO^-} \). The \( \mathrm{Cl_2} \) undergoes both oxidation and reduction, clearly demonstrating its nature as a redox reaction.

Alkaline Medium

An alkaline medium refers to an environment where the pH is greater than 7, typically involving the presence of hydroxide ions \( (\mathrm{OH^-}) \). This environment can significantly affect the course of chemical reactions, including disproportionation reactions like those discussed in this exercise. In a basic or alkaline medium, certain ions can be stabilized, which in turn promotes or discourages specific reactions, including redox reactions.
For instance, in the case of \( \mathrm{Cl_2} \), the basic environment facilitates its conversion into \( \mathrm{Cl^-} \) and \( \mathrm{ClO^-} \). Here’s why the alkaline conditions are so important:

• Stabilization of intermediates: The presence of hydroxide ions can stabilize the intermediates formed during the reactions.
• Shifts in equilibrium: Alkaline conditions can shift equilibria towards different products than would be favored in an acidic or neutral environment.
• Influencing potential reactions: Some species may only disproportionate in specific conditions, such as an alkaline medium, making the environment essential for these reactions to occur.

In summary, understanding the role of an alkaline medium is vital for predicting and controlling the outcomes of reactions like disproportionation.

Oxidation State Analysis

Oxidation state analysis is a critical tool in understanding and predicting chemical reactions, especially redox reactions. The oxidation state (or oxidation number) of an atom in a compound provides a framework to determine how electrons are distributed among atoms.
Let’s break down the oxidation state analysis:

• Determining oxidation states: The oxidation state is calculated based on a series of rules, such as assigning oxygen an oxidation state of -2 in most compounds.
• Tracking electron transfers: By analyzing changes in oxidation states, we can track where and how many electrons are transferred in redox reactions.
• Identifying disproportionation: In disproportionation reactions, one atom shows both an increase and decrease in oxidation state. For example, in the reaction involving \( \mathrm{NO_2} \), it can react to form both \( \mathrm{NO_3^-} \) (where nitrogen is oxidized) and \( \mathrm{NO^-} \) (where nitrogen is reduced).

This analysis allows us to identify which atoms are oxidized and reduced, providing insight into the pathways and feasibility of chemical reactions, especially in the context of redox and disproportionation reactions.