Problem 76 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Reduction of the metal centre in aqueous permanganate ion involves (a) 3 electrons in neutral medium (b) 5 electrons in neutral medium (c) 3 electrons in alkaline medium (d) 5 electrons in acidic medium

Step-by-Step Solution

Answer

(d) 5 electrons in acidic medium

Step 1: Identify the Reduction Reaction

We need to identify the reduction reaction of the permanganate ion \( \text{MnO}_4^- \) in different media (neutral, alkaline, acidic). Permanganate ion is known to change into different oxidation states depending on the medium. The problem asks about electron involvement, suggesting changes in oxidation states, specifically in an acidic medium.

Step 2: Define the Reaction in Acidic Medium

In an acidic medium, permanganate ion \( \text{MnO}_4^- \) is reduced to \( \text{Mn}^{2+} \). The chemical equation is: \[ \text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \]This equation shows that 5 electrons are needed for the reduction process in the acidic medium.

Step 3: Verify the Correct Answer

The correct reduction process involves 5 electrons in the acidic medium because the oxidation state of Mn changes from +7 to +2. This reduction correctly matches the condition given in option (d).

Key Concepts

Permanganate Ion

The **permanganate ion** is a purple, inorganic compound denoted as \( \text{MnO}_4^- \). It's well-known for its strong oxidizing properties, which means it has a high tendency to gain electrons during a chemical reaction. The manganese atom in \( \text{MnO}_4^- \) is in a high oxidation state, making it very reactive. When the permanganate ion undergoes a reaction, it typically involves either gaining electrons (reduction) or losing them. As a result, it changes oxidation states, which is central to its use in many redox reactions. Understanding the behavior of permanganate ions in different conditions (neutral, acidic, or alkaline) is crucial. It determines how many electrons are involved in the redox process, and what the final products of the reaction will be. This understanding is critical for predicting reaction outcomes and is used extensively in chemical synthesis and analytical chemistry.

Oxidation States

**Oxidation states** are a way to keep track of electrons in chemical reactions, especially in redox (reduction-oxidation) reactions. The oxidation state of an element refers to the charge it would have if all bonds were ionic. For a permanganate ion \( \text{MnO}_4^- \), manganese is in the +7 oxidation state. This is because each oxygen atom is considered to have an oxidation state of -2, and there are four oxygen atoms. In a redox reaction, the oxidation state of an element changes due to the transfer of electrons. In the example from the problem, manganese in \( \text{MnO}_4^- \) is reduced (gains electrons) and its oxidation state decreases from +7 to +2 when it forms \( \text{Mn}^{2+} \) in an acidic medium. This reduction process involves gaining 5 electrons:- Start: Mn in \( \text{MnO}_4^- \) with oxidation state +7- End: Mn in \( \text{Mn}^{2+} \) with oxidation state +2Such changes help in determining how and where electrons are transferred, which is critical in balancing chemical equations.

Acidic Medium

The **acidic medium** refers to the presence of acids during a chemical reaction, which significantly affects the behavior of reactants and thus the course of the reaction. In an acidic medium, there are plenty of \( \text{H}^+ \) ions available, which can participate in the reaction. This can lead to differences in the number of electrons involved and changes in the reduction or oxidation pathway.For example, in the given exercise, the reduction of the permanganate ion occurs in an acidic medium, which means there needs to be a sufficient presence of \( \text{H}^+ \) ions to facilitate the reaction. These ions help in balancing the charge and matter in the chemical equation:\[ \text{MnO}_4^- + 8\text{H}^+ + 5\text{e}^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O} \]The acidic environment thus allows the manganese ion to transition to a lower oxidation state by accepting 5 electrons. This distinctly shows how the medium affects electron transfer and how the reaction is completed, showcasing the necessity of knowing the medium when predicting the behavior of chemical reactions.