Problem 135 - A Complete Resource Book in Chemistry for JEE Main

Question

To the aqueous solution of the salt acidified potassium permanganate is added and its colour is discharged. It indicates the absence of (a) \(\mathrm{Sn}^{2+}\) (b) \(\mathrm{Be}^{2+}\) (c) \(\mathrm{Fe}^{2+}\) (d) \(\mathrm{NO}_{3}^{-}\)

Step-by-Step Solution

Answer

(b) Be^{2+} is absent since it doesn't discharge the color of KMnO_4.

Step 1: Identify the Reaction

When acidified potassium permanganate ( KMnO_4) is added to an aqueous solution, it usually acts as an oxidizing agent. It can be used to test for reducing agents, which undergo oxidation in the process, causing the purple color of the permanganate to be discharged to colorless or light pink due to the formation of Mn^{2+} ions.

Step 2: Determine Possible Reactions

The provided ions that could react with KMnO_4 are Sn^{2+}, Be^{2+}, Fe^{2+}, and NO_3^{-}. We need to determine which behaves as a reducing agent. Sn^{2+} and Fe^{2+} are known reducing agents that will react with permanganate, changing its color.

Step 3: Evaluate Non-Reacting Ions

For ions like Be^{2+} and NO_3^{-}, they do not function as reducing agents in this context. Be^{2+} is an ion of an alkaline earth metal and not easily oxidized. NO_3^{-} is an oxidizing agent itself, not a reducing agent. Therefore, they won't discharge the purple color of KMnO_4.

Step 4: Conclude the Conclusion

Since the color is originally discharged, it means there is a lack of a reducing agent in the solution. Since Be^{2+} does not react with KMnO_4 to discharge its color, its presence does not account for the reaction, indicating its absence.

Key Concepts

Oxidizing Agents

An oxidizing agent is a substance that gains electrons during a chemical reaction. This process is known as reduction. As such, oxidizing agents are central to redox (reduction-oxidation) reactions. Potassium permanganate, chemically represented as \(KMnO_4\), is a well-known oxidizing agent, often used in aqueous solutions.
In its purple crystalline form, potassium permanganate can oxidize various substances. When it encounters a reducing agent, which loses electrons, the characteristic purple color of permanganate fades to colorless or light pink due to the reduction of \(Mn^{7+}\) in permanganate to \(Mn^{2+}\) ions. This color change is visible evidence of its role as an oxidizing agent.
Understanding the behavior of oxidizing agents like \(KMnO_4\) helps in identifying other chemical substances present in a solution, as seen when the solution's color shifts upon interaction with reducing agents.

Reducing Agents

Reducing agents are substances that donate electrons in a chemical reaction, and in the process, they become oxidized themselves. Their role is crucial in reactions where electron transfer occurs. The chemical behavior of reducing agents makes them beneficial in reactions with oxidizing agents like potassium permanganate.
Common reducing agents in aqueous reactions include metals like magnesium or iron. Specifically, \(Sn^{2+}\) (tin) and \(Fe^{2+}\) (iron) are known to be reducing agents. These ions can react with \(KMnO_4\) in acidic solutions, causing the discharge of the permanganate's color due to their oxidation. The discharge of purple color to a lighter shade or colorless state results from the formation of \(Mn^{2+}\) ions during this redox reaction.
Identifying reducing agents in a solution helps chemists and students predict the outcome of interactions with substances like potassium permanganate, aiding in qualitative analysis or chemical test predictions.

Aqueous Chemistry Reactions

Aqueous chemistry focuses on reactions that occur in water-based solutions. These solutions are vital in chemical experiments and real-world applications because they allow ions to move freely, making it easier for them to interact.
When potassium permanganate is added to an aqueous solution, it exemplifies an aqueous reaction as it serves as an oxidizer in the solution, causing a visible change when acidic conditions are present. In these settings, the ions dissolve and react according to their chemical properties - oxidizers gain electrons, while reducers donate electrons. This dynamic makes aqueous solutions a powerful medium for predicting chemical reactions.
The reaction conditions, such as acidity or the presence of certain ions, vastly affect how chemicals will behave. Be it a neutral, acidic, or basic environment, solutions often yield different results. For example, acidic potassium permanganate is a more powerful oxidizing agent than when it's in a neutral or basic solution.
Thus, understanding aqueous chemical reactions involves recognizing how certain variables, like pH and ion presence, contribute to the outcome of reactions between oxidizing and reducing agents.