Problem 10 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as a reducing agent in acid medium? (a) \(\mathrm{MnO}_{4}^{-}\) (b) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) (c) \(\mathrm{SO}_{3}^{2-}\) (d) \(\mathrm{KI}\)

Step-by-Step Solution

Answer

H₂O₂ acts as a reducing agent with (a) MnO₄⁻ and (b) Cr₂O₇²⁻.

Step 1: Understand the Role of Reducing Agents

A reducing agent is a substance that donates electrons to another substance, causing the latter to be reduced while the reducing agent itself is oxidized. In the context of this problem, it involves identifying which reacting species causes the oxidation of hydrogen peroxide (H₂O₂) in an acid medium.

Step 2: Analyze Permanganate Ion ( \(\mathrm{MnO}_{4}^{-} \))

In an acidic medium, \(\mathrm{MnO}_{4}^{-}\) acts as a strong oxidizing agent. When it reacts with H₂O₂, it is reduced to Mn²⁺. Hence, in this reaction, H₂O₂ will be oxidized, acting as a reducing agent.

Step 3: Analyze Dichromate Ion ( \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-} \))

\(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) in an acidic medium is also a strong oxidizing agent, converting Cr⁶⁺ to Cr³⁺. H₂O₂ can act as a reducing agent by providing electrons for this reduction.

Step 4: Analyze Sulfite Ion ( \(\mathrm{SO}_{3}^{2-} \))

In an acidic medium, \(\mathrm{SO}_{3}^{2-}\) would more often react as a reducing agent, being oxidized to \(\mathrm{SO}_{4}^{2-}\). Therefore, it is less likely for H₂O₂ to act as a reducing agent in this scenario.

Step 5: Analyze Potassium Iodide ( \(\mathrm{KI} \))

In an acidic medium, KI can be oxidized to I₂ while H₂O₂ would be reduced. In this reaction, H₂O₂ acts as an oxidizing agent, not a reducing agent.

Step 6: Conclusion

Based on the analysis, H₂O₂ acts as a reducing agent when reacting with \(\mathrm{MnO}_{4}^{-}\) and \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\), as these species are reduced while H₂O₂ is oxidized.

Key Concepts

Oxidizing Agent

An oxidizing agent is a substance that accepts electrons from another chemical species during a redox reaction. This means it causes another species to be oxidized while it is reduced. Oxidizing agents are essential in redox reactions because they drive the transfer of electrons between substances. Some common examples include

• permanganate ions (\(\mathrm{MnO}_{4}^{-}\)),
• dichromate ions (\(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\)),
• and oxygen.

In the case of hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)), when it interacts with a strong oxidizing agent like \(\mathrm{MnO}_{4}^{-}\) or \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\), the latter compounds are reduced, thus transforming into Mn²⁺ and Cr³⁺ respectively.
This process includes a redox reaction where the oxidizing agent "pulls" electrons from hydrogen peroxide, which functions as the reducing agent by losing electrons.

Reducing Agent

A reducing agent is a substance that loses electrons in a redox reaction, thereby causing another substance to be reduced. When a reducing agent donates electrons, it itself becomes oxidized. This concept is the reverse of how oxidizing agents work; instead of gaining electrons, a reducing agent gives them away.

• Hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)) can act as a reducing agent when it reacts with strong oxidizing agents.
• This includes substances like permanganate ions and dichromate ions.

In reactions where \(\mathrm{MnO}_{4}^{-}\) or \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) are involved, \(\mathrm{H}_{2} \mathrm{O}_{2}\) donates electrons to these agents, resulting in the oxidation of \(\mathrm{H}_{2} \mathrm{O}_{2}\) itself.
The primary focus when identifying a reducing agent is to observe which substance in a reaction loses electrons during the chemical transformation.

Hydrogen Peroxide in Acidic Medium

Hydrogen peroxide (\(\mathrm{H}_{2} \mathrm{O}_{2}\)) is an interesting compound because it can function as either an oxidizing or reducing agent depending on the chemical environment and reactants present. It is particularly versatile when used in an acidic medium. Acidic conditions often facilitate and enhance redox reactions by providing the necessary H⁺ ions.
In an acidic medium, reactions with oxidizing agents like \(\mathrm{MnO}_{4}^{-}\) or \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\) result in hydrogen peroxide acting primarily as a reducing agent.

• The acidic medium helps in the efficient transfer of electrons.
• It also stabilizes the charged ions formed during these redox reactions.

The transformation of these oxidizing agents from higher oxidation states to lower ones, accompanied by the oxidation of \(\mathrm{H}_{2} \mathrm{O}_{2}\), showcases the dynamic role hydrogen peroxide plays in these reactions. Understanding how \(\mathrm{H}_{2} \mathrm{O}_{2}\) operates can help predict its behavior in various chemical settings.