Problem 7 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Hydrogen peroxide oxidises \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}\) to \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) in acidic medium but reduces \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3}\) to \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}\) in alkaline medium. The other products formed are respectively: (a) \(\left(\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\right)\) and \(\mathrm{H}_{2} \mathrm{O}\) (b) \(\left(\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\right)\) and \(\left(\mathrm{H}_{2} \mathrm{O}+\mathrm{OH}^{-}\right)\) (c) \(\mathrm{H}_{2} \mathrm{O}\) and \(\left(\mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\right)\) (d) \(\mathrm{H}_{2} \mathrm{O}\) and \(\left(\mathrm{H}_{2} \mathrm{O}+\mathrm{OH}^{-}\right)\)

Step-by-Step Solution

Answer

Option (b): \(\left(\mathrm{H}_2\mathrm{O} + \mathrm{O}_2\right)\) and \(\left(\mathrm{H}_2\mathrm{O} + \mathrm{OH}^-\right)\).

Step 1: Understanding the Reaction in Acidic Medium

In acidic medium, hydrogen peroxide acts as an oxidizing agent. It oxidizes \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) to \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\). During this process, hydrogen peroxide is reduced, typically forming water \(\mathrm{H}_2\mathrm{O}\) and oxygen \(\mathrm{O}_2\) as the other byproducts. Hence, the products in acidic medium are \(\left(\mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2}\right)\).

Step 2: Analyzing the Reaction in Alkaline Medium

In alkaline medium, hydrogen peroxide acts as a reducing agent. It reduces \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{3-}\) back to \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\). During this reduction process, hydrogen peroxide is broken down, resulting in the formation of water \(\mathrm{H}_2\mathrm{O}\) and hydroxide ions \(\mathrm{OH}^{-}\). Thus, the products in alkaline medium are \(\left(\mathrm{H}_{2} \mathrm{O} + \mathrm{OH}^{-}\right)\).

Step 3: Selecting the Correct Option

From the analysis, in acidic medium, the byproducts are \(\mathrm{H}_2\mathrm{O}\) and \(\mathrm{O}_2\), and in alkaline medium, the byproducts are \(\mathrm{H}_2\mathrm{O}\) and \(\mathrm{OH}^{-}\). Therefore, the correct option that matches these results is (b) \(\left(\mathrm{H}_{2} \mathrm{O} + \mathrm{O}_2\right)\) and \(\left(\mathrm{H}_{2} \mathrm{O} + \mathrm{OH}^{-}\right)\).

Key Concepts

Oxidizing agents

Oxidizing agents play an essential role in redox reactions by accepting electrons from other substances. This process allows them to oxidize other compounds while they themselves are reduced.
Hydrogen peroxide \((\mathrm{H}_2\mathrm{O}_2)\) is a classic example of an oxidizing agent. In the presence of hydrogen peroxide, compounds like \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\) will lose electrons and get oxidized to \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\).
This change is seen in acidic conditions, where hydrogen peroxide not only oxidizes the substance but also decomposes to form water \(\mathrm{H}_2\mathrm{O}\) and oxygen gas \(\mathrm{O}_2\).

• When acting as an oxidizing agent, hydrogen peroxide is reduced itself.
• It helps drive the redox reaction by removing electrons from the other compound.

Understanding oxidizing agents is crucial to grasping the fundamentals of redox chemistry.

Reducing agents

Reducing agents do the opposite of oxidizing agents—they donate electrons to other molecules. In doing so, they enable the reduction of the other substance while being oxidized themselves.
Hydrogen peroxide serves as an effective reducing agent when it is in an alkaline medium. During this reaction, hydrogen peroxide reduces \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\) back to \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\). Here, hydrogen peroxide decomposes to form water \(\mathrm{H}_2\mathrm{O}\) and hydroxide ions \(\mathrm{OH}^{-}\).
The behavior of hydrogen peroxide is fascinating because its capacity to act as either an oxidizing or reducing agent is determined by the medium (acidic or alkaline) it is in.

• In an alkaline medium, hydrogen peroxide gives electrons, promoting reduction.
• This dual nature provides versatility in chemical reactions.

The ability to switch roles makes hydrogen peroxide a unique reagent in chemical processes.

Acidic and alkaline mediums

The medium in which a reaction takes place significantly influences the behavior of reactants. Whether a medium is acidic or alkaline can determine if a substance will act as an oxidizing or reducing agent.
In acidic mediums, hydrogen peroxide exhibits its oxidizing properties, oxidizing compounds like \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\). This leads to the formation of oxidized products and the creation of oxygen gas \(\mathrm{O}_2\) along with water.
Conversely, an alkaline medium allows hydrogen peroxide to act as a reducing agent, transferring electrons to reduce \([\mathrm{Fe}(\mathrm{CN})_6]^{3-}\) back to \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\). This process yields hydroxide ions \(\mathrm{OH}^{-}\) as a byproduct.

• The medium affects the reduction-oxidation balance of a reaction.
• Acidity or alkalinity dictates the electron exchange process.

By altering only the pH, you can switch the nature of a reactant from oxidizing to reducing, showcasing the power and versatility of chemical mediums.