Problem 5 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

In which of the following reactions, hydrogen peroxide acts as an oxidizing agent ? [Main Online April 8, 2017] (a) \(\mathrm{HOCl}+\mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}+\mathrm{Cl}^{-}+\mathrm{O}_{2}\) (b) \(\mathrm{I}_{2}+\mathrm{H}_{2} \mathrm{O}_{2}+2 \mathrm{OH}^{-} \rightarrow 2 \mathrm{I}^{-}+2 \mathrm{H}_{2} \mathrm{O}+\mathrm{O}_{2}\) (c) \(2 \mathrm{MnO}_{4}^{-}+3 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2 \mathrm{MnO}_{2}+3 \mathrm{O}_{2}+2 \mathrm{H}_{2} \mathrm{O}+2 \mathrm{OH}^{-}\) (d) \(\mathrm{PbS}+4 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{PbSO}_{4}+4 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

In reaction (d), hydrogen peroxide acts as an oxidizing agent.

Step 1: Analyze Reaction (a)

In the reaction \( \mathrm{HOCl} + \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{H}_{3} \mathrm{O}^{+} + \mathrm{Cl}^{-} + \mathrm{O}_{2} \), hydrogen peroxide \(\mathrm{H}_{2} \mathrm{O}_{2}\) breaks down into \( \mathrm{O}_{2} \), indicating it loses electrons and therefore is oxidized. This means \(\mathrm{H}_{2} \mathrm{O}_{2}\) is reducing another substance, making it a reducing agent in this context, not an oxidizing agent.

Step 2: Analyze Reaction (b)

In the reaction \( \mathrm{I}_{2} + \mathrm{H}_{2} \mathrm{O}_{2} + 2 \mathrm{OH}^{-} \rightarrow 2 \mathrm{I}^{-} + 2 \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} \), hydrogen peroxide \(\mathrm{H}_{2} \mathrm{O}_{2}\) changes from \( -1 \) oxidation state in \(\mathrm{H}_{2}\mathrm{O}_{2}\) to \( 0 \) in \(\mathrm{O}_{2}\), indicating oxidation. The reduction of \( \mathrm{I}_{2} \) to \( \mathrm{I}^{-} \) implies that \(\mathrm{H}_{2} \mathrm{O}_{2}\) provides electrons, acting as a reducing agent rather than an oxidizing agent.

Step 3: Analyze Reaction (c)

In the reaction \( 2 \mathrm{MnO}_{4}^{-} + 3 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2\mathrm{MnO}_{2} + 3 \mathrm{O}_{2} + 2 \mathrm{H}_{2} \mathrm{O} + 2 \mathrm{OH}^{-} \), \(\mathrm{H}_{2} \mathrm{O}_{2}\) decomposes into \( \mathrm{O}_{2} \). Here, \( \mathrm{MnO}_{4}^{-} \) is reduced to \( \mathrm{MnO}_{2} \), as it accepts electrons. Thus, \(\mathrm{H}_{2} \mathrm{O}_{2}\) provides electrons and acts as a reducing—not oxidizing—agent.

Step 4: Analyze Reaction (d)

In the reaction \( \mathrm{PbS} + 4 \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{PbSO}_{4} + 4 \mathrm{H}_{2} \mathrm{O} \), \(\mathrm{H}_{2} \mathrm{O}_{2}\) oxidizes \( \mathrm{S}^{-2} \) in \( \mathrm{PbS} \) to \( \mathrm{S}^{+6} \) in \(\mathrm{PbSO}_{4}\). Here, \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as an oxidizing agent because it causes the oxidation of sulfur.

Key Concepts

Hydrogen Peroxide Chemistry

Hydrogen peroxide, known chemically as \( \mathrm{H}_2\mathrm{O}_2 \), is an important compound with unique chemical properties. It is often used in various chemical reactions because of its capability to act as either an oxidizing or a reducing agent.

Some key characteristics of hydrogen peroxide include:

• It is a pale blue liquid in its pure state, which appears colorless when diluted with water.
• It acts as a bleacher and antiseptic, showcasing its oxidizing abilities.
• At different concentrations, it is used in industries ranging from healthcare to textile manufacturing.

Hydrogen peroxide is known for its ability to break down into water \( \mathrm{H}_2\mathrm{O} \) and oxygen \( \mathrm{O}_2 \), which is an important feature in both biological and industrial processes. This comes particularly handy in reactions where the release of oxygen is beneficial.

Overall, hydrogen peroxide's fascinating chemistry is due to its unique ability to participate actively in chemical reactions, enhancing various processes across different fields.

Redox Reactions

Redox reactions, or reduction-oxidation reactions, are chemical processes where the oxidation state of atoms changes through the transfer of electrons. These reactions are fundamental to understanding many chemical processes, including those involving hydrogen peroxide.

In any redox reaction:

• Oxidation refers to the loss of electrons, resulting in an increase in oxidation state.
• Reduction refers to the gain of electrons, resulting in a decrease in oxidation state.
• The substance that donates electrons is known as the reducing agent.
• The substance that accepts electrons is called the oxidizing agent.

Hydrogen peroxide can participate in both roles, either by donating electrons and being oxidized itself, or by accepting electrons and acting as an oxidizing agent. Its dual functionality makes hydrogen peroxide a versatile player in numerous redox reactions. In the context of our exercise, hydrogen peroxide acts as an oxidizing agent in the reaction with \( \mathrm{PbS} \), promoting the oxidation of sulfur.

Chemical Reactions Analysis

Analyzing chemical reactions involves understanding the roles of different reactants and predicting the products formed. This is a crucial skill for understanding any chemical process, including those that involve redox reactions and oxidizing agents.

When analyzing reactions involving hydrogen peroxide, it is important to:

• Identify the oxidation states of the elements involved.
• Determine which reactant is reduced and which is oxidized.
• Predict the resulting products from the electron transfer.

In the original exercise, hydrogen peroxide served different roles in each reaction analyzed. By examining changes in oxidation states, we identified that hydrogen peroxide acted as an oxidizing agent in reaction \( \mathrm{PbS} + 4 \mathrm{H}_2\mathrm{O}_2 \rightarrow \mathrm{PbSO}_4 + 4 \mathrm{H}_2\mathrm{O} \). This reaction displays classic features of redox behavior, illustrating the importance of detailed chemical analysis for accurately interpreting chemical reactions.

Understanding the mechanisms behind reactions allows us to better predict and utilize chemical processes in practical applications.