Problem 117 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions, \(\mathrm{H}_{2} \mathrm{O}_{2}\) acts as an oxidant? (a) \(\mathrm{PbO}_{2}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{O}_{2}(\mathrm{aq}) \rightarrow \mathrm{PbO}(\mathrm{s})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l})+\mathrm{O}_{2}(\mathrm{~g})\) (b) \(\mathrm{KNO}_{2}\) (aq) \(+\mathrm{H}_{2} \mathrm{O}_{2}\) (aq) \(\rightarrow \mathrm{KNO}_{3}\) (aq) \(+\mathrm{H}_{2} \mathrm{O}\) (l) (c) \(2 \mathrm{KI}\) (aq) \(+\mathrm{H}_{2} \mathrm{O}_{2}\) (aq) \(\rightarrow 2 \mathrm{KOH}\) (aq) \(+\mathrm{I}_{2}(\mathrm{~s})\) (d) \(\mathrm{Na}_{2} \mathrm{SO}_{3}\) (aq) \(+\mathrm{H}_{2} \mathrm{O}_{2}\) (aq) \(\rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

\( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as an oxidant in (b), (c), and (d).

Step 1: Understanding the Concept of Oxidant

An oxidant (or oxidizing agent) is a substance that causes another substance to lose electrons and, in the process, gains electrons itself. In a reaction, the oxidant undergoes reduction.

Step 2: Analyzing Option (a)

In the reaction \( \mathrm{PbO}_{2} + \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{PbO} + \mathrm{H}_{2} \mathrm{O} + \mathrm{O}_{2} \), \( \mathrm{H}_{2} \mathrm{O}_{2} \) decomposes to water and oxygen, leading to the reduction of \( \mathrm{PbO}_{2} \) to \( \mathrm{PbO} \). Here, \( \mathrm{H}_{2} \mathrm{O}_{2} \) does not act as an oxidant.

Step 3: Analyzing Option (b)

In the reaction \( \mathrm{KNO}_{2} + \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{KNO}_{3} + \mathrm{H}_{2} \mathrm{O} \), \( \mathrm{H}_{2} \mathrm{O}_{2} \) helps \( \mathrm{KNO}_{2} \) to oxidize to \( \mathrm{KNO}_{3} \). This indicates that \( \mathrm{H}_{2} \mathrm{O}_{2} \) is acting as an oxidant by gaining electrons through the conversion from nitrite to nitrate.

Step 4: Analyzing Option (c)

In the reaction \( 2 \mathrm{KI} + \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow 2 \mathrm{KOH} + \mathrm{I}_{2} \), \( \mathrm{I}_{-} \) ions are oxidized to \( \mathrm{I}_{2} \), meaning \( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as an oxidant as it accepts electrons from iodide ions.

Step 5: Analyzing Option (d)

In the reaction \( \mathrm{Na}_{2} \mathrm{SO}_{3} + \mathrm{H}_{2} \mathrm{O}_{2} \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4} + \mathrm{H}_{2} \mathrm{O} \), \( \mathrm{SO}_{3}^{2-} \) is oxidized to \( \mathrm{SO}_{4}^{2-} \). This reaction shows that \( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as an oxidant by accepting electrons.

Step 6: Conclusion

By analyzing all options, \( \mathrm{H}_{2} \mathrm{O}_{2} \) acts as an oxidant in reactions (b), (c), and (d) because it causes another substance in the reaction to lose electrons by gaining electrons itself.

Key Concepts

Oxidizing Agents

In chemistry, oxidizing agents, also known as oxidants, play a vital role in many reactions. These substances have the ability to accept electrons from another substance. In doing so, they facilitate oxidation - a process where the other substance loses electrons. This transfer of electrons is crucial, as it drives the chemical changes that occur in a reaction.

• An oxidizing agent causes oxidation of another substance.
• It itself undergoes reduction by gaining electrons.
• They are commonly involved in redox reactions (oxidation-reduction reactions).

Understanding oxidizing agents is key in the study of redox reactions, as these agents dictate how electrons are transferred between reacting entities.

Hydrogen Peroxide

Hydrogen peroxide (\(\text{H}_2\text{O}_2\)) is a fascinating chemical compound that acts as an oxidizing agent in various reactions. It's unique because it can both gain and give up electrons, making it a versatile player in redox processes.

• It is composed of two hydrogen atoms and two oxygen atoms.
• Hydrogen peroxide decomposes into water and oxygen over time.
• It can act as both an oxidizing and reducing agent, but it frequently serves as an oxidizing agent in chemical reactions.

In many reactions, such as those found in the original exercise, \(\text{H}_2\text{O}_2\) accepts electrons, thus acting as an oxidant, as seen when it converts nitrite to nitrate or iodide ions to iodine.

Electron Transfer

The movement of electrons from one substance to another is central to the concept of redox reactions. Electron transfer is what makes redox reactions go, transforming the involved substances by oxidation and reduction.

• Oxidation involves the loss of electrons from a substance.
• Reduction involves the gain of electrons by a substance.
• The substance that loses electrons gets oxidized, while the one that gains electrons gets reduced.

Understanding electron transfer helps break down complex reactions into simpler steps. In the case of hydrogen peroxide acting as an oxidant, it gains electrons, which allows the other substances in the reaction to be oxidized.

Chemical Reactions Analysis

Analyzing chemical reactions involves closely examining how reactants are transformed into products. This is essential in identifying the roles that different compounds play, such as determining whether a substance is acting as an oxidant or reductant.

• Identify reactants, products, and their states of matter.
• Determine which substances are oxidized and reduced.
• Recognize the agents responsible for the oxidation and reduction processes.

In the example reactions, hydrogen peroxide serves as the oxidizing agent, accepting electrons to drive the transformations seen with lead dioxide, potassium nitrite, potassium iodide, and sodium sulfite. Analyzing each equation reveals how \(\text{H}_2\text{O}_2\) acts in its oxidizing capacity.