Problem 51 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{SO}_{2}\) behaves as a reducing agent when (a) passed through \(\mathrm{FeSO}_{4}\) solution (b) passed over hot \(\mathrm{CuO}\) (c) mixed with moist \(\mathrm{H}_{2} \mathrm{~S}\) (d) passed through acidified \(\mathrm{KMnO}_{4}\) solution

Step-by-Step Solution

Answer

(b) with \(\mathrm{CuO}\) and (d) with acidified \(\mathrm{KMnO}_{4}\).

Step 1: Analyze Each Option

Determine how sulfur dioxide, \(\mathrm{SO}_{2}\), behaves in each situation. \(\mathrm{SO}_{2}\) can act as a reducing agent by being oxidized itself, meaning it will donate electrons in a reaction.

Step 2: Examine \(\mathrm{FeSO}_{4}\) Interaction

When \(\mathrm{SO}_{2}\) is passed through \(\mathrm{FeSO}_{4}\), it generally does not cause any significant reaction that highlights its reducing behavior. \(\mathrm{SO}_{2}\) does not change the oxidation state of iron in \(\mathrm{FeSO}_{4}\).

Step 3: Check Reaction with \(\mathrm{CuO}\)

\(\mathrm{SO}_{2}\) passed over hot \(\mathrm{CuO}\) can act as a reducing agent. It reduces \(\mathrm{CuO}\) to \(\mathrm{Cu}\) by donating electrons, with \(\mathrm{SO}_{2}\) being oxidized to \(\mathrm{SO}_{3}\).

Step 4: Examine Mixture with \(\mathrm{H}_{2}\mathrm{S}\)

In this reaction, \(\mathrm{SO}_{2}\) oxidizes \(\mathrm{H}_{2}\mathrm{S}\) to elemental sulfur while it gets reduced. Thus, \(\mathrm{SO}_{2}\) acts as an oxidizing agent, not a reducing agent.

Step 5: Analyze \(\mathrm{KMnO}_{4}\) Interaction

\(\mathrm{SO}_{2}\) effectively acts as a reducing agent in an acidified \(\mathrm{KMnO}_{4}\) solution. It reduces \(\mathrm{KMnO}_{4}\) to \(\mathrm{Mn}^{2+}\) ions while being oxidized itself.

Step 6: Conclusion

Based on the analysis, \(\mathrm{SO}_{2}\) behaves as a reducing agent when passed over hot \(\mathrm{CuO}\) (option b) and when passed through acidified \(\mathrm{KMnO}_{4}\) solution (option d).

Key Concepts

Redox Reactions

Redox reactions are chemical processes involving electron transfer between substances. In these reactions, one substance gains electrons (is reduced) while another loses electrons (is oxidized).

• The substance that donates electrons acts as a reducing agent.
• Conversely, the one accepting electrons is the oxidizing agent.

These reactions play pivotal roles in various biological and chemical processes. Understanding the dual nature of reactions—how one component gets oxidized while the other is reduced—is crucial for mastering chemistry concepts. The interplay of oxidation and reduction is central, unveiling the foundational molecular changes involved in chemical transformations.

Sulfur Dioxide

Sulfur dioxide ( SO_{2} ) can serve as either an oxidizing or reducing agent. Its behavior largely depends on the substances it reacts with. As a gas, it has diverse applications, from food preservation to being a pivotal player in acid rain formation. In chemical reactions, SO_{2} can donate electrons, acting as a reducing agent. For example, through transforming copper oxide ( CuO ) into copper ( Cu ) by losing electrons and converting to sulfur trioxide ( SO_{3} ). Conversely, when it accepts electrons, it operates as an oxidizing agent, such as when it converts hydrogen sulfide ( H_{2}S ) into elemental sulfur. Grasping this dual capability helps in predicting chemical behaviors and outcomes in various industrial and experimental contexts.

Copper Oxide Reduction

Copper oxide ( CuO ) reduction is a chemical reaction where CuO is transformed into copper metal ( Cu ). This transformation is achieved through a reducing agent, like sulfur dioxide ( SO_{2} ), that donates electrons.

• During the reaction, SO_{2} donates electrons and is oxidized to SO_{3} .
• Simultaneously, CuO accepts these electrons and reduces to copper metal ( Cu ).

This process is an excellent example of a redox reaction, showcasing the specific role of a reducing agent within the chemical transformation of metal oxides. Such reactions are pertinent in metallurgy and refining processes, highlighting the role of electron transfer in material recovery.

Acidified Potassium Permanganate

Acidified potassium permanganate ( KMnO_4 ) is a powerful oxidizing agent used in various chemical reactions. When SO_{2} is introduced to an acidified KMnO_4 solution, it acts as a reducing agent.

• In this context, KMnO_4 gets reduced to Mn^{2+} ions.
• Meanwhile, SO_{2} is oxidized to SO_{3} .

This redox reaction exemplifies how reactive compounds like KMnO_4 can facilitate electron exchanges in chemical transformations. Such solutions are instrumental in titrations and various analytical chemistry techniques, aiding in the determination of substance concentration through their oxidative prowess.