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

Question

Which one of the following cannot function as an oxidising agent ? (a) \(\mathrm{I}^{-}\) (b) \(\mathrm{S}(\mathrm{s})\) (c) \(\mathrm{NO}_{3}^{-}\)(aq) (d) \(\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}\)

Step-by-Step Solution

Answer

The iodide ion \(\mathrm{I}^-\) cannot function as an oxidizing agent.

Step 1: Define Oxidizing Agent

An oxidizing agent is a substance that gains electrons and is reduced in a chemical reaction. They facilitate the oxidation of another substance.

Step 2: Evaluate Iodide Ion

Consider the iodide ion, \(\mathrm{I}^-\). It commonly donates an electron to become iodine, \(\mathrm{I}_2\). Therefore, \(\mathrm{I}^-\) typically acts as a reducing agent, not an oxidizing agent.

Step 3: Evaluate Sulfur

Sulfur \(\mathrm{S(s)}\) can gain electrons to form sulfide ions \(\mathrm{S^{2-}}\), therefore it can act as an oxidizing agent.

Step 4: Evaluate Nitrate Ion

The nitrate ion \(\mathrm{NO}_3^-\) is a common oxidizing agent. It is reduced to nitrogen dioxide \(\mathrm{NO}_2\), or other nitrogen oxides, in redox reactions.

Step 5: Evaluate Dichromate Ion

The dichromate ion \(\mathrm{Cr_2O_7^{2-}}\) is also a known oxidizing agent. It is reduced to chromium ions \(\mathrm{Cr^{3+}}\) in redox reactions.

Step 6: Conclusion

Among the substances considered, only \(\mathrm{I}^-\) cannot function as an oxidizing agent since it acts as a reducing agent.

Key Concepts

Redox Reactions

Redox reactions, short for reduction-oxidation reactions, are fundamental chemical processes where the transfer of electrons occurs between two substances. These reactions are characterized by the movement of electrons from one molecule, atom, or ion to another. This transfer is crucial because it changes the oxidation states of the elements involved.

• Reduction refers to the gain of electrons by a substance, decreasing its oxidation state.
• Oxidation is the loss of electrons, which increases the oxidation state of the element.

A typical redox reaction involves two half-reactions: one for reduction and one for oxidation. These complementary processes ensure that the electrons lost by the oxidized species are gained by the reduced one. Understanding these reactions is key to grasping many processes in chemistry and biology.

Electron Transfer

Electron transfer is a core component of redox reactions. It represents the actual movement of electrons from one chemical entity to another. This is the fundamental process through which redox reactions occur.

• Oxidizing agents accept electrons and are reduced.
• Reducing agents donate electrons and are oxidized.

The ability of a substance to accept or donate electrons defines its role as either an oxidizing or a reducing agent. This transfer mechanism is essential in many natural and industrial processes, including energy production in living cells and metal extraction in metallurgy.

Iodide Ion

The iodide ion ( I^- ) is an interesting player in redox chemistry. It typically acts as a reducing agent, meaning it readily donates an electron to other substances. When I^- loses an electron, it becomes iodine ( I_2 ), thus participating in oxidation.

• This behavior is why iodide ions are generally not considered oxidizing agents.
• They primarily facilitate the reduction process of other substances by undergoing oxidation themselves.

In redox reactions, I^- takes the role of giving electrons, rather than accepting them, which is crucial in identifying its function in various chemical processes.

Sulfur

Sulfur, represented as ( S(s) ), has the capability to act as an oxidizing agent in certain reactions. Sulfur can gain electrons and transform into sulfide ions ( S^{2-} ), thereby undergoing reduction.

• This property allows sulfur to accept electrons from other substances, facilitating their oxidation.
• The transformation showcases sulfur's ability to shift roles in reactions, depending on the chemistry involved.

Understanding sulfur's behavior in redox reactions helps explain its versatility and the various forms it can take in chemical processes.

Nitrate Ion

The nitrate ion ( NO_3^- ) is a well-known oxidizing agent, especially in aqueous environments. It readily participates in redox reactions by accepting electrons.

• In these reactions, NO_3^- often gets reduced to nitrogen dioxide ( NO_2 ) or other nitrogen oxides.
• Its capacity to accept electrons makes it an important substance in chemical processes like combustion and cellular respiration.

Nitrate ions are involved in a range of biological and environmental processes, emphasizing their significance in chemical reactions.

Dichromate Ion

The dichromate ion ( Cr_2O_7^{2-} ) is a potent oxidizing agent widely utilized in various chemical processes. It gains electrons during redox reactions, transforming into chromium ions ( Cr^{3+} ).

• This reduction of dichromate typically involves the oxidation of other substances.
• Its strong oxidizing ability makes it valuable in procedures like organic synthesis and analytical chemistry.

The versatile nature of dichromate ions in facilitating electron transfer highlights their importance in both educational and practical chemistry settings.