Problem 103 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following reactions does ammonia act as an oxidizing agent? (a) \(3 \mathrm{NaClO}+2 \mathrm{NH}_{3} \longrightarrow 3 \mathrm{NaCl}+\mathrm{N}_{2}+3 \mathrm{H}_{2} \mathrm{O}\) (b) \(3 \mathrm{CaOCl}_{2}+2 \mathrm{NH}_{3} \longrightarrow 3 \mathrm{CaCl}_{2}+\mathrm{N}_{2}+3 \mathrm{H}_{2} \mathrm{O}\) (c) \(4 \mathrm{NH}_{3}+5 \mathrm{O}_{2} \longrightarrow 4 \mathrm{NO}+6 \mathrm{H}_{2} \mathrm{O}\) (d) none of these

Step-by-Step Solution

Answer

Ammonia does not act as an oxidizing agent in any reaction; option (d) is correct.

Step 1: Understanding the Role of an Oxidizing Agent

An oxidizing agent is a substance that gains electrons and is reduced in a chemical reaction. To determine if ammonia acts as an oxidizing agent, we need to check if nitrogen from ammonia gains electrons (is reduced) during the reaction.

Step 2: Analyzing Reaction (a)

In reaction (a): \(3 \mathrm{NaClO}+2 \mathrm{NH}_{3} \rightarrow 3 \mathrm{NaCl}+\mathrm{N}_{2}+3 \mathrm{H}_{2} \mathrm{O}\), ammonia \( \mathrm{NH}_3 \) produces nitrogen gas \( \mathrm{N}_2 \). Here, nitrogen in \( \mathrm{NH}_3 \) has a -3 oxidation state and in \( \mathrm{N}_2 \) it becomes 0. Thus, ammonia is oxidized, not reduced, so it cannot be the oxidizing agent.

Step 3: Analyzing Reaction (b)

In reaction (b): \(3 \mathrm{CaOCl}_{2}+2 \mathrm{NH}_{3} \rightarrow 3 \mathrm{CaCl}_{2}+\mathrm{N}_{2}+3 \mathrm{H}_{2} \mathrm{O}\), similar to reaction (a), ammonia forms nitrogen gas \( \mathrm{N}_2 \). Ammonia is oxidized from a -3 to a 0 oxidation state, meaning it does not act as an oxidizing agent.

Step 4: Analyzing Reaction (c)

In reaction (c): \(4 \mathrm{NH}_{3}+5 \mathrm{O}_{2} \rightarrow 4 \mathrm{NO}+6 \mathrm{H}_{2} \mathrm{O}\), the nitrogen in \( \mathrm{NH}_3 \) has an oxidation state of -3 and it goes to +2 in \( \mathrm{NO} \). Again, ammonia is oxidized rather than reduced.

Step 5: Conclusion

Since in reactions (a), (b), and (c) ammonia is oxidized instead of reduced, it does not act as an oxidizing agent in any of these reactions. Therefore, option (d) 'none of these' is correct.

Key Concepts

Oxidation States

In chemistry, understanding oxidation states is crucial for figuring out how electrons are transferred during a reaction.
The oxidation state represents the number of electrons an atom has gained or lost to form a compound. For example:

• In ammonia, NH extsubscript{3}, nitrogen has an oxidation state of -3.
• When nitrogen changes to form nitrogen gas, N extsubscript{2}, or nitrogen oxide, NO, its oxidation states are 0 and +2, respectively.

Identifying these states helps determine if a substance is oxidized or reduced.
This concept is key to understanding whether a reactant can be an oxidizing agent.

Chemical Reactions

Chemical reactions often involve the transfer of electrons, resulting in the formation of new substances. These reactions are represented by balanced chemical equations.
Key aspects to consider in a reaction include:

• The reactants, which are the starting materials.
• The products, which are formed at the end.
• Tracking the change in oxidation states of elements to understand electron transfer.

A chemical reaction where ammonia is supposed to act as an oxidizing agent requires that it gains electrons, thereby reducing itself. However, in the reactions given, ammonia consistently loses electrons.

Ammonia Chemistry

Ammonia (NH extsubscript{3}) is a simple molecule consisting of nitrogen and hydrogen.
It is commonly used in fertilizers and industrial applications. In chemical reactions, ammonia can act as both a reducing agent and a reactant.

• Its ability to donate electrons makes it a reducing agent.
• Despite its common uses, ammonia rarely acts as an oxidizing agent because it does not usually gain electrons.

In the exercise's examples, ammonia is oxidized, losing electrons and thus failing to act as an oxidizing agent.

Electron Transfer

Electron transfer is a fundamental concept in chemistry that involves the movement of electrons from one chemical species to another.
Understanding this movement helps in identifying oxidizing and reducing agents.

• An oxidizing agent is a substance that gains electrons, and hence, is reduced.
• Contrastingly, a reducing agent loses electrons and is oxidized.

In the given reactions, ammonia loses electrons, indicating that it is oxidized and acts as a reducing agent instead.

Reduction and Oxidation

The process of reduction and oxidation occurs simultaneously in chemical reactions and is collectively known as redox reactions.
In any redox reaction:

• Reduction involves the gain of electrons, decreasing the oxidation state.
• Oxidation involves the loss of electrons, increasing the oxidation state.

For ammonia to be an oxidizing agent, it needs to undergo reduction by gaining electrons, but in the given reactions, it undergoes oxidation.

Nitrogen Oxidation State

The concept of nitrogen oxidation state is essential in understanding chemical reactions involving nitrogen compounds, such as ammonia.
As nitrogen transitions through different oxidation states, it undergoes various chemical transformations:

• In NH extsubscript{3}, nitrogen has the lowest oxidation state of -3, which means it can be easily oxidized.
• Through chemical reactions, it can shift to states such as 0 in N extsubscript{2} or +2 in NO by losing electrons.

Recognizing these shifts helps in understanding the role of nitrogen as a reducing agent in reactions, confirming that in the analyzed reactions, ammonia acts as the oxidized substance, not the oxidizing agent.