Problem 10 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{Zn}\) gives \(\mathrm{H}_{2}\) gas with \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and \(\mathrm{HCl}\) but not with \(\mathrm{HNO}_{3}\) because (a) \(\mathrm{Zn}\) acts as oxidizing agent when reacts with \(\mathrm{HNO}_{3}\) (b) \(\mathrm{HNO}_{3}\) is weaker acid then \(\mathrm{H}_{2} \mathrm{SO}_{4}\) and \(\mathrm{HCl}\) (c) in electrochemical series \(\mathrm{Zn}\) is above hydrogen (d) \(\mathrm{NO}_{3}\) is reduced in preference to hydronium ion

Step-by-Step Solution

Answer

(d) \(\mathrm{NO}_{3}^{-}\) is reduced in preference to hydronium ion.

Step 1: Understand the Problem

The question asks why zinc (\(\mathrm{Zn}\)) does not release hydrogen gas (\(\mathrm{H}_{2}\)) when reacting with nitric acid (\(\mathrm{HNO}_{3}\)), unlike with hydrochloric acid (\(\mathrm{HCl}\)) and sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)). We're given four options to choose the correct explanation.

Step 2: Analyze Reaction Dynamics

Compare the reactivity of \(\mathrm{HCl}\) and \(\mathrm{H}_{2}\mathrm{SO}_{4}\) with \(\mathrm{HNO}_{3}\). Typically, zinc displaces hydrogen in \(\mathrm{HCl}\) and \(\mathrm{H}_{2}\mathrm{SO}_{4}\) because they are non-oxidizing acids, but \(\mathrm{HNO}_{3}\) is a strong oxidizing agent, leading to it undergoing reduction before hydrogen ions can be displaced.

Step 3: Consider Electrochemical Series

Examine the placement of zinc and hydrogen in the electrochemical series. Zinc is above hydrogen, meaning zinc can displace hydrogen in reactions with non-oxidizing acids. However, \(\mathrm{HNO}_{3}\)'s oxidizing nature changes this dynamic, not because of the position in the series but because \(\mathrm{NO}_{3}^{-}\) ions are preferentially reduced.

Step 4: Evaluate the Options

Option (a) suggests zinc acts as an oxidizing agent, which is incorrect; zinc acts as a reducing agent. Option (b) is incorrect because \(\mathrm{HNO}_{3}\)'s strength is irrelevant—its oxidizing nature is key. Option (c) addresses position in the electrochemical series but does not explain the reduction behavior. Option (d) correctly identifies that \(\mathrm{NO}_{3}^{-}\) ions are reduced instead of hydrogen, explaining the lack of \(\mathrm{H}_{2}\) gas.

Step 5: Conclusion

Based on the above analysis, the correct answer to the question is option (d): "\(\mathrm{NO}_{3}^{-}\) is reduced in preference to hydronium ion." Thus, zinc does not release \(\mathrm{H}_{2}\) gas with \(\mathrm{HNO}_{3}\).

Key Concepts

Zinc and Acid Reactions

Zinc (\(\mathrm{Zn}\)) is a fascinating metal, known for its ability to react with certain acids to produce hydrogen gas. When zinc comes into contact with hydrochloric acid (\(\mathrm{HCl}\)) or sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)), it undergoes a chemical reaction that releases hydrogen gas (\(\mathrm{H}_{2}\)). This is because these acids contain ions that facilitate the displacement of hydrogen atoms by zinc. However, when it comes to nitric acid (\(\mathrm{HNO}_{3}\)), the situation is different. Although it is a strong acid, \(\mathrm{HNO}_{3}\)) is also a strong oxidizing agent. The nitric acid tends to react with zinc differently, preventing the release of hydrogen gas and instead undergoing other chemical changes. Understanding why only certain acids lead to the production of \(\mathrm{H}_{2}\)) gas when reacting with zinc is key in grasping broader chemical reaction principles.

Oxidizing Agents

An oxidizing agent is a substance that can accept electrons in a chemical reaction. Therefore, it oxidizes other substances. In the context of zinc reactions, nitric acid (\(\mathrm{HNO}_{3}\)) plays a crucial role as an oxidizing agent. Unlike hydrochloric and sulfuric acids, \(\mathrm{HNO}_{3}\)) does not simply provide hydrogen ions (\(\mathrm{H}^{+}\)) for zinc to react with. Instead, its nitrate ions (\(\mathrm{NO}_{3}^{-}\)) have a stronger tendency to accept electrons from zinc, leading to their reduction. This shift in behavior is essential to understand why zinc does not displace hydrogen in such cases.
The reaction with \(\mathrm{HNO}_{3}\)) often leads to the formation of nitrogen oxides and other zinc compounds, rather than hydrogen gas.- Nitric acid's oxidizing property is more emphasized with metals like zinc.- The tendency of nitrate ions to undergo reduction means that they "outcompete" the hydrogen ions.

Electrochemical Series

The electrochemical series is a useful tool that ranks elements based on their standard electrode potentials. It helps to predict how different substances will react in redox reactions. In this series, zinc is placed above hydrogen, indicating that it has a higher tendency to lose electrons compared to hydrogen.
This typically means that zinc can successfully displace hydrogen from acids, thereby releasing hydrogen gas. However, the presence of a strong oxidizing agent like \(\mathrm{HNO}_{3}\)), changes the expected outcomes. Understanding this deviation is important because it highlights how certain chemical properties can override the basic predictions based simply on the electrochemical series. Additionally, knowing where an element lies on the electrochemical series can help in predicting and understanding other chemical behaviors and reactions.
Key points include:

• Zinc's higher position allows it to donate electrons easily in some environments.
• The presence of strong oxidizing agents can alter expected reactivity outcomes.

Hydrogen Displacement

Hydrogen displacement is a reaction where a more reactive metal displaces hydrogen from an acid, releasing hydrogen gas. Zinc reacting with hydrochloric and sulfuric acids is an excellent example. However, the reaction changes when nitric acid is introduced.

When zinc reacts with \(\mathrm{HCl}\)) or \(\mathrm{H}_{2}\mathrm{SO}_{4}\)), the metal donates electrons to hydrogen ions to form hydrogen gas. Yet, in the case of \(\mathrm{HNO}_{3}\)), the acid's nitrate ions preferentially reduce, interrupting the typical hydrogen displacement process. This highlights how the nature of the acid can dictate the type of reaction and the products formed.
Understanding hydrogen displacement is fundamental for predicting how metals behave with various acids. It also illustrates the concept of "competition" among ions during reactions. Such knowledge is crucial for applications in battery technology and other fields where redox reactions play a critical role.