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: Understanding the Reaction

When zinc (\(\mathrm{Zn}\)) reacts with non-oxidizing acids like hydrochloric acid (\(\mathrm{HCl}\)) and sulfuric acid (\(\mathrm{H}_{2}\mathrm{SO}_{4}\)), it produces hydrogen gas (\(\mathrm{H}_{2}\)). However, when zinc reacts with nitric acid (\(\mathrm{HNO}_{3}\)), it does not release hydrogen gas.

Step 2: Identify the Role of Nitric Acid

Nitric acid (\(\mathrm{HNO}_{3}\)) is a strong oxidizing agent. Instead of producing hydrogen gas when reacting with zinc, \(\mathrm{HNO}_{3}\) gets reduced. The nitrate ion (\(\mathrm{NO}_{3}^{-}\)) is often reduced to nitrogen oxides (such as \(\mathrm{NO}\)). Therefore, the reduction of nitrate ions is preferred over the reduction of hydronium ions to hydrogen gas.

Step 3: Analyze the Electrochemical Series

In the electrochemical series, zinc is above hydrogen, meaning it can displace hydrogen from acids like \(\mathrm{HCl}\) and \(\mathrm{H}_{2}\mathrm{SO}_{4}\) to release hydrogen gas. \(\mathrm{HNO}_{3}\)’s strong oxidizing ability prevents this displacement as it reduces \(\mathrm{NO}_{3}^{-}\) instead.

Step 4: Conclusion and Correct Answer

Based on the behavior of zinc with \(\mathrm{HNO}_{3}\), it is not the weakness of \(\mathrm{HNO}_{3}\) as an acid but its oxidizing property that prevents \(\mathrm{H}_{2}\) production. Therefore, the correct option explaining why \(\mathrm{Zn}\) does not liberate \(\mathrm{H}_{2}\) with \(\mathrm{HNO}_{3}\) is (d) \(\mathrm{NO}_{3}^{-}\) is reduced in preference to hydronium ion.

Key Concepts

Electrochemical Series

The electrochemical series is a fundamental tool in chemistry that helps us understand the relative tendency of different substances to gain or lose electrons. Often visualized as a list of reduction potentials, this series ranks elements and compounds based on their ability to be reduced.

This list is critical when explaining reactions involving metals and acids. For instance, zinc (Zn) finds its place above hydrogen in the electrochemical series. This positioning indicates that zinc can displace hydrogen from solutions like hydrochloric acid (HCl) and sulfuric acid (H₂SO₄) to produce hydrogen gas. It's all about who can "steal" the electrons faster, and in this case, zinc outcompetes hydrogen, liberating it from the acid in gas form.

However, being higher in the series doesn't always guarantee hydrogen gas production, particularly in the presence of a strong oxidizing agent such as nitric acid (HNO₃). Here, the magic of the series comes into play again, explaining why an expected reaction does not happen.

Oxidizing Agents

When a substance acts as an oxidizing agent, it accepts electrons in a chemical reaction. In essence, it gets reduced while causing the other substance to get oxidized.

Nitric acid (HNO₃) is a prime example of a strong oxidizing agent. Instead of allowing reactions that generate hydrogen gas, it prefers to utilize its oxidizing strength. When zinc reacts with nitric acid, the nitrate ions (NO₃⁻) in HNO₃ are reduced to nitrogen oxides, such as NO or NO₂, instead of producing hydrogen gas. This powerful reduction ability of the nitrate ions means that the reduction of hydrogen ions to form hydrogen gas does not occur.

This competition for electrons ensures that HNO₃ acts differently compared to non-oxidizing acids like HCl or H₂SO₄. Understanding this concept is crucial for grasping why certain reactions favor the production of gasses other than hydrogen.

Hydrogen Gas Production

Understanding hydrogen gas production in chemical reactions requires a firm grasp on both the nature of the reactants and the products formed.

In typical reactions between metals and acids, such as zinc with hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), hydrogen gas (H₂) is released. This occurs because the metal displaces the hydrogen ions in the acid, liberating them as gas. Such reactions are straightforward when both reactants are non-oxidizing.

However, the story changes when nitric acid (HNO₃) enters the picture. As we know, HNO₃ is more interested in reducing itself as an oxidizing agent, as opposed to allowing hydrogen gas to form. Instead, it forces a different set of chemical reactions, leading to products like nitrogen oxides rather than hydrogen gas.

The absence of hydrogen gas in reactions with nitric acid is a striking example of how the chemical nature of reactants dictates the pathway of a chemical reaction.