Problem 40 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

(b) NO₃⁻ ion is reduced in preference to hydronium ion.

Step 1: Understand Reaction of Zn with Different Acids

Zinc (Zn) reacts with acids to produce hydrogen gas. The general reaction with an acid like HCl or H₂SO₄ can be written as: \( \text{Zn} + 2\text{H}^+ \rightarrow \text{Zn}^{2+} + \text{H}_2 \). However, the reaction with nitric acid (HNO₃) is different due to the presence of the \(\text{NO}_3^-\) ion, which itself requires reduction.

Step 2: Analyze Zinc's Reactivity in Electrochemical Series

In the electrochemical series, zinc is placed above hydrogen, which means zinc can displace hydrogen from acids like HCl or H₂SO₄ to release hydrogen gas. Therefore, options (a) suggest that zinc being above hydrogen is why it reacts with HCl and H₂SO₄ to produce H₂ gas.

Step 3: Consider NO₃⁻ Ion Reduction by Zn with HNO₃

When zinc reacts with HNO₃, the nitrate ion (NO₃⁻) is reduced instead of hydronium ions because NO₃⁻ is a stronger oxidizing agent than the H⁺ ions. This occurs because \( \text{NO}_3^- \) can be reduced to form nitrogen-containing compounds like NO₂. Thus, hydrogen gas is not produced. This aligns with option (b).

Step 4: Evaluate Other Options

Option (c) states that zinc acts as an oxidizing agent, which is incorrect since zinc acts as a reducing agent, donating electrons to reduce the NO₃⁻ ion. Option (d) suggests that HNO₃ is a weaker acid, which is incorrect as HNO₃ is actually a strong acid.

Step 5: Conclusion

The given statements need to explain why Zn gives H₂ gas with HCl and H₂SO₄ but not with HNO₃. Zinc's inability to produce H₂ with HNO₃ is best explained because the NO₃⁻ ion is reduced in preference to the hydronium ion, aligning with option (b).

Key Concepts

Electrochemical Series

The electrochemical series is a table that ranks elements based on their ability to be oxidized or reduced. In this series, zinc (Zn) is positioned above hydrogen. This positioning indicates that zinc can easily release electrons and reduce hydronium ions, which are present when acids like hydrochloric acid (HCl) or sulfuric acid (H₂SO₄) are dissolved in water. The general reaction can be represented as:

• Zinc releases two electrons as it is oxidized: \( \text{Zn} \rightarrow \text{Zn}^{2+} + 2e^- \)
• Hydronium ions gain electrons: \( 2\text{H}^+ + 2e^- \rightarrow \text{H}_2 \)

As a result of this exchange, hydrogen gas is released when zinc reacts with these acids. This fundamental principle guides the understanding of zinc's behavior in the presence of acids, which ultimately revolves around electron exchange dictated by the electrochemical series.

Reduction of Nitrate ion

When zinc reacts with nitric acid (HNO₃), the presence of nitrate ions (\(\text{NO}_3^-\)) significantly alters the typical reaction pathway observed with other acids. Instead of producing hydrogen gas, zinc facilitates the reduction of nitrate ions due to its electron-donating ability.

• Nitrate ions act as strong oxidizing agents.
• They are reduced in preference to the hydronium ions found in other acids.
• During the reaction, products like nitrogen dioxide \((\text{NO}_2)\) can form instead, without releasing any hydrogen gas.

The reduction of \(\text{NO}_3^-\) tends to proceed because it is thermodynamically more favorable given the conditions of the reaction with zinc, aligning with how strong oxidizing agents behave.

Comparative Reactivity

To understand why zinc reacts differently with various acids, it's essential to consider the reactivity or strength of the acids involved. While all acids can release \(\text{H}^+\) ions:

• HCl and H₂SO₄ release \(\text{H}^+\) ions which, thanks to zinc's position in the electrochemical series, can be successfully reduced by zinc to produce hydrogen gas.
• In contrast, HNO₃ is a strong acid but uniquely has \(\text{NO}_3^-\) ions, which behave as strong oxidizing agents.
• This means zinc's electrons are more effectively utilized in reducing the nitrate ions rather than hydrogen ions.

Hence, the reactivity of zinc in these reactions is not merely about the acid's strength, but also about the solution chemistry and the nature of ions present. This aspect of reactivity ties into zinc's unique chemical interactions, underlying the importance of recognizing the available ions for reaction.