Problem 121 - 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 \(\mathrm{HNO}_{4}\) (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: Evaluate Reaction with H2SO4 and HCl

Zinc (Zn) reacts with both sulfuric acid (H2SO4) and hydrochloric acid (HCl) to produce hydrogen gas. The reactions are as follows: Zn + H2SO4 → ZnSO4 + H2↑ and Zn + 2HCl → ZnCl2 + H2↑. These equations show that Zn displaces hydrogen from acids to release H2 gas.

Step 2: Consider Electrochemical Series and Hydrogen Generation

In the electrochemical series, zinc is above hydrogen, which means Zn can displace hydrogen ions from acids like HCl and H2SO4, leading to the evolution of H2 gas. This is consistent with the reactions observed in Step 1.

Step 3: Investigate Reaction with HNO3

When Zinc reacts with nitric acid (HNO3), H2 gas is not produced. Instead, a different reaction occurs where nitrate ions (NO3^-) are reduced to gases like NO or NO2. In this process, Zn is oxidized to Zn2+.

Step 4: Surface NO3- Reduction Preference

Unlike H2SO4 and HCl, nitric acid (HNO3) contains the nitrate ion (NO3^-), which is a strong oxidizing agent. NO3^- is reduced preferentially over the hydrogen ions to form nitrogen oxides, thus preventing the formation of hydrogen gas.

Step 5: Compare Acids' Oxidizing Strength

Nitric acid is a much stronger oxidizing agent compared to sulfuric acid and hydrochloric acid. This is because NO3^- can accept electrons more readily than the hydronium ions in H2SO4 and HCl, thus hindering H2 gas evolution.

Key Concepts

Zinc Reaction with Acids

When zinc (Zn) comes into contact with acids like sulfuric acid (\(\mathrm{H}_{2} \mathrm{SO}_{4}\)) and hydrochloric acid (\(\mathrm{HCl}\)), a reaction occurs that produces hydrogen gas (\(\mathrm{H}_{2}\)). This happens because zinc displaces or replaces the hydrogen ions present in the acid. For example, in these reactions:
\(\mathrm{Zn} + \mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{ZnSO}_{4} + \mathrm{H}_{2}\uparrow\)
\(\mathrm{Zn} + 2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_{2} + \mathrm{H}_{2}\uparrow\).

• In each case, zinc takes the place of hydrogen ions, leading to the release of hydrogen gas, seen as bubbles.
• This type of reaction is classified as a single displacement reaction, where zinc displaces hydrogen, demonstrating its activity.

Reaction with these acids is straightforward for zinc because it is more reactive than hydrogen in the electrochemical series, allowing for easy hydrogen displacement.

Displacement Series

The displacement series, also known as the electrochemical series, orders metals by their reactivity with respect to hydrogen. In this series, zinc (Zn) is positioned above hydrogen, meaning zinc is more reactive and can replace hydrogen ions from an acid.

• This reactivity ranking is crucial when predicting displacement reactions, as it explains why zinc can successfully react with acids like \(\mathrm{HCl}\) and \(\mathrm{H}_{2} \mathrm{SO}_{4}\) to generate hydrogen gas.
• The higher placement of zinc above hydrogen in the series is due to its greater tendency to lose electrons and form positive ions (\(\mathrm{Zn}^{2+}\)), making it an effective reducing agent in these reactions.

This placement underlines zinc's capability to push out hydrogen from its compounds, leading to the formation of \(\mathrm{H}_{2}\). Understanding the position of elements in the electrochemical series helps predict their interactions with acids and other substances.

Oxidizing Agents

Oxidizing agents are substances that have the ability to accept electrons from other substances during a chemical reaction, facilitating oxidation. In the context of zinc and nitric acid (\(\mathrm{HNO}_{3}\)), the key player is the nitrate ion (\(\mathrm{NO}_{3}^{-}\)).

• Unlike sulfuric and hydrochloric acids, \(\mathrm{HNO}_{3}\) does not release hydrogen gas when reacting with zinc. Instead, the nitrate ion acts as a potent oxidizing agent.
• \(\mathrm{NO}_{3}^{-}\) ions are reduced, typically to nitrogen oxides such as nitric oxide (\(\mathrm{NO}\)) or nitrogen dioxide (\(\mathrm{NO}_{2}\)), rather than allowing free hydrogen ions to form \(\mathrm{H}_{2}\) gas.

The presence of nitrate ions leads to the oxidation of zinc, forming \(\mathrm{Zn}^{2+}\) ions, without the evolution of hydrogen gas. This strong oxidizing behavior of \(\mathrm{HNO}_{3}\) makes it capable of altering the usual outcome seen in other acid reactions, highlighting the distinct chemical properties of different acids.