Problem 40 - 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) 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

The correct answer is (b): \(\mathrm{NO}_3^-\) ion is reduced in preference to the hydronium ion.

Step 1: Understanding the Reaction Conditions

When zinc, \(\mathrm{Zn}\), reacts with acids like hydrochloric acid \(\mathrm{HCl}\), or sulfuric acid \(\mathrm{H}_2\mathrm{SO}_4\), it displaces the hydrogen ions in these acids to produce hydrogen gas \(\mathrm{H}_2\). The reaction occurs because zinc is higher than hydrogen in the electrochemical series, meaning zinc can displace hydrogen ions from acids.

Step 2: Reaction with Nitric Acid

Unlike \(\mathrm{HCl}\) and \(\mathrm{H}_2\mathrm{SO}_4\), nitric acid (\(\mathrm{HNO}_3\)) behaves differently. \(\mathrm{HNO}_3\) is a strong oxidizing agent, so when zinc reacts with it, a different reaction pathway occurs. Instead of releasing \(\mathrm{H}_2\) gas, the \(\mathrm{NO}_3^-\) ions present in \(\mathrm{HNO}_3\) are preferentially reduced, preventing the production of hydrogen gas.

Step 3: Analyzing the Options

Let's analyze the options: - Option (a) is correct because zinc's position above hydrogen in the electrochemical series allows it to release \(\mathrm{H}_2\) gas in reactions with certain acids.- Option (b) is correct because with \(\mathrm{HNO}_3\), \(\mathrm{NO}_3^-\) is reduced instead of hydrogen ions being displaced.- Option (c) is incorrect because zinc acts as a reducing agent with \(\mathrm{HNO}_3\), not oxidizing.- Option (d) is incorrect as \(\mathrm{HNO}_3\) is a strong acid, often considered stronger than \(\mathrm{H}_2\mathrm{SO}_4\) and \(\mathrm{HCl}\).

Key Concepts

Electrochemical Series

The electrochemical series, also known as the activity series, is a chart that ranks elements based on their ability to displace hydrogen ions from solutions. This list is crucial in predicting the outcomes of reactions, especially with acids. When an element is placed higher than another in the series, it means that it is more reactive.

• Zinc is positioned above hydrogen in this series, which implies that it can displace hydrogen ions from acids.
• This displacement results in the release of hydrogen gas, a common feature in reactions with metals like zinc.

Understanding the electrochemical series helps in predicting not only whether a reaction will occur, but also the nature of the products formed. It simplifies the determination of whether metals will react with acids by directly comparing their positions relative to hydrogen.

Redox Reactions

Redox (reduction-oxidation) reactions are fundamental chemical processes where electrons are transferred between substances. These reactions involve two key components: oxidation, where a substance loses electrons, and reduction, where a substance gains electrons.

• In reactions with nitric acid ( HNO_3 ), instead of hydrogen ions being displaced, the NO_3^- ions are reduced.
• This is distinctively different from what happens with hydrochloric ( HCl ) or sulfuric acid ( H_2SO_4 ), where H_2 gas is formed instead.

Nitric acid is a strong oxidizing agent, so it promotes an entirely different reaction pathway, showcasing the selective reduction of nitrate ions over hydrogen ions. Thus, understanding redox reactions is crucial in determining the behavior of metals in various acidic environments.

Acid-Base Reactions

Acid-base reactions form a cornerstone of chemical interactions and are identified by the transfer of protons ( H^+ ions) between reactants. Typically, when an acid reacts with a metal, hydrogen gas is produced.

• While HCl and H_2SO_4 are straightforward in their interactions, allowing for direct displacement of hydrogen ions, HNO_3 behaves differently.
• Despite being a strong acid, HNO_3 facilitates a reaction pathway where it acts as an oxidizing agent.

Thus, nitric acid's unique behavior deviates from the typical acid-metal interaction due to its oxidizing properties, preventing the formation of hydrogen gas. Comprehending acid-base reactions and the distinct characteristics of varying acids aids in predicting the specific outcomes of their reactions with metals such as zinc.