Problem 69 - A Complete Resource Book in Chemistry for JEE Main

Question

The gas which is liberated when \(\mathrm{PbO}_{2}\) reacts with concentrated \(\mathrm{HNO}_{3}\) (a) \(\mathrm{NO}_{2}\) (b) \(\mathrm{O}_{2}\) (c) \(\mathrm{N}_{2} \mathrm{O}\) (d) \(\mathrm{N}_{2}\)

Step-by-Step Solution

Answer

The gas liberated is \(\mathrm{NO}_2\); therefore, the answer is option (a).

Step 1: Understand the Reaction Conditions

We are given that \(\mathrm{PbO}_2\) is reacting with concentrated \(\mathrm{HNO}_3\). This is an oxidation-reduction reaction because \(\mathrm{PbO}_2\) will act as an oxidizing agent and \(\mathrm{HNO}_3\) will act as both an oxidizing and nitrating agent.

Step 2: Write and Balance the Chemical Equation

The reaction of \(\mathrm{PbO}_2\) with concentrated \(\mathrm{HNO}_3\) typically produces \(\mathrm{Pb(NO}_3)_2\), \(\mathrm{H}_2\mathrm{O}\), and \(\mathrm{NO}_2\) gas. The balanced chemical equation is:\[ \mathrm{PbO}_2 + 4\mathrm{HNO}_3 \rightarrow \mathrm{Pb(NO}_3)_2 + 2\mathrm{H}_2\mathrm{O} + 2\mathrm{NO}_2 \]

Step 3: Identify the Gas Liberated

From the balanced chemical equation, we observe that the gaseous product formed during the reaction is \(\mathrm{NO}_2\).

Step 4: Confirm with Options

The options given are (a) \(\mathrm{NO}_2\), (b) \(\mathrm{O}_2\), (c) \(\mathrm{N}_2\mathrm{O}\), and (d) \(\mathrm{N}_2\). From Step 3, we identified \(\mathrm{NO}_2\) as the gas, which matches option (a).

Key Concepts

Chemical Equations

A chemical equation is a symbolic representation of a chemical reaction. In these equations, the reactants are on the left side, the products are on the right, and an arrow points from reactants to products indicating the direction of the reaction.
Simple symbols and numbers denote elements and molecules:

• The chemical formula (like \(\mathrm{PbO}_2\) for lead(IV) oxide) represents the compounds.
• Subscripts show the number of atoms in a molecule.

Such an equation describes what happens during a chemical reaction by specifying the reactants and the resulting products. It also often gives insight into the type of reaction (such as synthesis, decomposition, or redox reactions). Understanding the symbols and numbers allows us to grasp the nature of the substances involved and their transformations. In our example, the reaction between \(\mathrm{PbO}_2\) and \(\mathrm{HNO}_3\) is a redox reaction where lead(IV) oxide acts as an oxidizing agent.

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry, ensuring that the number of atoms for each element is the same on both sides of the equation. This upholds the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction. To balance a chemical equation, follow these steps:

• Write the unbalanced equation. For example, \(\mathrm{PbO}_2 + \mathrm{HNO}_3 \rightarrow \mathrm{Pb(NO}_3)_2 + \mathrm{H}_2\mathrm{O} + \mathrm{NO}_2\).
• Count the number of atoms for each element on both sides.
• Adjust coefficients to get equal numbers of each type of atom on both sides. For example, making sure there are four \(\mathrm{HNO}_3\) molecules balances the equation: \(\mathrm{PbO}_2 + 4\mathrm{HNO}_3 \rightarrow \mathrm{Pb(NO}_3)_2 + 2\mathrm{H}_2\mathrm{O} + 2\mathrm{NO}_2\).
• Check your work to ensure the equation is balanced.

Balancing is key in predicting the amounts of products formed or reactants needed and is crucial for accurate experimental setups.

Gas Identification in Reactions

Identifying gases released during chemical reactions is integral to understanding these reactions fully. Different reactions produce different gases as byproducts, which can give crucial insights into the chemical processes occurring. For instance, in our given reaction between \(\mathrm{PbO}_2\) and \(\mathrm{HNO}_3\), we determined that \(\mathrm{NO}_2\) gas is liberated. Identifying gases involves:

• Inspecting the balanced chemical equation to see which substances on the product side are gases. In this case, it's \(\mathrm{NO}_2\).
• Understanding the reaction conditions, since the presence of heat, pressure, or catalysts could affect which gases are produced.
• Using knowledge of similar reactions and the properties of reactants to predict products.

This step is crucial in laboratory settings for maintaining safety and ensuring thorough comprehension of the reaction mechanisms.