Problem 122 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following statement (s) is /are correct? (a) Ammonia is oxidized to \(\mathrm{NO}_{2}\) by oxygen at \(800^{\circ} \mathrm{C}\) in presence of a catalyst platinum (b) Nitric acid on standing slowly turns yellow. (c) Colloidal sulphur is formed when \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through nitric acid solution (d) \(\mathrm{N}_{2} \mathrm{O}_{3}\) gas dissolves in water giving a pale blue solution.

Step-by-Step Solution

Answer

Statements (b) and (c) are correct.

Step 1: Understanding Statement (a)

The reaction involves the oxidation of ammonia (\(\mathrm{NH}_3\)) to \(\mathrm{NO}_{2}\) using oxygen with a platinum catalyst. The correct reaction should be the formation of \(\mathrm{NO}\) instead of \(\mathrm{NO}_{2}\), typically done at \(800^{\circ}\,\mathrm{C}\). Therefore, statement (a) is not correct.

Step 2: Evaluation of Statement (b)

Over time, nitric acid may turn yellow due to the formation of nitrogen dioxide gas (\(\mathrm{NO}_{2}\)), which slightly dissolves in the solution and gives it a yellow tint. This statement is correct.

Step 3: Analysis of Statement (c)

Passing \(\mathrm{H}_2\mathrm{S}\) gas through nitric acid results in the formation of colloidal sulphur. The reaction occurs as the \(\mathrm{H}_2\mathrm{S}\) is oxidized by the strong oxidizing nature of nitric acid to form sulphur. Thus, this statement is correct.

Step 4: Review of Statement (d)

When \(\mathrm{N}_2\mathrm{O}_3\) gas is dissolved in water, it forms a mixture of nitrous acid, not a pale blue solution. Therefore, this statement is not correct.

Key Concepts

Ammonia Oxidation

Ammonia (NH_3) oxidation is an important process in the industrial production of nitric acid. During this chemical process, ammonia is oxidized by oxygen in the presence of a catalyst. A platinum catalyst is typically used, which facilitates the conversion of ammonia mainly into nitrogen monoxide (NO) at a high temperature of about 800^\circ\mathrm{C}.
This exothermic reaction is a crucial step in the Ostwald process, which is a major industrial method for producing nitric acid. The balanced chemical equation representing this reaction is:
\[4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}\]
It's important to note that not NO_2 but NO is produced primarily, contrary to some misconceptions.

Nitric Acid Properties

Nitric acid (HNO_3) is a highly corrosive and strong mineral acid. When exposed to light or high temperatures, nitric acid can decompose over time, leading to the formation of nitrogen dioxide (NO_2), a red-brown gas. This phenomenon is responsible for the yellowing of nitric acid upon standing.

Key properties of nitric acid include:

• Highly corrosive nature
• Strong oxidizing ability
• Density greater than that of water
• Fuming in moist air due to nitrogen dioxide escape

The yellow coloration is evident because the dissolved NO_2 imparts this tint, confirming statement (b) of the exercise.

Colloidal Sulphur Formation

Colloidal sulphur forms through a chemical reaction that involves hydrogen sulfide gas (H_2S) and nitric acid. When H_2S gas is introduced into a solution of nitric acid, H_2S gets oxidized by the strong oxidizing agent, nitric acid. This results in the formation of sulphur in a colloidal form, which is a suspension of tiny particles dispersed in a medium.

The reaction can be represented as:
\[3\text{H}_2\text{S} + 2\text{HNO}_3 \rightarrow 3\text{S} + 2\text{NO} + 4\text{H}_2\text{O}\]
The colloidal form of sulphur arises because the newly formed sulphur particles are extremely small, not settling out, which gives the mixture a turbid appearance.

Chemical Reactions Analysis

Analysis of chemical reactions involves understanding the reactants, products, conditions, and processes involved. Each reaction follows specific pathways based on electron exchange and molecular interactions. In the context of this exercise, each statement asked the student to apply knowledge about chemistry reactions and conditions.

Correct chemical reactions rely on several factors:

• Proper identification of reactants and products
• Understanding reaction conditions such as temperature and catalysts required
• Balancing chemical equations
• Recognizing oxidation-reduction aspects

Understanding these dynamics allows us to evaluate reactions like those involving N_2O_3 when it's dissolved in water, leading to the formation of nitrous acid and not a pale blue solution, as commonly misconstrued.