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: Analyze Statement (a)

Statement (a) states that ammonia is oxidized to \(\mathrm{NO}_{2}\) by oxygen at \(800^{\circ} \mathrm{C}\) in the presence of a platinum catalyst. This is incorrect because ammonia is oxidized to \(\mathrm{NO}\), not \(\mathrm{NO}_{2}\), in the presence of platinum at high temperature. Hence, statement (a) is incorrect.

Step 2: Analyze Statement (b)

Statement (b) states that nitric acid turns yellow upon standing. This is correct because nitric acid can decompose, releasing nitrogen dioxide, a brown gas that can dissolve back into the acid, giving it a yellow color.

Step 3: Analyze Statement (c)

Statement (c) indicates that colloidal sulphur is formed when \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through a nitric acid solution. This is correct as the reaction between \(\mathrm{H}_{2} \mathrm{~S}\) and \(\mathrm{HNO}_{3}\) leads to the oxidation of hydrogen sulfide, resulting in the precipitation of colloidal sulphur.

Step 4: Analyze Statement (d)

Statement (d) claims that \(\mathrm{N}_{2} \mathrm{O}_{3}\) dissolves in water to give a pale blue solution. This is incorrect because \(\mathrm{N}_{2} \mathrm{O}_{3}\) dissolves in water to form nitric acid (\(\mathrm{HNO}_{2}\)), which is colorless. The pale blue color mentioned in the statement is incorrect.

Key Concepts

Ammonia oxidation

Ammonia oxidation is a critical step in chemical processes, often carried out at high temperatures in the presence of a catalyst. In industrial applications, this reaction primarily utilizes a platinum-rhodium catalyst and happens at temperatures around 800°C. The critical outcome of ammonia (\( \text{NH}_3 \)) oxidation is forming nitric oxide (\( \text{NO} \)), not nitrogen dioxide (\( \text{NO}_2 \)) as some might misinterpret.

The simplified reaction equation is:

• 4 \( \text{NH}_3 \) + 5 \( \text{O}_2 \) → 4 \( \text{NO} \) + 6 \( \text{H}_2\text{O} \)

Here, ammonia acts as the reducing agent while oxygen functions as the oxidizing agent. The product, \( \text{NO} \), is crucial in synthesizing nitric acid, a vital industrial chemical. Understanding the correct outcome of ammonia oxidation helps avoid errors in chemical production and experimentation.

Nitric acid decomposition

Nitric acid (\( \text{HNO}_3 \)) is known to decompose upon standing, turning a yellowish color. This color change happens because nitric acid breaks down logically over time, especially if exposed to light.

The decomposition process produces nitrogen dioxide (\( \text{NO}_2 \)), a brown gas that can dissolve back into the acid, thereby altering its color.

• The decomposition reaction can be represented as:\( 4 \text{HNO}_3 → 4 \text{NO}_2 + 2 \text{H}_2\text{O} + \text{O}_2 \)

The presence of \( \text{NO}_2 \) not only changes the appearance of nitric acid but also its properties. The yellow tint is typically more pronounced in older samples or ones stored in less ideal conditions.

Colloidal sulphur formation

Colloidal sulphur formation is an interesting process observed when hydrogen sulfide (\( \text{H}_2\text{S} \)) gas interacts with nitric acid. This is a characteristic redox reaction where \( \text{H}_2\text{S} \) undergoes oxidation, leading to the production of elemental sulfur.

Here's what happens step-by-step:

• When \( \text{H}_2\text{S} \) is passed through \( \text{HNO}_3 \), the \( \text{H}_2\text{S} \) is oxidized, which results in the formation and precipitation of sulfur as a colloid.
• The overall chemical equation for this interaction is:\( 3 \text{H}_2\text{S} + 2 \text{HNO}_3 → 3 \text{S} + 2 \text{NO} + 4 \text{H}_2\text{O} \)

Colloidal sulphur, which is made up of tiny sulfur particles suspended in solution, is often visible as a cloudy, milky precipitate.

Nitrogen oxides reactions

Nitrogen oxides are a variety of compounds that contain nitrogen and oxygen in different ratios. Common examples of these are \( \text{NO}, \text{NO}_2, \text{N}_2\text{O}, \text{N}_2\text{O}_3 \), and \( \text{N}_2\text{O}_5 \).

Each of these oxides behaves differently when dissolved in water. For instance, \( \text{N}_2\text{O}_3 \), comprised of nitric oxide and more nitrogen dioxide in equilibrium, forms a solution that transforms into nitrous acid (\( \text{HNO}_2 \)), assuming an incorrect color association of pale blue might arise due to misconceptions or experimental errors.

• The equation for \( \text{N}_2\text{O}_3 \) in water is: \( \text{N}_2\text{O}_3 + \text{H}_2\text{O} → 2 \text{HNO}_2 \)

A mastery of how nitrogen oxides interact provides insights into larger applications, including atmospheric chemistry and pollution management.