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) says Ammonia is oxidized to \(\mathrm{NO}_{2}\) by oxygen at \(800^{\circ} \mathrm{C}\) in the presence of a platinum catalyst. The correct formation is that ammonia is oxidized to \(\mathrm{NO}\), not \(\mathrm{NO}_{2}\) under these conditions. Thus, this statement is incorrect.

Step 2: Analyze Statement (b)

Statement (b) states that nitric acid on standing slowly turns yellow. This is correct. Nitric acid decomposes slowly in the presence of light or heat to form oxides of nitrogen, which impart a yellow color.

Step 3: Analyze Statement (c)

Statement (c) tells us that colloidal sulfur is formed when \(\mathrm{H}_{2} \mathrm{~S}\) gas is passed through nitric acid. This is correct. The reaction oxidizes hydrogen sulfide to sulfur, forming a colloidal solution.

Step 4: Analyze Statement (d)

Statement (d) claims that \(\mathrm{N}_{2} \mathrm{O}_{3}\) gas dissolves in water giving a pale blue solution. This is incorrect. \(\mathrm{N}_{2} \mathrm{O}_{3}\) actually forms a mixture of nitrous acid and nitric acid instead, and does not form a notably colored solution.

Key Concepts

Ammonia Oxidation

Ammonia oxidation is a crucial industrial process used primarily in the production of nitric acid. In this oxidation process, ammonia (\(\text{NH}_3\)) is reacted with oxygen in the presence of high temperatures and a catalyst, usually platinum. This chemical reaction transforms ammonia into nitric oxide (\(\text{NO}\)), not nitrogen dioxide (\(\text{NO}_2\)) as mistakenly believed in some contexts. The formula for this oxidation is:
\[4 \text{NH}_3 + 5 \text{O}_2 \rightarrow 4 \text{NO} + 6 \text{H}_2\text{O}\]This process typically occurs at approximately 800°C, where platinum serves as an efficient catalyst to speed up the reaction without undergoing any permanent chemical transformation itself.

• Platinum enables the reaction to proceed quickly and at a lower temperature than normally required.
• This is a highly exothermic reaction, meaning it releases a significant amount of energy.

These properties make the platinum-catalyzed ammonia oxidation a cornerstone process in chemical manufacturing.

Nitric Acid Decomposition

Nitric acid (\(\text{HNO}_3\)) is a strong acid commonly used in laboratories and industries. One interesting property of nitric acid is its tendency to slowly decompose over time or when exposed to light and heat. Upon decomposition, it produces nitrogen dioxide (\(\text{NO}_2\)), water (\(\text{H}_2\text{O}\)), and oxygen (\(\text{O}_2\)), which gives the acid a yellowish tint.The decomposition of nitric acid can be expressed as:
\[2 \text{HNO}_3 \rightarrow \text{NO}_2 + \frac{1}{2} \text{O}_2 + \text{H}_2\text{O}\]

• The release of nitrogen dioxide is responsible for the change in color as it imparts the yellow hue.
• This reaction is also an example of how light can drive chemical changes, known as photochemical reaction.

When handling nitric acid, it's important to store it properly to minimize decomposition. Doing so will maintain its concentration and effectiveness in various applications.

Catalysis in Chemistry

Catalysis is a process by which the rate of a chemical reaction is increased, often using a substance known as a catalyst. In many reactions, like the oxidation of ammonia, catalysts are key for improving both the speed and efficiency of the reaction.

A catalyst remains unchanged at the end of the reaction, able to influence the process by providing an alternate reaction pathway that has a lower activation energy. Platinum is a common catalyst used in ammonia oxidation due to its high catalytic activity and ability to withstand high temperatures.

• Using catalysts often allows industrial processes to operate more economically and with greater control.
• Catalysts have widespread applications in everything from industrial production to our own biological processes, like the enzymes speeding up metabolism in the human body.

Though catalysts don't change the quantities of products formed, they make industrial processes like ammonia oxidation scalable and viable for mass production.