Problem 63 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{NH}_{4} \mathrm{Cl}(\mathrm{s})\) is heated in a test tube. Vapours are brought in contact with red litmus paper, which changes to blue and then to red. It is because of (a) formation of \(\mathrm{NH}_{3}\) and \(\mathrm{HCl}\) (b) formation of \(\mathrm{NH}_{4} \mathrm{OH}\) and \(\mathrm{HCl}\) (c) greater diffusion of \(\mathrm{HCl}\) than \(\mathrm{NH}_{3}\) (d) greater diffusion of \(\mathrm{NH}_{3}\) than \(\mathrm{HCl}\)

Step-by-Step Solution

Answer

Option (a): formation of \(\mathrm{NH}_3\) and \(\mathrm{HCl}\).

Step 1: Understand the Chemical Reaction

When ammonium chloride (\(\mathrm{NH}_4\mathrm{Cl}\)) is heated, it decomposes into ammonia (\(\mathrm{NH}_3\)) and hydrogen chloride (\(\mathrm{HCl}\)) gases. The chemical equation for this decomposition is: \[ \mathrm{NH}_4\mathrm{Cl(s)} \rightarrow \mathrm{NH}_3(g) + \mathrm{HCl}(g) \]

Step 2: Interaction with Litmus Paper

Both \(\mathrm{NH}_3\) and \(\mathrm{HCl}\) gases ascend. \(\mathrm{NH}_3\) is a basic gas, which turns red litmus paper blue when it comes into contact. Conversely, \(\mathrm{HCl}\) is an acidic gas, which turns blue litmus paper back to red.

Step 3: Analyze the Sequence of Reactions

Initially, the red litmus paper turns blue when exposed to the basic \(\mathrm{NH}_3\). As \(\mathrm{HCl}\) reaches the litmus paper next, it reacts with the litmus to turn it back to red. These sequential reactions indicate the presence of both \(\mathrm{NH}_3\) and \(\mathrm{HCl}\).

Step 4: Evaluate the Options

Option (a) - formation of \(\mathrm{NH}_3\) and \(\mathrm{HCl}\), explains why the litmus paper changes color twice. Options (b), (c), and (d) either involve incorrect reactions or explanations that don't fully support the observed changes with red litmus paper.

Key Concepts

Chemical Reaction

The decomposition of ammonium chloride is a fascinating demonstration of a chemical reaction. In essence, when ammonium chloride (\(\mathrm{NH}_4\mathrm{Cl}\) ) is heated, it breaks down into two separate gases — ammonia (\(\mathrm{NH}_3\)) and hydrogen chloride (\(\mathrm{HCl}\)). This process can be represented by the following chemical equation: \[\mathrm{NH}_4\mathrm{Cl(s)} \rightarrow \mathrm{NH}_3(g) + \mathrm{HCl}(g)\] This decomposition involves the splitting of bonds in the solid ammonium chloride, allowing it to transition into its gaseous components. When considering why this reaction happens, it's all about the energy provided by heating, which enables the bonds to break and form new substances — the hallmark of a chemical reaction.

• It's important to remember that such reactions transform substances into new forms, showing how matter interacts and changes.

Litmus Test

The litmus test is a simple, yet powerful experiment used to determine the acidity or basicity of a substance. It involves using litmus paper, which comes in two colors — red and blue. Red litmus paper turns blue when exposed to a basic substance, whereas blue litmus paper turns red when it comes into contact with an acidic substance.
The scenario with ammonium chloride's decomposition connects beautifully with the litmus test. Initially, the emitted ammonia gas turns red litmus paper blue, indicating its basic nature. As the hydrogen chloride reaches the paper, it restores the red color, confirming the presence of an acidic compound.

• This test is straightforward, but its applications are crucial in identifying chemical properties in various experiments.

Ammonia Gas

Ammonia gas (\(\mathrm{NH}_3\)) is one of the key products of ammonium chloride decomposition. It is known for its pungent smell and is classified as a basic gas. This means that ammonia can neutralize acids and will turn red litmus paper blue, showcasing its basicity.
Ammonia's role in experiments extends far beyond its behavior with litmus paper. It plays a critical part in fertilization as it's converted into ammonium compounds that benefit plant growth.

• Understanding ammonia helps in studying how gases interact in chemical reactions, influencing outcomes in chemical synthesis and industrial applications.

Hydrogen Chloride

Hydrogen chloride (\(\mathrm{HCl}\)) is produced alongside ammonia in the decomposition of ammonium chloride. In contrast to ammonia, hydrogen chloride is an acidic gas. Upon contact with blue litmus paper, it turns red, clearly illustrating its acidic properties.
This gas releases into the air and can dissolve in water to form hydrochloric acid, a significant chemical in industrial and laboratory settings.

• Its importance lies in diverse applications ranging from pH regulation to the digestion process in the human stomach, even extending to the cleaning industry.

Acid-Base Indicator

Acid-base indicators, like litmus paper, reveal the acidity or basicity of substances. They are invaluable in experiments and real-world applications to identify what's happening at a molecular level when acids and bases interact.
Litmus is one of the oldest indicators, changing colors due to the chemical bonds formed or broken by acidic or basic substances. In the experiment involving ammonium chloride, the litmus paper's color change vividly demonstrates the dynamic interplay of ammonia and hydrogen chloride.

• Utilizing acid-base indicators is essential in chemistry, offering a visual representation of invisible molecular changes, which is crucial for students and scientists alike.