Problem 43 - A Complete Resource Book in Chemistry for JEE Main

Question

A bottle of dry ammonia and a bottle of dry hydrogen chloride connected through a long tube are opened simultaneously at both ends, the white ammonium chloride ring first formed will be (a) at the centre of the tube (b) near the hydrogen chloride bottle (c) near the ammonia bottle (d) throughout the length of the tube

Step-by-Step Solution

Answer

(b) near the hydrogen chloride bottle.

Step 1: Identifying the Gases

First, we need to identify the gases involved: Ammonia (NH₃) and Hydrogen Chloride (HCl). When these gases meet, they react to form a white solid, Ammonium Chloride (NH₄Cl). We must determine where these gases meet and react within the tube.

Step 2: Understanding Gas Diffusion

Gases diffuse at different rates depending on their molecular weights. According to Graham's law of effusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. Calculate the molar masses: Ammonia (NH₃) = 14 (N) + 3 (H × 1) = 17 g/mol, Hydrogen Chloride (HCl) = 1 (H) + 35.5 (Cl) = 36.5 g/mol.

Step 3: Calculating Diffusion Rates

Using Graham's Law, the ratio of diffusion rates of Ammonia to Hydrogen Chloride is given by: \[\frac{{\text{rate of NH}_3}}{{\text{rate of HCl}}} = \sqrt{\frac{{M_{\text{HCl}}}}{{M_{\text{NH}_3}}}} = \sqrt{\frac{{36.5}}{{17}}} \approx 1.47\]This means Ammonia diffuses faster than Hydrogen Chloride.

Step 4: Determining the Reaction Point

Since Ammonia diffuses faster, it will travel further down the tube before meeting Hydrogen Chloride. Therefore, the white ring will form closer to the Hydrogen Chloride bottle where the two gases meet.

Step 5: Drawing the Conclusion

With both gases moving towards each other, and given their diffusion rates, the ammonium chloride ring will form near the end where Hydrogen Chloride started.

Key Concepts

Gas Diffusion

Gas diffusion occurs when gas molecules spread from areas of high concentration to areas of lower concentration. Unlike liquids or solids, gases diffuse freely due to their high kinetic energy. This process is crucial in many natural and industrial processes. In the experiment with ammonia (NH₃) and hydrogen chloride (HCl), each gas starts at opposite ends of the tube. When both bottles are opened, these gases begin to diffuse towards one another.
The rate at which they diffuse depends on their molecular properties. This concept is governed by Graham's Law of Effusion. It's a fundamental part of understanding how gases behave in various environments. Gas diffusion ensures that molecules eventually mix evenly in any given space, which is especially significant in chemical reactions. By analyzing gas diffusion rates, scientists and students can predict where reactions like the formation of ammonium chloride will occur.

Molecular Weight Comparison

Molecular weight plays a vital role in gas diffusion rates. According to Graham’s Law, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass. This means lighter gases diffuse faster than heavier ones.
In our example, we compare the molecular weights of ammonia (17 g/mol) and hydrogen chloride (36.5 g/mol). Since ammonia has a lower molecular weight, it diffuses more quickly than hydrogen chloride. This characteristic allows NH₃ to travel further in the tube before meeting HCl, leading to a reaction zone closer to the hydrogen chloride source.
Understanding how molecular weight affects diffusion is critical in predicting how gases will mix and react, particularly when designing experiments and industrial processes.

Reaction Point Determination

Determining where gases will meet and react requires understanding their diffusion rates. For the discussed scenario, determining the reaction point where ammonium chloride forms relies on calculating the diffusion rates of ammonia and hydrogen chloride.
Using Graham's Law, we find that ammonia diffuses roughly 1.47 times faster than hydrogen chloride. Hence, it travels further in the same amount of time, causing the reaction to occur nearer the hydrogen chloride end of the tube.
This knowledge helps predict and control reaction sites in various chemical processes. Recognizing where gases will meet can optimize the efficiency and safety of these reactions. It's crucial in laboratory settings and industrial applications where reactions between gases are necessary.

Ammonium Chloride Formation

When ammonia and hydrogen chloride gases meet, they react quickly to form ammonium chloride ( ext{NH}_4 ext{Cl}), a solid white substance. This reaction is a classic demonstration of gas diffusion and reaction.
In the tube experiment, the point of ammonium chloride formation is visually represented by a white ring along the tube's length. This visual cue indicates where the two gases have successfully met and reacted. The rapid formation of the solid is because ammonium chloride precipitates directly from the gaseous interaction.
Understanding how and where ammonium chloride forms from ammonia and hydrogen chloride is a crucial aspect of chemical education. It demonstrates the practical application of theoretical principles like gas diffusion and molecular interactions. This experiment helps solidify the students' understanding of these essential chemistry concepts.