Problem 99 - A Complete Resource Book in Chemistry for JEE Main

Question

In the Dewar's method of separation of noble gases, the mixture of noble gases is kept in contact with coconut charcoal at \(173 \mathrm{~K}\). Which one of the following gaseous mixtures is not adsorbed on to the charcoal? (a) \(\mathrm{He}, \mathrm{Ne}\) (b) \(\mathrm{Xe}, \mathrm{Kr}\) (c) \(\mathrm{Ar}, \mathrm{Kr}\) (d) \(\mathrm{Xe}, \mathrm{Ar}\)

Step-by-Step Solution

Answer

(a) He, Ne is not adsorbed.

Step 1: Understanding the Adsorption Process

Adsorption is a process where molecules of a substance (such as gases) accumulate on a solid surface. In Dewar's method, charcoal is used as the adsorbent for noble gases. The effectiveness of adsorption is influenced by factors like temperature and the surface area of the adsorbent.

Step 2: Relating Adsorption to Temperature and Atomic Mass

At a given temperature, gases with higher atomic masses are more likely to be adsorbed than those with lower atomic masses because they have stronger van der Waals forces. This means that noble gases like Xe and Kr are more effectively adsorbed than lighter ones like He and Ne.

Step 3: Analyzing Given Temperatures and Gases

The temperature given in the question is 173 K, which is relatively low. Under such conditions, gases with lower atomic masses (He and Ne) are less likely to be adsorbed compared to heavier gases (Xe, Ar, Kr) on the charcoal.

Step 4: Comparing Options

Check each option for the gases more likely to not be adsorbed. Since He and Ne have the smallest atomic masses among noble gases, a gaseous mixture containing both, such as option (a), is least likely to be adsorbed onto the charcoal at 173 K.

Key Concepts

Noble Gases

Noble gases are a group of chemical elements with very low reactivity due to their complete valence electron shells. This group consists of helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These gases are colorless, odorless, and tasteless, and they exist as monatomic gases under standard conditions. Due to their stable electron configurations, noble gases are not prone to forming chemical bonds, which makes them quite unique in the periodic table.

Noble gases are often used in lighting and lasers, as well as in creating inert atmospheres required for various industrial processes. The fact that they remain inert under most conditions makes them highly valuable in applications where reactions must be prevented. In Dewar's method, the behavior of noble gases like Xe, Kr, Ar, and the lighter He and Ne is important for determining which gases are more likely to be adsorbed by coconut charcoal under specific temperatures.

Adsorption Process

The adsorption process is a surface phenomenon where molecules of gases or liquids form a film on the surface of a solid. The solid on which these molecules gather is called the adsorbent, while the accumulated molecules are the adsorbate. Adsorption occurs due to various forces like van der Waals forces, which attract the adsorbate molecules onto the surface of the adsorbent.

Temperature and molecular weight play crucial roles in adsorption. Generally, lower temperatures favor adsorption due to reduced kinetic energy, making it easier for particles to adhere to the adsorbent surface. Heavier gases, with stronger interatomic van der Waals forces, are more readily adsorbed. In the Dewar’s method, adsorption on coconut charcoal increases with molecular weight. This means gases like xenon and krypton are likely to adhere to the surface more readily than lighter gases such as helium and neon.

Coconut Charcoal

Coconut charcoal is a type of activated carbon derived from coconut shells. It is valued for having a high surface area and porous structure, making it excellent for adsorption processes. The ability of coconut charcoal to adsorb gases or impurities is linked to its incredibly porous nature, which provides numerous binding sites for molecules.

In Dewar's method, coconut charcoal is used to selectively adsorb noble gases based on their atomic mass and the temperature conditions. While heavier noble gases like Xe and Kr are efficiently adsorbed due to their higher atomic masses and increased interaction with the surface, lighter gases such as He and Ne are not as effectively captured, especially at lower temperatures like 173 K. This selectivity makes coconut charcoal an effective tool in separating gases based on their molecular characteristics, exploiting the principles of adsorption.