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

The mixture 0201(0201 ext{He, Ne}02010201) is not adsorbed at 173 K.

Step 1: Understand the Adsorption Process

Adsorption is a process where substances adhere to the surface of materials. In Dewar's method, noble gases are adsorbed onto coconut charcoal. This process is influenced by factors such as temperature and the specific adsorption characteristics of each gas.

Step 2: Consider Temperature Effects on Adsorption

At lower temperatures, like the given 173 K, gases with higher atomic weight are generally more easily adsorbed because they have stronger van der Waals forces. This means heavier noble gases will be adsorbed more proficiently than lighter gases.

Step 3: Review Noble Gas Properties

The noble gases in order of increasing molecular weight are: He (Helium), Ne (Neon), Ar (Argon), Kr (Krypton), Xe (Xenon). With increasing molecular weight, the van der Waals forces increase, thus affecting adsorption.

Step 4: Identify the Least Adsorbed Mixture

Among the options, He and Ne are the lightest noble gases. Due to their low molecular weights and weak van der Waals forces, they are the least adsorbed at 173 K on coconut charcoal compared to the other pairs that include heavier noble gases.

Key Concepts

Noble Gases

Noble gases are a set of elements located in Group 18 of the periodic table. They include helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe). Noble gases are unique because they have a complete outer shell of electrons, making them extremely stable and unreactive under normal conditions. This characteristic also results in them existing as monoatomic gases instead of forming molecules. Due to their lack of chemical reactivity, noble gases were historically difficult to identify and isolate.

• Helium is the lightest and second most abundant element in the universe, primarily found in stars.
• Neon is famous for its bright spectral lines, hence its use in neon signs.
• Argon makes up about 1% of the Earth's atmosphere, being the most abundant noble gas on Earth.
• Krypton and xenon are less common but are used in specialized lighting and imaging technologies.

Understanding these properties helps in applications like Dewar's method where the separation and identification of these gases can be executed effectively.

Adsorption

Adsorption is a process where one substance becomes attached to the surface of another substance. This phenomenon differs from absorption, where a substance permeates or is dissolved by a liquid or solid. In the context of Dewar's method, adsorption plays a crucial role as noble gases adhere to the surface of coconut charcoal at a given temperature. Several factors influence adsorption:

• Temperature: Lower temperatures generally increase adsorption, as molecules move slower, allowing more to adhere to the surface.
• Surface Area: Larger surface areas provide more space for gases to adhere.
• Gas-Surface Interactions: Stronger van der Waals forces between the gas and the surface will result in increased adsorption.

In the Dewar's method, the focus is on exploiting these forces to effectively separate noble gases as they are introduced to coconut charcoal.

Coconut Charcoal

Coconut charcoal, also known as activated carbon derived from coconut shells, plays a pivotal role in Dewar's method of separation by serving as the adsorbent material. It's noted for having a high surface area due to its porous structure, which makes it ideal for adsorbing gases. Some benefits of coconut charcoal include:

• Environmentally friendly: Sourced from renewable coconut shells.
• High adsorption capacity: The large surface area from pore structures allows more gas molecules to adhere.
• Versatility: Used in air purification, water treatment, and gas separation processes.

This efficiency in gas adsorption makes coconut charcoal a preferred material in not just the separation of noble gases but in various filtration applications as well.

Atomic Weight

Atomic weight, which is sometimes called atomic mass, refers to the average mass of atoms of an element, measured in atomic mass units (u). The atomic weight is a crucial factor in determining how gases interact with materials like coconut charcoal in adsorption processes. How atomic weight impacts adsorption:

• Heavier gases have a greater tendency to adsorb due to their larger van der Waals forces.
• Lighter gases, such as helium and neon, are less affected by van der Waals forces and thus not readily adsorbed.
• In Dewar's method, gases with a higher atomic weight, such as krypton and xenon, will be more strongly adsorbed compared to lighter gases.

Understanding atomic weight and its role in adsorption helps in effectively separating noble gases, thereby influencing the choice of method and materials used.

van der Waals Forces

Van der Waals forces are weak attractive forces that arise between atoms or molecules, particularly non-polar entities like noble gases. These forces come into play significantly during adsorption processes employed in Dewar's method. They originate from the temporary dipoles created when electrons around an atom or molecule are not symmetrically distributed. Key points about van der Waals forces include:

• These forces increase with the size and weight of the molecule, meaning heavier noble gases experience stronger van der Waals interactions.
• As van der Waals forces strengthen, gases are more easily adsorbed onto surfaces, like that of coconut charcoal.
• Since helium and neon have small sizes and low atomic weights, they exhibit the weakest van der Waals forces, making them less likely to be adsorbed.

In practical applications, understanding these forces helps in designing efficient systems for gas separation and enhances the understanding of intermolecular forces overall.