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

Option (a) He, Ne is not adsorbed on the charcoal.

Step 1: Understanding Dewar's Method

In Dewar's method, noble gases are separated based on their adsorption onto coconut charcoal at low temperatures, such as 173 K. The adsorption tendency depends on the size and polarizability of the gas molecules.

Step 2: Adsorption Tendencies

At 173 K, larger and more polarizable gases like Xenon (Xe) and Krypton (Kr) are preferentially adsorbed onto coconut charcoal, while smaller and less polarizable gases like Helium (He) and Neon (Ne) tend to remain unadsorbed.

Step 3: Analyzing the Options

Let's examine each option:- For (a) \( \mathrm{He, Ne} \), both He and Ne are small, less polarizable gases, hence they are less likely to adsorb.- For (b) \( \mathrm{Xe, Kr} \), both Xe and Kr are larger, more polarizable gases, likely to adsorb.- For (c) \( \mathrm{Ar, Kr} \), Ar and Kr have moderate adsorption characteristics, but still more than He and Ne.- For (d) \( \mathrm{Xe, Ar} \), Xe and Ar are larger noble gases likely to adsorb.

Step 4: Identifying the Non-adsorbed Mixture

Based on adsorption tendencies, the mixture of \( \mathrm{He, Ne} \) (option a) remains largely unadsorbed on the charcoal due to their smaller size and low polarizability.

Key Concepts

Noble Gases

Noble gases are a group of elements found in Group 18 of the periodic table. They include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). These gases are known for their chemical inertness, meaning they typically do not react with other elements. This is due to their complete outer shell of electrons, which makes them stable and unreactive.
One interesting property of noble gases is their variance in size and polarizability, which affects their behavior in physical processes. For example, helium and neon are much smaller and less polarizable compared to xenon and krypton, which influences how they interact with materials such as coconut charcoal in adsorption processes.
Noble gases are used in various applications due to their unique properties, like providing an inert atmosphere in welding, or creating bright lights in advertising signs. Their stability and unique characteristics make them an essential area of study in chemistry.

Adsorption

Adsorption is a process where atoms, ions, or molecules from a gas, liquid, or dissolved solid adhere to a surface. It is a surface phenomenon, as opposed to absorption, which involves the entire volume of the material.
This process can be influenced by various factors, including temperature, surface area of the adsorbent, and the properties of the substance being adsorbed. In Dewar's method, adsorption is used to separate noble gases by taking advantage of their different tendencies to stick to coconut charcoal at low temperatures.
The type of adsorption occurring can be physical or chemical. Physical adsorption, or physisorption, involves weak forces such as van der Waals forces, and is generally reversible. Chemical adsorption, or chemisorption, involves stronger chemical bonds, making it less reversible. In the context of noble gases and Dewar's method, physisorption is primarily at play.

Coconut Charcoal

Coconut charcoal is a form of activated carbon made from coconut shells. It is favored in adsorption processes because of its high surface area and porosity, which allow it to effectively trap various molecules.
Coconut charcoal is used in Dewar's method for separating noble gases. At 173 K, the charcoal acts as an adsorbent, preferentially holding onto larger, more polarizable noble gases like xenon and krypton, while smaller gases like helium and neon are not easily retained. This selective adsorption is crucial for the separation process.
This type of charcoal is chosen not only for its effectiveness but also for its eco-friendly production process. Made from renewable resources, coconut charcoal stands as a sustainable choice in industrial applications.

Polarizability

Polarizability is the ability of an atom or molecule to be distorted by an electric field, resulting in an induced dipole moment. This property depends on the size of the electron cloud in the atom or molecule—larger clouds are generally more polarizable.
In terms of noble gases, polarizability increases from helium to xenon. This is important in processes like adsorption used in Dewar's method. Larger and more polarizable gases, such as xenon and krypton, have stronger van der Waals forces acting on them, which enhances their ability to adhere to surfaces like coconut charcoal.
Understanding polarizability helps explain the separation of noble gases. Since smaller and less polarizable gases like helium and neon do not easily adsorb, they can be effectively separated from their larger counterparts using the adsorption method.