Problem 157 - A Complete Resource Book in Chemistry for JEE Main

Question

\(\mathrm{H}_{2}\) gas is adsorbed on activated charcoal to a very little extend in comparison to easily liquefiable gases due to (a) Very strong van der Waal's interaction (b) Very high critical temperature (c) Very low critical temperature (d) Very weak van der Waals forces

Step-by-Step Solution

Answer

(d) Very weak van der Waals forces

Step 1: Understanding the Problem

We're asked to identify why H_{2} gas is weakly adsorbed on activated charcoal compared to other gases. Adsorption is influenced by the interactions between gas molecules and the surface they are adsorbing onto.

Step 2: Defining Key Terms

Critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied. Van der Waals forces are weak intermolecular forces between molecules.

Step 3: Explaining the Options

Option (a) suggests strong van der Waals forces, leading to high adsorption. Option (b) involves a high critical temperature, indicating molecules are easily liquefied. Option (c) indicates a low critical temperature, while option (d) suggests weak van der Waals forces, leading to poor adsorption.

Step 4: Analyzing Hydrogen Gas

H_{2} is a light, nonpolar molecule with very weak van der Waals forces and very low critical temperature. As a result, it is less easily liquefiable compared to heavier gases.

Step 5: Choosing the Correct Answer

The limited adsorption of H_{2} on activated charcoal can be attributed to its very weak van der Waals forces, making option (d) the correct answer.

Key Concepts

Van der Waals forces

Van der Waals forces are the weak attractive forces that exist between all molecules, whether they are polar or nonpolar. These forces come into play due to the temporary fluctuations in electron density within molecules, creating temporary dipoles.
These temporary dipoles can induce dipoles in nearby molecules leading to an attractive interaction. Although these forces are relatively weak compared to other types of intermolecular forces like hydrogen bonds or ionic bonds, they are crucial in determining the physical properties of molecules, particularly gases.

• Van der Waals forces are directly related to the molecular size; larger molecules exhibit stronger forces.
• Though weak, they play a significant role in adsorption processes, influencing how gases interact with surfaces like activated charcoal.
• Molecules like hydrogen ( H_{2} ) being light and nonpolar, have minimal van der Waals interactions, resulting in lower adsorption rates.

Critical temperature

The critical temperature of a substance is the highest temperature at which it can be converted to a liquid state by applying pressure. Above this temperature, the kinetic energy of the molecules is too high to allow intermolecular attractions to bring them together into a liquid phase.
Critical temperature is an intrinsic property of substances that significantly affects their adsorption behavior. Generally, substances with higher critical temperatures are more easily liquefiable and thus may adsorb better on surfaces.

• Non-polar gases, like hydrogen ( H_{2} ), tend to have low critical temperatures.
• The low critical temperature implies that even at high pressures, hydrogen cannot easily condense into liquid, reducing its adsorption ability.
• In contrast, gases with higher critical temperatures exhibit stronger physical adsorption due to their ability to liquefy more easily.

Activated charcoal

Activated charcoal, also known as activated carbon, is a form of carbon that is highly porous and offers a large surface area for adsorption. This material is commonly used for gas adsorption because its porous structure provides numerous sites for molecules to adhere to.
Activated charcoal works effectively with gases that can be easily liquefied due to the presence of significant van der Waals attractions.

• It is especially good at adsorbing toxins and impurities, which tend to have larger molecules with stronger van der Waals forces.
• The effectiveness of activated charcoal in adsorbing gases depends largely on the nature of the gas, including its molecular weight, polarity, and critical temperature.
• Although gases like H_{2} exhibit weak adsorption on activated charcoal due to their lower molecular size and weaker van der Waals forces, activated charcoal remains a versatile adsorbent in various applications.