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 Process

To solve the problem, we need to understand the adsorption process. Adsorption is the accumulation of substances at the surface rather than in the bulk of a medium. In this context, \(H_2\) gas is adsorbed on activated charcoal, a common adsorbent.

Step 2: Analyzing Critical Temperature

Critical temperature is the temperature above which a gas cannot be liquefied, regardless of pressure. Gases with low critical temperatures, like \(H_2\), are difficult to condense and are generally not easily adsorbed.

Step 3: Evaluating van der Waals Forces

Van der Waals forces are the intermolecular forces present between molecules. Stronger van der Waals forces lead to better adsorption performance.

Step 4: Understanding Hydrogen Gas Properties

Hydrogen (H_2) has very weak van der Waals forces and a very low critical temperature, making it less easily adsorbed.

Step 5: Selecting the Appropriate Explanation

Given the properties of \(H_2\), it is adsorbed to a lesser extent mainly due to its very weak van der Waals forces as compared to easily liquefiable gases.

Key Concepts

van der Waals Forces

In the simplest terms, van der Waals forces are weak attractions between molecules that do not involve a covalent bond or the sharing of electrons. These forces play a crucial role in the process of adsorption, which is the adherence of gas molecules onto a surface like activated charcoal.

There are different types of van der Waals forces,:

• London Dispersion Forces: Fluctuations in the electron cloud lead to these forces. They are generally weak and short-range.
• Dipole-Dipole Interactions: Seen in molecules with permanent dipoles, contributing stronger attractions.
• Hydrogen Bonds: Sometimes included as a special type of dipole-dipole interaction, these are typically stronger.

For gases like hydrogen \(H_2\), which have weak van der Waals forces, adsorption on materials like activated charcoal is minimal. Understanding these forces is key to predicting how different gases will interact with surfaces.

Critical Temperature

Critical temperature is a fundamental property of a substance that greatly influences its ability to be adsorbed or condensed. It is defined as the highest temperature at which a gas can be converted into a liquid, regardless of pressure.

For hydrogen \(H_2\), the critical temperature is extremely low, at about 33 K (-240 °C). This low critical temperature means:

• Hydrogen remains a gas under normal conditions.
• It is challenging to liquefy, making adsorption onto surfaces difficult.

The critical temperature helps determine how well a gas can be adsorbed on materials such as activated charcoal. Gases with higher critical temperatures, which are easier to liquefy, typically exhibit better adsorption characteristics.

Activated Charcoal

Activated charcoal is a form of carbon processed to have small, low-volume pores that increase its surface area. This large surface area makes it highly effective for adsorption processes.

In the context of gas adsorption:

• The porous structure allows gases to adhere to the surface more efficiently.
• Activated charcoal is often used to adsorb impurities or certain gases due to these properties.
• It works best with gases that have higher van der Waals forces and critical temperatures.

However, because hydrogen \(H_2\) gas has low van der Waals forces and critical temperature, it is not readily adsorbed. Despite being a potent adsorbent, activated charcoal has limitations with gases that do not interact strongly with its surface.