Question

Which among the following gases is least adsorbed on solid at similar conditions of temperature and pressure?

• A. $\text{Cl}_{2}$
• B. $\text{NH}_{3}$
• C. $\text{SO}_{2}$
• D. $\text{H}_{2}$

Hint:

Adsorption Extent $\propto$ Critical Temperature ($T_c$) $\propto$ Intermolecular Force Strength $\propto$ Ease of Liquefaction. $\text{SO}_2$ and $\text{NH}_3$ are the undisputed champions of adsorption in standard exam questions!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks us to identify the gas that exhibits the lowest extent of physical adsorption (physisorption) onto a solid surface under identical environmental conditions.

Step 2: Detailed Explanation:
The extent of physical adsorption of a gas on a solid surface depends heavily on the strength of the intermolecular forces (van der Waals forces) between the gas molecules and the solid surface.
Stronger intermolecular forces lead to more ready liquefaction of the gas and, consequently, a much higher degree of adsorption.
A highly reliable physical indicator of intermolecular force strength is the gas's critical temperature ($T_c$).
- Gases with a high critical temperature ($\text{SO}_2, \text{NH}_3, \text{Cl}_2$) are highly polar or highly polarizable. They possess strong intermolecular forces and are very easily liquefied. Because of these strong forces, they are strongly and rapidly adsorbed onto surfaces.
- Gases with a very low critical temperature (the so-called "permanent" gases like $\text{H}_2, \text{N}_2, \text{O}_2$) are small, highly non-polar, and possess extremely weak van der Waals forces. Because these attractive forces are so weak, they are very difficult to liquefy and are very weakly and poorly adsorbed.
Among the given options, Hydrogen ($\text{H}_2$) is a small, non-polar diatomic gas with the lowest critical temperature and the weakest intermolecular forces. Therefore, it will be the least adsorbed.

Step 3: Final Answer:
The gas least adsorbed is $\text{H}_{2}$, matching option (d).