Problem 132 - A Complete Resource Book in Chemistry for JEE Main

Question

On the basis of data given below predict which of the following gases shows least adsorption on a definite amount of charcoal? Gas \(\mathrm{CO}_{2} \quad \mathrm{SO}_{2} \quad \mathrm{CH}_{4} \quad \mathrm{H}_{2}\) Critical temp./K \(\quad 304 \quad 630\) \(190 \quad 33\) (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{CH}_{4}\) (c) \(\mathrm{H}_{2}\) (d) \(\mathrm{CO}_{2}\)

Step-by-Step Solution

Answer

\( H_2 \) shows the least adsorption due to its lowest critical temperature of 33 K.

Step 1: Understanding adsorption and critical temperature

Adsorption is the process where a gas accumulates on the surface of a solid. A key factor affecting adsorption is the critical temperature of the gas. The higher the critical temperature, the greater the extent of adsorption, because gases closer to their critical temperature are more easily liquefied, enhancing the interaction with the solid surface.

Step 2: Analyze critical temperatures of the gases

The critical temperatures of the given gases are as follows: \( CO_2: 304\, K \), \( SO_2: 630\, K \), \( CH_4: 190\, K \), \( H_2: 33\, K \). Lower critical temperature means weaker adsorption properties.

Step 3: Identify the gas with the lowest critical temperature

Among the given gases, \( H_2 \) has the lowest critical temperature of 33 K. This indicates that \( H_2 \) will show the least adsorption on charcoal because it is the least likely to be liquefied or adsorbed under normal conditions compared to the other gases.

Key Concepts

Critical Temperature

In the context of adsorption, the critical temperature of a gas plays a vital role. Critical temperature is the highest temperature at which a substance can exist as a liquid. Above this temperature, the substance cannot be liquefied, regardless of pressure. Understanding this concept helps us determine how likely a gas is to be adsorbed onto a surface.
For gases, those with higher critical temperatures are generally more prone to being adsorbed onto surfaces such as charcoal. This is because they are

• closer to their liquefaction point,
• tend to condense easier,
• have stronger intermolecular forces near surfaces.

Therefore, when comparing gases, their critical temperatures provide significant insight into their adsorption tendencies on solid surfaces.

Gas Adsorption

Gas adsorption is the process where gas molecules accumulate on solid surfaces. This occurs due to interactions between the gas molecules and the solid's surface, which can be physical or chemical in nature.
Key factors that affect gas adsorption include:

• Surface Area: Larger surfaces offer more sites for adsorption.
• Temperature: Lower temperatures usually enhance adsorption because they lower kinetic energy, helping the gas molecules adhere to the solid surface.
• Pressure: Higher pressures drive more gas molecules to the surface, increasing adsorption.

Moreover, specific characteristics of the gas, such as polarity, also contribute to how effective the adsorption process is. These factors collectively determine how strongly and extensively adsorption occurs.

Surface Chemistry

Surface chemistry focuses on chemical processes at interfaces, where phases meet and interact. It's highly relevant in adsorption as it describes the molecular interactions between gases and solid surfaces.
In adsorption, surface properties are essential:

• Surface Energy: High surface energy materials tend to attract and hold gas molecules more effectively.
• Surface Functional Groups: Specific chemical groups on the surface can promote stronger chemical adsorption through bonding.
• Porosity: Highly porous materials increase available surface area for gases to adsorb.

Thus, a thorough understanding of surface chemistry helps predict and control gas adsorption processes, which are crucial in various industrial and environmental applications. These insights are invaluable for designing materials with specific adsorption capabilities.