Problem 89 - A Complete Resource Book in Chemistry for JEE Main

Question

The amount of gas adsorbed physically on charcoal (a) increases with pressure and decreases with temperature (b) increases with temperature and decreases with pressure (c) increases with temperature and pressure (d) increases either temperature or pressure.

Step-by-Step Solution

Answer

(a) Increases with pressure and decreases with temperature.

Step 1: Understanding Adsorption

Adsorption refers to the process where a solid surface accumulates gas molecules or vapor. Charcoal, being porous, is used to adsorb gases due to its high surface area.

Step 2: Analyzing Effect of Pressure

According to the principles of physical adsorption, the amount of gas adsorbed on a surface increases as the pressure of the gas increases, due to the higher likelihood of gas molecules interacting with the surface.

Step 3: Analyzing Effect of Temperature

Physical adsorption is generally exothermic. As temperature increases, kinetic energy of gas molecules increase, making them less likely to adhere to the surface. Therefore, physical adsorption decreases with an increase in temperature.

Step 4: Choosing the Correct Option

Based on the characteristics of adsorption regarding pressure and temperature, we conclude that the correct behavior describes an increase in gas adsorbed with increased pressure and a decrease with increased temperature. This matches option (a).

Key Concepts

Adsorption on Charcoal

Charcoal, an intriguing material, is often used for adsorption because of its extremely porous structure. This structure provides a vast surface area, allowing it to effectively capture gas molecules. Adsorption occurs when gas molecules adhere to the solid surface of the charcoal, forming a thin layer. This process is physical in nature and is primarily driven by weak van der Waals forces.
These forces are not permanent, which means that adsorption can be easily reversed. Charcoal's capacity to adsorb gases makes it useful in numerous applications, including air purification and gas masks. It acts like a sponge, trapping gases between its fibers. The efficiency of adsorption on charcoal depends on several conditions such as pressure and temperature, which we will explore further.

Effect of Pressure on Adsorption

Pressure significantly impacts the process of adsorption, especially in the context of gases interacting with a solid surface like charcoal. As gas pressure increases, more molecules are forced onto the surface of the charcoal.
This is because higher pressure means more gas molecules are moving in a confined space and have a higher probability of colliding with and adhering to the surface.

• Increased pressure leads to more frequent contact of gas molecules with charcoal, enhancing adsorption.
• This effect can be likened to squeezing a sponge, pushing more liquid (or gas, in this case) into it.
• More pressure equates to more interaction, resulting in higher adsorption rates.

Thus, under high-pressure conditions, the amount of gas adsorbed onto charcoal increases, making pressure a crucial factor in optimizing adsorption processes.

Effect of Temperature on Adsorption

Temperature plays a critical role in the process of physical adsorption. Unlike pressure, increasing temperature tends to have an opposite effect. This is because physical adsorption is an exothermic process, meaning it releases heat when gas molecules attach to the surface.
As the temperature rises, the kinetic energy of gas molecules also increases, causing them to move more vigorously.

• With greater movement, molecules are less likely to settle on the surface, thus reducing adsorption.
• Think of it as trying to catch bouncing balls—the hotter (more energetic) they are, the harder they are to catch.
• This property makes temperature control vital in applications where adsorption is necessary.

Consequently, higher temperatures lead to decreased adsorption, so cooler conditions are preferred when using charcoal for absorbing gases.