Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

The adsorption of a gas, on a solid surface, varies with pressure of the gas in which of the following manner? (a) slow \longrightarrow fast \(\longrightarrow\) independent of the pressure (b) fast \(\longrightarrow\) slow \(\longrightarrow\) independent of the pressure (c) independent of the pressure \(\longrightarrow\) slow \(\longrightarrow\) fast (d) independent of the pressure \(\longrightarrow\) fast \(\longrightarrow\) slow

Step-by-Step Solution

Answer

The correct pattern is (b) fast \(\rightarrow\) slow \(\rightarrow\) independent of the pressure.

Step 1: Understanding Adsorption

Adsorption is the process where gas molecules accumulate on a solid surface. The rate and extent of adsorption can vary with the pressure of the gas.

Step 2: Adsorption at Low Pressure

At low pressures, the surface of the solid is mostly empty. Adsorption occurs quickly because there are many available sites for the gas molecules.

Step 3: Adsorption at Intermediate Pressure

As pressure increases, the surface of the solid begins to fill with gas molecules. The rate of adsorption slows down because there are fewer available sites.

Step 4: Adsorption at High Pressure

At very high pressures, most of the adsorption sites on the solid surface are occupied. The rate of adsorption becomes independent of pressure as maximum adsorption has been achieved.

Step 5: Identifying the Correct Pattern

The described pattern is fast at low pressure, slow at intermediate pressure, and independent at high pressure. This matches option (b): fast \(\rightarrow\) slow \(\rightarrow\) independent of the pressure.

Key Concepts

Gas Pressure and Adsorption

When we talk about the pressure of a gas, we're referring to the force that the gas molecules exert on the walls of their container. Pressure is a key player in the process of adsorption. Picture gas molecules as tiny particles that are constantly moving and colliding with surfaces. When the pressure is low, only a few molecules are present, allowing them to quickly find and stick to available spaces on a solid surface.

As the pressure increases, though, more molecules are in the air. This means that more of them are trying to settle on the surface, leading to a situation where the surface starts to "fill up" with gas molecules. This crowding reduces the speed at which new molecules are able to find a place to adsorb.

At high pressures, most spots on the surface are already occupied, and new molecules find it challenging to squeeze in. Here the process of adsorption becomes independent of pressure because all available sites are essentially occupied.

Adsorption Kinetics

Adsorption kinetics helps us understand the rate at which gas molecules bind to a solid surface. Initially, when gases are at low pressure, the kinetics indicate a rapid adsorption rate because the solid has plenty of unoccupied sites. It’s like an empty parking lot where cars can easily find spots to park.

Once the pressure increases, leading to more gas molecules in the vicinity, fewer spaces are available, and the adsorption rate declines. This is akin to a nearly full parking lot where fewer spots mean cars take longer to park.

Finally, at very high pressures, the surface sites become saturated with gas molecules. The adsorption process levels off and becomes pressure-independent, as almost all sites are occupied, and the system has reached a state of equilibrium.

Surface Chemistry and Adsorption

Surface chemistry examines how the surface properties influence adsorption. The ability of a solid surface to adsorb gas depends largely on the characteristics of that surface, such as surface area, porosity, and chemical nature.

• **Surface Area**: Larger surface areas provide more sites for gas molecules to adhere to, boosting the adsorption process.
• **Porosity**: A porous surface means more intricate pathways and tunnels where molecules can be trapped, enhancing adsorption efficiency.
• **Chemical Nature**: The specific composition of the surface can influence how strongly gases are attracted. For instance, a surface with specific functional groups might have stronger interactions with certain gas molecules.

The efficiency of adsorption is greatly influenced by the interaction between the gas molecules and the solid's surface. When these factors are aligned, adsorption can be optimized, showcasing the intricate dance of physical interactions and surface properties.