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

Option (b): fast → slow → independent of pressure.

Step 1: Understanding Adsorption Behavior

The process of adsorption involves the accumulation of gas molecules on the surface of a solid. The rate at which adsorption occurs can be affected by the pressure of the gas. At low pressures, adsorption is generally faster because the surface has many available sites for the gas molecules. As pressure increases, these sites fill up, causing the adsorption rate to slow down until it reaches a point where the rate becomes independent of pressure, because most sites are occupied.

Step 2: Analyzing Each Option

Given the progression of adsorption rates affected by gas pressure, we analyze each option: - **Option (a)**: slow → fast → independent of pressure. Incorrect because it should initially be fast. - **Option (b)**: fast → slow → independent of pressure. Correct, as initially many sites are available, which then fill up. - **Option (c)**: independent → slow → fast. Incorrect progression. - **Option (d)**: independent → fast → slow. Incorrect progression.

Step 3: Selecting the Correct Answer

Based on the theoretical understanding, the correct progression is that adsorption starts fast at low pressure, slows down as pressure increases, and eventually becomes independent of pressure. Thus, the correct choice aligns with *Option (b)* which is: fast → slow → independent of pressure.

Key Concepts

Gas-Solid Interactions

Adsorption is a fascinating process occurring at the interface between a solid and a gas. This involves gas molecules sticking to the surface of a solid.
It is driven by forces such as Van der Waals forces or chemical bonding. These interactions are crucial for many industrial applications, like catalysis and filtration.

• Van der Waals Forces: These are weak forces that help the gas molecules adhere to the solid surface. They are essential, especially for physically adsorbed gases.
• Chemical Bonding: In some cases, gas molecules form stronger bonds by reacting chemically with the surface, leading to stronger adsorption.

It's important to remember that these interactions determine how well and how quickly the gas molecules can attach to the surface of the solid.

Adsorption Rate

The adsorption rate is how quickly the gas molecules adhere to a solid surface. Initially, when the pressure is low, many sites are available on the solid for gas molecules, so adsorption is rapid.
As these sites start to fill, fewer places remain for gas molecules to occupy. This leads to the adsorption rate decreasing over time as the pressure increases.

• Fast at Low Pressure: Many free sites on the surface mean gas molecules can quickly attach.
• Slows Over Time: As more sites fill, there is less opportunity for attachment, making the process slower.
• Reaches Equilibrium: Eventually, most sites are filled, and the adsorption rate becomes constant, independent of further pressure changes.

Understanding how this rate changes with pressure helps in predicting the efficiency of adsorption in various applications.

Pressure Effects on Adsorption

Pressure has a significant impact on adsorption processes. At the outset, low pressure means high availability of sites for adsorption. This means the gas molecules can swiftly attach to the solid surface. As the pressure increases, this starts to change.

• Initial Fast Rate: At low pressures, plenty of sites are available. This allows a quick attachment of gas molecules.
• Slowing Down: As pressure builds, fewer sites are left empty. This slows down the rate of adsorption.
• Independence of Pressure: Eventually, once all sites are nearly occupied, the adsorption rate does not increase with pressure anymore. The rate levels off and becomes pressure-independent.

Grasping these pressure effects helps in optimizing adsorption setups for industrial applications, ensuring maximum efficiency.