Problem 9 - A Complete Resource Book in Chemistry for JEE Main

Question

Physical adsorption is inversely proportional to (a) temperature (b) volume (c) concentration (d) all of these

Step-by-Step Solution

Answer

Physical adsorption is inversely proportional to temperature.

Step 1: Understand Physical Adsorption

Physical adsorption, also known as physisorption, involves the adhesion of gas molecules onto a solid surface through weak van der Waals forces. It is typically a reversible process and depends heavily on external conditions, such as temperature, pressure, and the nature of the gas and solid.

Step 2: Analyzing Temperature Dependency

Physisorption decreases with an increase in temperature because the added heat provides energy for the adsorbed molecules to overcome van der Waals forces, returning to the gaseous state. Hence, it is inversely proportional to temperature.

Step 3: Examine Volume Impact

The volume itself does not directly affect physisorption. While increasing volume can relate to decreasing pressure (given a constant quantity of gas), the direct relationship of volume to physisorption is minimal compared to temperature effects.

Step 4: Consideration of Concentration

Concentration typically refers to the amount of gas in a given volume. Physisorption generally increases with higher gas concentration up to a saturation point, as more molecules are available to adhere to the solid surface, making concentration not inversely proportional to physisorption.

Step 5: Evaluate All Proposed Options

Having examined each option, physical adsorption is inversely related primarily to temperature. Volume and concentration have either minimal or non-inverse relationships to the process.

Key Concepts

Temperature Dependency in Adsorption

In the realm of physical adsorption, also known as physisorption, temperature plays a pivotal role. As you raise the temperature, the amount of physical adsorption tends to decrease. This is because the increase in temperature provides gaseous molecules with the energy to break free from the weak van der Waals forces that bind them to a surface.

In essence, temperature and physisorption share an inverse relationship. A lower temperature favors physisorption since the kinetic energy of the gas molecules is reduced, making them more likely to stick to the solid surface. Conversely, higher temperatures supply heat energy that enables the molecules to escape back into the gaseous state.

• This process is quite similar to people warming up and wanting to shed their extra layer of clothes when the temperatures rise.
• Adsorption is thus more efficient in cooler conditions, where the gas molecules leisurely adhere to the solid surface.

Understanding this temperature dependency is crucial when designing systems that utilize physisorption. Whether it's in catalytic converters or air purification systems, controlling temperature can optimize the performance and efficiency of these systems.

Van der Waals Forces

Van der Waals forces are the unsung heroes of physical adsorption. These forces are weak compared to chemical bonds but vital for the adhesion of molecules to surfaces during physisorption. They arise from transient dipoles and instantaneous polarization that occur within molecules.

Let's break it down: molecules constantly fluctuate due to electron movement. This fluctuation causes temporary uneven charges within the molecule, forming a dipole. When one molecule with a dipole encounters another, they attract each other through van der Waals forces. These forces are significant enough to hold gas molecules onto a solid but still weak, enabling easy reversibility.

• They are more noticeable when you deal with large molecules or surfaces with higher molecular weight.
• These forces operate over a short range, making proximity critical for adsorption to occur effectively.

Van der Waals forces, even in their simplicity, are essential for many practical applications, including gas storage and separation processes. Recognizing their role helps to better harness their potential in industrial applications and understand the temporal nature of physical adsorption.

Reversible Adsorption Process

Physical adsorption distinguishes itself with a highly reversible nature, unlike chemical adsorption. In physisorption, this reversibility means under appropriate conditions, adsorbed molecules can readily detach from solid surfaces and return to their gaseous state. Reversibility is largely due to the weak van der Waals forces involved.

In practical applications, this makes physical adsorption particularly useful. For instance, in gas masks, harmful gases can be rapidly adsorbed, protecting individuals efficiently. Once the mask is no longer needed, the gas can be desorbed safely and the mask reused.

• Reversibility implies easy regeneration of adsorbent materials, making them economically sustainable.
• Conditions such as increased temperature or decreased pressure can trigger desorption, allowing for controlled release of adsorbed gases.

This feature offers flexibility in industrial systems where conditions might vary. Physisorption's reversible nature allows for adjustments in process parameters to either maximize adsorption or facilitate desorption, creating highly versatile systems beneficial for both commercial and environmental uses.