Question

Extent of physisorption of a gas increases with (a) Increase in temperature (b) Decrease in temperature (c) Decrease in strength of van der Walls forces (d) Decrease in surface area of adsorbent.

Short answer

(b) Decrease in temperature

Step-by-step solution

Understanding Physisorption

Physisorption involves the physical adsorption of gas molecules onto a solid surface, relying on weak van der Waals forces. It typically results in the formation of a multilayer of gas molecules on the adsorbent surface.

Temperature Influence on Physisorption

Physisorption is an exothermic process, which means it releases heat. According to Le Chatelier’s principle, if we increase the temperature, the equilibrium will shift to absorb heat. Hence, physisorption decreases with an increase in temperature.

Effects of Lower Temperature

A decrease in temperature favors the exothermic nature of physisorption. Thus, reducing the temperature typically increases the extent of physisorption because it shifts the equilibrium toward the adsorption side.

Role of Van der Waals Forces

Strength of van der Waals forces plays a crucial role in physisorption. However, decreasing these forces would reduce the interaction between the surface and gas molecules, decreasing physisorption. Thus, stronger van der Waals forces enhance physisorption.

Surface Area and Adsorption

Surface area is directly related to adsorption. A decrease in surface area would result in fewer sites available for gas molecules to adsorb, hence reducing the extent of physisorption.

Key concepts

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that play a crucial role in physisorption. These forces are responsible for the weak attraction between gas molecules and the adsorbent surface. Unlike chemical bonds, van der Waals forces do not involve the sharing or exchange of electrons, making them much weaker. However, their collective action can significantly stabilize the adsorption of gas molecules.

In the context of physisorption, these forces must be sufficiently strong to facilitate gas molecule attachment to the surface, yet weak enough to allow for easy desorption when conditions change. This delicate balance makes van der Waals forces essential for reversible adsorption processes. If these forces weaken, the likelihood of gas molecules remaining on the surface decreases, resulting in reduced physisorption. Conversely, stronger van der Waals forces enhance the interaction and lead to increased physisorption efficiency.

Exothermic Process

Physisorption is considered an exothermic process, which means it releases energy in the form of heat. During the adsorption of gas molecules onto the surface, energy is released as the gas molecules move to a lower energy state. This release of energy is a hallmark of exothermic processes and is crucial for understanding how temperature affects physisorption.

Being exothermic is particularly important, as it means that physisorption will generally decrease with an increase in temperature. This is due to Le Chatelier's principle, which dictates that systems will adjust to counteract imposed changes. Therefore, increasing the temperature provides energy to the system, promoting desorption rather than adsorption. This is why lower temperatures favor physisorption, as they encourage the equilibrium shift to the adsorption side, promoting gas molecule attachment to the adsorbent.

Effect of Temperature on Adsorption

Temperature is a key factor influencing the extent of physisorption. Since physisorption is exothermic, the interaction between gas molecules and the adsorbent surface becomes less favorable with rising temperatures. At higher temperatures, the kinetic energy of gas molecules is increased, allowing them to overcome the weak van der Waals forces and desorb from the surface.

In contrast, decreasing the temperature enhances the likelihood of adsorption. Lower temperatures reduce the kinetic energy of gas molecules, facilitating their attachment to the surface due to weaker thermal agitation. Additionally, the exothermic nature of physisorption is more pronounced at lower temperatures, aligning with Le Chatelier's principle that pushes the equilibrium towards adsorption.

To optimize physisorption, it's crucial to maintain a lower temperature, maximizing the interaction between gas molecules and the adsorbent surface. By understanding this temperature relationship, one can better control and predict the efficiency of the adsorption process.