Rate of physisorption increases with decrease in temperature

Physisorption, also known as physical adsorption, is a type of adsorption where the adsorbate molecules are held onto the surface of the adsorbent by weak intermolecular forces, such as van der Waals forces. The rate of physisorption is influenced by several factors, including temperature.

Effect of temperature on physisorption rate:
The rate of physisorption generally increases with a decrease in temperature. This can be explained by the following factors:

1. Activation energy:
Physisorption is a spontaneous process that occurs due to the attractive forces between the adsorbate and adsorbent. However, there is still an energy barrier that needs to be overcome for the adsorbate molecules to be adsorbed onto the surface. This energy barrier is known as activation energy. At higher temperatures, the kinetic energy of the adsorbate molecules increases, allowing them to overcome the activation energy more easily and therefore increasing the rate of desorption. On the other hand, at lower temperatures, the kinetic energy of the adsorbate molecules decreases, making it more difficult for them to overcome the activation energy and desorb from the surface. As a result, the rate of physisorption increases with a decrease in temperature.

2. Collision frequency:
The rate of physisorption is also influenced by the collision frequency between the adsorbate molecules and the adsorbent surface. At higher temperatures, the adsorbate molecules have higher kinetic energy, leading to increased collision frequency with the adsorbent surface. This higher collision frequency increases the chances of successful adsorption, resulting in a higher rate of physisorption. Conversely, at lower temperatures, the adsorbate molecules have lower kinetic energy, leading to a decreased collision frequency with the adsorbent surface. This lower collision frequency reduces the chances of successful adsorption, resulting in a lower rate of physisorption.

Conclusion:
In conclusion, the rate of physisorption increases with a decrease in temperature. This is due to the decreased kinetic energy of the adsorbate molecules, which makes it more difficult for them to overcome the activation energy and desorb from the surface. Additionally, the lower temperature leads to a decreased collision frequency between the adsorbate molecules and the adsorbent surface, further reducing the chances of successful adsorption. Hence, a decrease in temperature enhances the rate of physisorption.