Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. The extent of adsorption increases with the increase of surface are per unit mass of the adsorbent at a given temperature and pressure. Another important factor featuring adsorption is the heat of adsorption. During adsorption, there is always a decrease in residual forces of the surface, therefore, it is invariably an exothermic process or \(\Delta H\) and \(\Delta S\) are always negative. There are mainly two types of adsorption of gases on solids. In physisorption, the attractive forces are mainly van der Waals forces while in cemisorption, actual bonding occurs between the particles of adsorbate and adsorbent. Generally, easily liquifying gases are adsorbed more easily on the surface of a solid as compared to the gases which are liquified with difficulty. Freundlich gave an emperical relationship between the quantity of gas adsorbed by unit mass of solid adsorbent and pressure, at a particular temperature.

Rate of physisorption increases with

decrease in temperature

The correct answer is option 4. decrease in temperature.

Physisorption is the adsorption of molecules onto a surface through weak van der Waals forces, which are weaker than the covalent or ionic bonds involved in chemisorption. In physisorption, the following factors influence the adsorption rate:

i. Surface Area of the Adsorbent

A larger surface area of the adsorbent provides more sites for adsorption. Therefore, the rate of physisorption increases with an increase in surface area because there are more locations where the adsorbate molecules can adhere. Decreasing the surface area will reduce the rate of physisorption. This means that a decrease in surface area is not favorable for physisorption.

ii. Pressure of the Adsorbate

Physisorption is influenced by the availability of adsorbate molecules. According to the Langmuir adsorption isotherm, the amount of adsorption increases with an increase in pressure because more gas molecules are available to interact with the adsorbent surface. Decreasing pressure will reduce the rate of physisorption as there are fewer adsorbate molecules in contact with the adsorbent surface.

iii. Temperature

Physisorption is generally an exothermic process, which means that it releases heat when the adsorbate molecules adhere to the surface. The effect of temperature on physisorption can be understood through the following points. For exothermic processes, lower temperatures favor the adsorption process because the release of heat helps drive the adsorption equilibrium. Lower temperatures reduce the kinetic energy of the adsorbate molecules, making them more likely to stick to the adsorbent surface. Increasing temperature increases the kinetic energy of the adsorbate molecules. Higher kinetic energy means the molecules are moving faster and are more likely to overcome the weak van der Waals forces holding them to the adsorbent. This reduces the likelihood of adsorption. Higher temperatures typically decrease the rate of physisorption because the adsorbate molecules have higher kinetic energy and are less likely to adhere to the adsorbent surface.

iv. Decrease in Temperature

As discussed, lowering the temperature reduces the kinetic energy of the adsorbate molecules, making it easier for them to be adsorbed by the adsorbent. Since physisorption is an exothermic process, a decrease in temperature enhances the adsorption rate by making the process more favorable. Decreasing the temperature generally increases the rate of physisorption. This is because the reduced kinetic energy of the adsorbate molecules leads to a higher probability of adsorption, and the exothermic nature of the process is favored at lower temperatures.

Summary

Surface Area: Increasing surface area increases physisorption rate.

Pressure: Increasing pressure increases physisorption rate.

Temperature: Increasing temperature decreases physisorption rate because it provides higher kinetic energy to the adsorbate molecules, making them less likely to be adsorbed.

Decrease in Temperature: A decrease in temperature generally increases the rate of physisorption because it reduces the kinetic energy of the adsorbate molecules and favors the exothermic adsorption process.

Thus, the correct answer to which factor increases the rate of physisorption is a decrease in temperature.