Question

Which of the following statements is incorrect regarding physisorption?

A. It occurs because of the van der Waal forces

B. Liquefiable gases are adsorbed more easily

C. Enthalpy of adsorption $(\Delta {H_{adsorption}})$is high and negative

D. Enthalpy of adsorption $(\Delta {H_{adsorption}})$ is low and positive

Answer 1

Hint : Adsorption is the phenomenon of accumulation of a substance on the surface of other substances. As solid has unbalanced molecular forces on their surface so they invite particles of other substances to accumulate on their surface and thus, adsorption of gases on solids is a common phenomenon.

Complete step by step solution:

Step 1:

Depending upon the types of forces that are responsible for the attachment of the molecules of the adsorbate on the surface of the adsorbent, the adsorption of gases on solids is classified into physisorption and chemisorption.

Step 2:

Physisorption takes place when the intermolecular force of attraction between the gas and the solid molecule is weak van der Waal force. The characteristic features of physisorption are as follows:

As they have weak van der Waal forces so the adsorption is non-specific.

As the forces are weak so this type of adsorption is generally reversible.

Physical adsorption though is an exothermic process but the heat of adsorption is quite low and negative to the range of $- 20$ to $- 40kJ/mol$.

This type of adsorption having non-specific nature is usually multi-layered.

With an increase in temperature, the extent of physisorption also increases.

The extent of physisorption increases with an increase in the pressure of the gas.

It also depends upon the nature of the gas. Easily liquefiable gas, i.e., gases having higher critical temperatures are more readily adsorbed as compared to those which are liquefied with difficulty.

Hence, option C) Enthalpy of adsorption $(\Delta {H_{adsorption}})$ is high and negative and option D) Enthalpy of adsorption $(\Delta {H_{adsorption}})$ is low and positive are the two statements which are incorrect about physisorption.

Therefore, Option (C) and (D) is the correct answer.

Note : Physisorption and Chemisorption are two types of adsorption. Physisorption takes place when the force between the gas and the solid is weak van der Waal forces. Chemisorption, on the contrary, takes place when the gas molecules get attached to the surface of the adsorbent through strong valence bond forces.