Question

The heat of physisorption lies in the range of:
A.\[{\text{1 to 10 KJ mo}}{{\text{l}}^{{\text{ - 1}}}}\]
B.\[{\text{20 to 40 KJ mo}}{{\text{l}}^{{\text{ - 1}}}}\]
C.\[{\text{40 to 200 KJ mo}}{{\text{l}}^{{\text{ - 1}}}}\]
D.\[{\text{200 to 400 KJ mo}}{{\text{l}}^{{\text{ - 1}}}}\]

Answer 1

Hint: Physisorption occurs when gas adsorbs on the surface but no chemical change occurs. The energy required is very less for physisorption.

Complete step by step answer:
Adsorption is a topic of surface chemistry. It is a process in which a high concentration of adsorbate molecules is present on the surface than in the bulk of the adsorbent molecule. For example, silica gel absorbs water molecules on its surface. Adsorbate is a molecule which gets adsorbed, such as water molecules as in the previous example. Adsorbent is that molecule on which adsorbate gets attached. Adsorbate and adsorbent are just like solute and solvent.
In physical adsorption no chemical bond formation occurs. There is weak Van der waal interaction between the molecules. Whenever there is interaction or bond formation energy is released by the molecules. But as we know there is no chemical bond form in physisorption, very less energy is released due to weak interactions. The value energy released is \[{\text{20 to 40 KJ mo}}{{\text{l}}^{{\text{ - 1}}}}\].
Physisorption is a reversible process, that is the adsorbate molecule attached to adsorbent can be removed easily by providing heat or any other method. It is not specific in nature. Multilayer formation occurs in case of physisorption.

Hence, the correct option is B.

Note:
There is another term referred to as sorption, which is a combination of adsorption and absorption. The best example of sorption is water accumulated on sponge. The concentration of molecules is present on the surface of sponge as well as inside that bulk of sponge. Physisorption is not selective in nature and occurs faster than chemisorption.