Question

Assertion: In physical adsorption, enthalpy of adsorption is very low.
Reason: In physical adsorption, attraction between gas molecules and solid surfaces is due to weak Van der Waals forces.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(C) Assertion is correct but the reason is incorrect.
(D) both Assertion and reason are incorrect.

Answer 1

Hint: In order to know why the enthalpy of adsorption is very low in the physical adsorption, we must first have an idea about what an adsorption is and also about the types of adsorption present.

Complete Solution :
Let us first understand what an adsorption is. Adsorption is said to be a surface phenomenon. Adsorption being a surface process, it involves deposition of the molecular species in higher concentration on the surface. There are two types of adsorption present, they are:
- Physical adsorption
- Chemical adsorption.
Let us now understand about the physical adsorption. Physical adsorption is also known as the physisorption. When the adsorption of a solid occurs on the surface due to weak Vander Waals, then it is called physical adsorption. The physical adsorption is a reversible process and this is not specific in nature. Enthalpy of physical adsorption is very low because of the accumulation of substance on the surface. This accumulation is low due to the presence of the weak Van Der Waals force. Therefore, the Assertion and the reason given is correct. Hence the reason which is given is the correct explanation for the assertion. So, the correct answer is “Option A”.

Note: Let us see how physisorption is different from the chemisorption.

PHYSISORPTION	CHEMISORPTION
When the physical forces are acting to cause adsorption, it is called Physical adsorption.	When the chemical forces are acting to cause adsorption, it is called Chemical adsorption.
It is a weak process.	It is a strong process.
It is a Multilayer process.	It is a single layer process.
It is not a specific process.	It is a specific process.
Energy of activation of the physisorption is found to be very low.	Energy of activation of the chemisorption is found to be very high.