Answer the question on the basis of passage given below:
Adsorption is a surface phenomenon which can be expressed by means of an empirical relationship known as Freundlich Adsorption isotherm. The relationship is given by
\(\frac{x}{m} = Kp^{\frac{1}{n}}\left(n > 1\right)\)
\(x = \) Mass of the gas adsorbed
\(m = \) Mass of adsorbent
\(p = \) Pressure at which adsorption takes place
\(K\) and \(n\) are constant
Choose the incorrect statement with reference to physical adsorption:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Enthalpy of adsorption value \(= 80 - 240 kJ/mol\)
Physical adsorption arises from weak van der Waals forces, so its enthalpy of adsorption is low, about 20–40 kJ mol−1. A value of 80–240 kJ mol−1 belongs to chemisorption, where chemical bonds are formed.
Option (1) It is non-specific in nature — A feature of physical adsorption. Any gas is adsorbed to some extent on any solid.
Option (2) Enthalpy of adsorption value = 80 – 240 kJ/mol — Not a feature of physical adsorption. Physisorption has a low enthalpy of 20–40 kJ mol−1; 80–240 kJ mol−1 is the range for chemisorption.
Option (3) It is reversible in nature — A feature of physical adsorption. Gas taken up at higher pressure is released when the pressure is lowered.
Option (4) It arises due to van der Waals forces — A feature of physical adsorption. These weak forces are the cause of physisorption.
NCERT (Pre-rationalised edition, 2022-23):
Types of Adsorption – …
