Question

Physical adsorption is appreciable at:
(A) Higher temperature
(B) Lower temperature
(C) Room temperature
(D) $ {100^0}\,{\text{C}} $

Answer 1

Hint :An adsorption is a surface phenomenon, it is defined as the deposition of molecular species on the surface. The molecular species that get adsorbed on the surface is known as adsorbate and the surface on which the adsorption takes place is known as adsorbent. There are two types of adsorption: physical adsorption also called physisorption and chemical adsorption also known as chemisorption.

Complete Step By Step Answer:
We already discussed that there are two types of adsorption: physical adsorption and chemical adsorption. Physical adsorption involves adsorption of gases on a solid surface through Van der Waals forces. Whereas in chemical adsorption the gas molecules or atoms are held on the solid surface via chemical bonds.
Now we will look at the main characteristics of physical adsorption.
In physical adsorption particles of an adsorbate are held to the surface of an adsorbent by the physical forces such as Van der Waals forces.
The process of physical adsorption is generally reversible and therefore equilibrium is reached rapidly .
The attractive forces between adsorbent and adsorbate molecules are weak and therefore enthalpy of adsorption is low.
The gases which are easily liquefied are adsorbed readily
All the gases are adsorbed on all solids to the same extent, it is because a given surface of adsorbent doesn’t show any preference for particular gas as the Van der Waals forces are universal.
Adsorption process is an exothermic process so that physical adsorption occurs at low temperature and decreases with increase in temperature according to Le-Chatelier's principle.
Now we will discuss the effect of temperature on physical adsorption
Temperature plays an important role in the study of adsorption. In general we can say that physical adsorption being an exothermic process at a lower temperature is favourable to larger adsorption. It is because if the temperature increases the kinetic energy of gas molecules increases there by the gas molecules leave the surface of adsorbent. So that lower temperature favours physical adsorption.
Hence here the correct answer is option B.

Note :
 Since physisorption is a surface phenomenon , the rate of physical adsorption increases initially than it decreases. It is an exothermic process so that the enthalpy change is always negative. Also concentration of adsorbate on the surface of adsorbent is different from that in the bulk. More the surface is, the more will be the rate of adsorption.