Question

The enthalpy of chemisorptions is _____ than the enthalpy of physisorption.
A. Equal
B. Lesser
C. Greater
D. None of these

Answer 1

The phenomenon of attracting and retaining the molecules of a substance on the surface of solid or liquid results in higher concentration of molecules on the surface is termed as adsorption. The experimental study of the adsorption of gases on solid shows two types of adsorption namely physisorption and chemisorptions.

Complete step by step answer: We have learnt that there are two types of adsorption namely physisorption and chemisorptions. Physisorption or physical adsorption involves weak Van Der Waals forces while forces acting in chemisorption are similar to chemical bonds.
Physical adsorption is reversible in nature (desorption of gas occurs by increasing the temperature or decreases with decrease in temperature) while chemisorption is irreversible.
No new compounds are formed in physisorption while surface compounds are formed in chemisorptions.
Physisorption does not require any activation energy while activation energy is required in chemisorption.
On increasing pressure, physisorption increases while chemisorption increases have no effect of increasing pressure.
So, due to the weak Vander Waals forces present in physisorption, the heat of adsorption are low i.e. about $20 - 40\;{\rm{kJ}}\;{\rm{mo}}{{\rm{l}}^{ - 1}}$. In chemisorptions, the chemical forces are strong and required high heat of adsorption i.e. $40 - 400\;{\rm{kJ}}\;{\rm{mo}}{{\rm{l}}^{ - 1}}$.
The enthalpy of chemisorptions is greater than the enthalpy of physisorption.
So, the correct answer is “Option C”.

Note:
As adsorption involves attraction of molecules of the adsorbate on the surface of the adsorbent, surface energy decreases and appears as heat. That is why the heat of adsorption is always exothermic. Physical adsorption involves weak forces of attraction, heat evolved is less whereas chemisorption involves chemical reaction, and heat evolved is much higher.