Question

Write three distinct features of chemisorptions which are not found in physisorptions.

Answer 1

Hint: The adsorption in the adsorbed substance that is held together by the chemical bonds is known as chemisorption or chemical adsorption. The adsorption due to weak intermolecular forces or the van der Waals forces on the surface of the adsorbent is known as physisorption or physical adsorption.

Complete answer:
The three distinct features of chemisorptions which are not found in physisorptions are as follows:
> Chemisorption occurs due to the formation of chemical bonds. Chemical bonds are not found in physisorption. Physisorption occurs due to the presence of van der Waals forces.
> Chemisorption is irreversible in nature. After chemisorption, the adsorbed substances cannot be recovered due to the chemical reaction that occurs. Physisorption is reversible in nature. The adsorbed substances can be recovered in physisorption as no chemical reactions occur.
> Chemisorption is highly specific in nature. But physisorption is not specific.
> Example of chemisorption: Adsorption of gases like nitrogen or hydrogen on the surface of adsorbent like ferrous catalyst at higher temperature.
> Example of physisorption: Adsorption of gases like nitrogen or hydrogen on the surface of adsorbent like charcoal at lower temperature.

Note: Other distinctive features of chemisorptions which are not found in physisorptions are:
The adsorption enthalpy of chemisorption is higher. The adsorption enthalpy of chemisorption is ${\text{80 - 240 kJ mo}}{{\text{l}}^{ - 1}}$ whereas the adsorption enthalpy of physisorption is ${\text{20 - 40 kJ mo}}{{\text{l}}^{ - 1}}$.
Chemisorption occurs at higher temperatures whereas physisorption occurs at lower temperatures.
The extent of chemisorption increases with an increase in temperature. But physisorption decreases as the temperature increases.
Chemisorption results in the formation of a unimolecular layer while physisorption results in the formation of a multimolecular layer.
The activation energy of chemisorption is higher than that of physisorption.