Question

Distinguish between chemical and physical adsorption.

Answer 1

In chemical adsorption, chemical bonds are formed between adsorbate and adsorbent whereas in physical adsorption, Vander Waals forces are found between adsorbate and adsorbent

Complete answer:
Adsorption is defined as “the phenomenon of deposition of higher concentration of any molecular species on any surface in comparison to the bulk phases”. The substance on the surface of which adsorption takes place is called adsorbent and the molecular species which gets adsorbed is termed as adsorbate.
On the basis of forces existing between adsorbate molecule and adsorbent, adsorption is of two type:
- Physical adsorption, also known as physisorption.
- Chemical adsorption, also known as chemisorption.
Comparison between physisorption and chemisorption are as follows:

It has low heat of adsorption usually in the range of 20-40 KJ/mol.	It has high heat of adsorption in the range of 80-240 KJ/mol.
The force of attraction between adsorbate and adsorbent are Van der Waals forces.	The force of attraction between adsorbate and adsorbent are chemical bond forces.
It can be easily reversed by heating or decreasing the pressure.	This type of adsorption cannot be easily reversed as it involves breaking of chemical bonds.
It usually takes place at low temperature and decreases with increasing temperature.	It takes place at high temperature and increases on increasing the temperature.
It forms multimolecular layers of adsorbed gases.	It forms monomolecular layers.
Generally, more easily liquefiable gases are absorbed easily.	Only those gases which can react with adsorbent shows chemical adsorption.
It is not very specific i.e. does not depend on the nature of gas to be adsorbed,	It is highly specific.
It does not require any activation energy as no chemical bonds are formed in physisorption	It requires activation energy.

Note:
Both physisorption and chemisorption are exothermic in nature, that is they liberate heat. And also remember that both physisorption and chemisorption increases with increase in surface area of the adsorbent.