Quick Revision: Surface Chemistry | Chemistry Class 12 - NEET PDF Download

 Page 1


ADSORPTION
The phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid
resulting into higher concentration of the molecules on the surface is called adsorption.
There are two types of adsorption
(i) Physical adsorption
(ii) Chemical adsorption
Physical adsorption 
or 
Physisorption 
Chemical adsorption 
or 
Chemisorption 
1. Enthalpy of adsorption usually 
is of the order of –20 kJ mol
–1
 
i.e. exothermic. 
2. Molecule of adsorbate and 
adsorbent are held by weak  
van der Waal’s interaction. 
3. It usually takes place at low 
temperature and decreases with 
increasing temperature. 
4. It is not very specific i.e. all 
gases are adsorbed on all 
solids to some extent. 
5. Multimolecular layers may be 
formed on adsorbent. 
6. It is reversible in nature. 
7. It does not require activation 
energy. 
1. Enthalpy of adsorption is of 
the order of –200 kJ mol
–1
. 
2. Molecules of adsorbate and 
adsorbent are held by 
chemical bonds. 
3. It takes place at relatively 
high temperature. 
4. It is highly specific and take 
place when there is some 
possibility of compound 
formation between adsorbate 
and adsorbent molecule. 
5. Usually monomolecular layer 
is formed on the adsorbent. 
6. It is usually irreversible in 
nature. 
7. It requires activation energy. 
ADSORPTION ISOTHERM
A relation or graph between x/m (x are number of moles of adsorbate and m is the mass of adsorbent) and
the pressure (P) of the gas at a constant temperature is called adsorption isotherm.