| Physical Adsorption (Physisorption) |
Chemical Adsorption (Chemisorption) |
| When the particles of the adsorbate are held to the surface of the adsorbent by the physical forces such as van der Walls forces, the adsorption is called physical adsorption. |
When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces or chemical bonds, the adsorption is called chemical adsorption. |
| It has low enthalpy of adsorption of the order of 20 to 40 kJ mol-1. |
It has high enthalpy of adsorption of the order of 80 to 240 kJ mol-1. |
| It usually occurs at low temperature and decreases with increasing temperature. |
It occurs at high temperature and increases with the increase of temperature. |
| It is reversible in nature. |
It is irreversible. |
| The extent of adsorption depends upon the ease of liquefication of the gas. More easily liquefiable gases are adsorbed readily. |
There is no correlation between extent of adsorption and the ease of liquefication of gas. It also depends on the nature of gas. Gases which can react with the adsorbent show chemisorption. |
| It is not specific in nature i.e. all gases are adsorbed on the surface of a solid to some extent. |
It is highly specific in nature and occurs only when there is bond formation between adsorbent and adsorbate molecules. |
| No appreciable activation energy is needed. |
High activation energy is sometimes needed. |
| The state of adsorbate is same as in the bulk. |
The state of adsorbate molecules may be different from that in the bulk. |
| It forms multimolecular layers. |
It forms mono-molecular layer. |
| The rate of adsorption increases with increase in pressure of adsorbate. |
The rate of adsorption usually decreases as the pressure increases. |
