1. Caused by intermolecular Vander Waal’s forces.
2. Depends on nature of gas. Easily liquefiable gases are adsorbed readily.
3. Heat of adsorption is small (about 5 kcal per mol)
5. Forms multimolecular layers on adsorbent surface.
6. Occurs at low temperature; decreases with increase in temperature.
7. Increase of pressure increases adsorption
8. Equilibrium is attained readily and it is reversible.
Caused by chemical bond formation.
Much more specific and depends upon the nature of the both the adsorbate and adsorbent.
Very large (20-100 kcal per mol).
Forms unimolecular layer.
Increases with increase of temperature
High pressure is favourable. Decrease of pressure does not cause desorption.
Equilibrium is attained slowly and mostly not reversible.
Enthalpy of adsorption:
The adsorption of one substance on the surface of another leads to the existence of new types of forces between them. Therefore, it is an exothermic process and is accompanied by the release of energy. The enthalpy or heat of adsorption is defined as the heat energy evolved when one mole of adsorbate is adsorbed on the surface of adsorbent. Since physical adsorption involves weak forces of attraction between the molecules of the adsorbent and the adsorbate, the heat of physisorption is generally low, of the order of 20-40 kJ mol–1. Chemical adsorption, on the other hand involves strong chemical bond formation and the heat of chemisorptions is quite high, of the order of 80-400 kJ mol–1.