Depending on the nature of attractive forces existing between the adsorbate and adsorbent, adsorption can be classified as:
i) Physical adsorption
ii) Chemical adsorption
In physical adsorption, the forces of attraction between the molecules of the adsorbate and the adsorbent are of the weak van der Waals' type. Since the forces of attraction are weak, the process of physisorption can be easily reversed by heating or decreasing the pressure of the adsorbate (as in the case of gases).
In chemisorption, the forces of attraction between the adsorbate and the adsorbent are very strong; the molecules of adsorbate form chemical bonds with the molecules of the adsorbent present in the surface.Adsorption is generally accompanied by release of energy, that is, most adsorption processes are exothermic in nature. Adsorption is a spontaneous process; therefore its free energy change is negative (DG<0). However, the entropy change associated with adsorption is generally negative because the adsorbate molecules lose their translation freedom when they get attached to the surface of the adsorbent. Therefore, in order for DG to be negative, the enthalpy change (DH) must be sufficiently negative, such that, (DG=DH-TDS)<0. This explanation accounts for exothermic adsorption processes. In cases, where endothermic adsorption occurs as in the case of hydrogen adsorption on glass, the entropy change DS is sufficiently positive such that DG remains negative. Enthalpy of adsorption, which is the enthalpy change for the adsorption of one mole of an adsorbate on an adsorbent surface, is usually in the range of 20 kJ/mole to 40kJ/mole while for chemisorption, the values are an order of magnitude high, that is, 200 kJ/mole to 400 kJ/mole.
The differences between physisorption and chemisorption are summarized in the below table.
|Forces of attraction are vander Waals’ forces||Forces of attraction are chemical bond forces|
|Low enthalpy of adsorption (20 - 40 k.J/mole)||High enthapy of adsorption (200 - 400 k.J/mole)|
|This process is observed under conditions of low temperature||This process takes place at high temperatures|
|It is not specific||It is highly specific|
|Multi-molecular layers may be formed||Generally, monomolecular layer is formed|
|This process is reversible||This process is irreversible|