Question

Write the differences between physisorption and chemisorption with respect to the following:
(i)Specificity (ii) Temperature dependence (iii) Reversibility and (iv) Enthalpy change

Answer 1

Hint: In physical adsorption or physisorption, a gas is adsorbed by a solid through weak van der Waals forces and so there is no formation of any chemical bond between the gas adsorbate and the solid adsorbent.
In chemical adsorption or chemisorption, a gas is adsorbed by a solid by forces similar to those of a chemical bond, which may be ionic or covalent in nature.

Complete step by step answer:
  (i)Specificity:
-Physisorption is simply just a chemical change as there is no formation of chemical bond. Since van der Waals forces are universal in nature, every gas is adsorbed on every solid adsorbent to some extent. Only the extent of adsorption may vary. In other words, the surface of the solid does not have any preference for a particular type of gas in physisorption. Hence, it is non-specific in nature.
-Chemisorption is highly specific in nature as it takes place only if there is a chance of chemical bond formation between the gas adsorbate and the solid adsorbent. For example, oxygen will be chemisorbed only on those solids with which it can form oxides.
(ii) Temperature dependence:
-As physisorption involves no chemical reaction, very low or no activation energy is required for physisorption to occur and so it takes place at low temperature and decreases with increase of temperature.
-As chemisorption involves a chemical reaction, very high activation energy is required for chemisorption to occur and so it takes place at low temperature and increases with increase in temperature.
 (iii) Reversibility:
-Physisorption is a reversible process which means that desorption of the gas takes place by increasing the temperature or by decreasing the pressure.
-Chemisorption involves the formation of a compound, so, any effort to free the adsorbed gas will release the compound instead of the free gas. Thus, it is irreversible in nature.
(iv) Enthalpy change:
-Heat of adsorption is always negative, i.e., adsorption is always exothermic. But physisorption involves van der Waals forces of attraction and no chemical change, so the enthalpy of adsorption is very low.
-On the other hand, chemisorption involves chemical reaction, so it is accompanied with evolution of a large amount of heat. Thus, the enthalpy of adsorption is very high.

Note:
There are also some other differences between physisorption and chemisorptions:
-The amount of gas adsorbed in physisorption is related to the ease of liquefaction of the gas but there is no such correlation in case of chemisorption.
-Layers of the gas can be adsorbed one over the other by van der Waals forces in physisorption. As a result, multimolecular layers are formed in physisorption. But in chemisorption, chemical bonds can be formed only with the layers of molecules coming in direct contact with the adsorbent surface and so a unimolecular layer is formed in chemisorption.