2.2 Types of Adsorption

S. No.

Physical adsorption

Chemisorption

1.

It involves physical forces, i.e., weak van der Waal's forces.

It involves transfer of electrons between gas and solids, i.e., the forces operating are similar to those in a chemical bond.

2.

Adsorption is multilayer.

Adsorption is only in the upper layer.

3.

Heat of adsorption is low $\left( {1 - 10\;kcal\;mo{l^{ - 1}}} \right)$.

Heat of adsorption is high $\left( {20 - 100\;kcal\;mo{l^{ - 1}}} \right)$.

4.

The process is reversible, i.e., desorption of the gas occurs by decreasing the pressure or increasing the temperature.

The process is irreversible. Efforts to free the adsorbed gas give some definite chemical compound formed between gas and adsorbent.

5.

It does not show specificity.

It is highly specific, e.g., ${H_2}$ is adsorbed on the transition metals leading hydride formation; ${O_2}$ is adsorbed on the metals.

6.

More easily liquefiable and more soluble gas is adsorbed more easily.

Gas having potentiality of compound formation with the adsorbent are chemisorbed.

7.

It decreases with increase in temperature.

It increase with increase in temperature.

8.

It does not require appreciable activation energy.

Activation energy is involved in this process.