Heat of adsorption is greater for chemisorptions than physisorption. Why?

Explanation

In chemical adsorption, gas molecules are held to the surface by formation of chemical bonds. Since strong bond is formed, nearly 400 KJ / mole is given out as heat of absorption. Whereas in physical adsorption, physical forces like van der Waals force of attraction exist between adsorbent and adsorbate. The various other forces that can cause physical adsorption are
(i) dipole - dipole interaction
(ii) Dispersion forces
As these forces are weak, heat of adsorption is low, hence physical adsorption occurs at low temperatures.