as physical adsorption and chemical adsorption, depending on the nature of forces acting between adsorbent and adsorbate. In chemical adsorption, gas molecules are held to the surface by formation of chemical bonds. Since strong bond is formed, nearly KJ / mole is given out as heat of adsorption. Examples • Adsorption of O on tungsten, Adsorption of H on nickel, Adsorption of ethyl alcohol vapours on nickel.
In physical adsorption, physical forces such as van der waals force of attraction, dipole - dipole interaction, dispersion forces etc., exist between adsorbent and adsorbate. As these forces are weak, heat of adsorption is low, hence physical adsorption occurs at low temperatures. Examples (a) Adsorption of N on mica. (b) Adsorption of gases on charcoal.
The following table . illustrates the distinction between chemical and physical adsorption. Table . Distinction between chemical and physical adsorption Chemical adsorption or Chemisorption or Activated adsorption Physical adsorption or van der waals adsorption or Physisorption .
It is very slow . It is instantaneous . It is very specific depends on nature of adsorbent and adsorbate. .
It is non-specific XII U10-Surface XII U10-Surface - - - - . Chemical adsorption is fast with increase pressure, it can not alter the amount. . In Physisorption, when pressure increases the extent of adsorption increases.
. When temperature is raised chemisorption first increases and then decreases. . Physisorption decreases with increase in temperature.
. Chemisorption involves transfer of electrons between the adsorbent and adsorbate. . No transfer of electrons .
Heat of adsorption is high i.e., from - 400kJ/mole. . Heat of adsorption is low in the order of 40kJ/mole. .
Monolayer of the adsorbate is formed. . Multilayer of the adsorbate is formed on the adsorbent. .
Adsorption occurs at fixed sites called active centres. It depends on surface area . It occurs on all sides. .
Chemisorption involves the formation of activated complex with appreciable activation energy. . Activation energy is insignificant. .
. Factors affecting adsorption The adsorption is well understood by considering the various factors affecting it.