Q.What is the difference between physisorption and chemisorption?

Adsorption is classified by the nature of the forces binding the adsorbate to the adsorbent.

Physisorption (physical adsorption) — the adsorbate is held by weak van der Waals forces:

• Low enthalpy of adsorption (~20-40 kJ/mol) and reversible.
• Not specific — any gas adsorbs on any solid to some extent.
• Favoured by low temperature; it decreases as temperature rises because the weak van der Waals attractions cannot hold the gas molecules once they gain thermal energy and escape from the surface.
• Increases with the ease of liquefaction of the gas and forms multi-molecular layers.

Chemisorption (chemical adsorption) — the adsorbate is held by strong chemical (covalent) bonds. Two characteristics:

• High enthalpy of adsorption (~80-240 kJ/mol) and it is essentially irreversible and highly specific.
• Forms only a unimolecular (monolayer) layer; it first increases with temperature (like a chemical reaction needing activation energy) and is not reversed easily.