Physisorption and chemisorption. Give one example.

Physisorption :

1. In physisorption, the forces operating are weak van der Waals forces.

2. It is not specific in nature as all gases adsorb on all solids. 

For example,

All gases adsorb on charcoal.

3. The heat of adsorption is low and lies in the range 20-40 kJ mol^-1.

4. It occurs at low temperature and decreases with an increase of temperature.

5. It is reversible. 

6. Physisorbed layer may be multimolecular layer of adsorbed particles under high pressure. 

e.g.,

At low temperature N₂ gas is physically adsorbed on iron.

Chemisorption :

1. In chemisorption, the forces operating are of chemical nature (covalent or ionic bonds).

2. It is highly specific and occurs only when chemical bond formation is possible between adsorbent and adsorbate. 

For example,

Adsorption of oxygen on tungsten, hydrogen on nickel, etc.

3. The heat of adsorption is high and lies in the range 40-200 kJ mol^-1.

4. It is favoured at high temperature, however, the extent of chemical adsorption is lowered at very high temperature due to bond breaking.

5. It is irreversible. 

6. Chemisorption forms monomolecular layer of adsorbed particles.

e.g., 

N₂ gas chemically adsorbed on iron at high temperature forms a layer of iron nitride, which desorbs at very high temperature.