Physisorption

Chemisorption

1. The forces of attraction between adsorbent and adsorbate are of Van der Waals type (weak forces).

2. This predominates at low temperatures.

3. Almost all gases show this type of adsorption at low temperatures.

4. The heat of adsorption is low and has a value of about 40 kJ per mole or less.

5. This type of adsorption attains equilibrium very rapidly on changing the temperature and pressure of the system.

6. This is reversible in nature.

7. The activation energy involved in this adsorption is small and is often less than 5 kJ. It is for this reason, this is even attained at low temperatures.

8. Adsorption in this case is often multilayer.

1. The forces of attraction between adsorbent and absorbate are of a chemical nature (strong forces).

2. This usually occurs at high temperatures.

3. It is highly specific in nature.

4. The heat of adsorption is high and has a value of the order of 80 to 420 kJ per mole.

5. This type of adsorption is relatively slower.

6. This is usually irreversible in nature. For example, O₂ adsorbed on charcoal, when adsorbed also releases CO and CO₂.

7. The activation energy involved in this adsorption is high. It is for this reason, it is attained only at high temperatures.

8. Adsorption in this case in monolayer.