Physisorption and Chemisorption - Practice Questions & MCQ

You will come across mainly two types of adsorptions namely Physical and Chemical adsorption.

Let us talk about Physical adsorption.

If the accumulation of gas on the surface of a solid occurs on account of weak van der Waals’ forces, the adsorption is termed as physical adsorption or physisorption. This type of adsorption is reversible with the temperature usually occurs at low temperature.

What are the Characteristics of Physical adsorption?

1. A given surface of an adsorbent does not show any preference for a particular gas as the van der Waals’ forces are universal.

2. Physical adsorption of a gas by a solid is generally reversible. Thus,

   solid+gas  gas/solid + heat 

   More gas is adsorbed when pressure is increased as the volume of the gas decreases (Le–Chateliers’s principle) and the gas can be removed by decreasing pressure. Since the adsorption process is exothermic, the physical adsorption occurs readily at low temperatures and decreases with increasing temperature (Le-Chatelier’s principle).

3. It is reversible with temperature and usually occurs at low temperatures.

4. No doubt, physical adsorption is an exothermic process but its enthalpy of adsorption is quite low. This is because the attraction between gas molecules and solid surface is only due to weak van der Waals’ forces

5. It is exothermic, about 20-40 kJ/mole heat energy is given out.

Example of Physical Adsorption - Sticking of the water vapours around a cold glass full of water due to condensation.

Now as you have learned about Physical Adsorption or Physiosorption, let us talk about Chemical adsorption or Chemisorption.

When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption. The chemical bonds may be covalent or ionic in nature. Chemisorption involves high energy of activation and is, therefore, often referred to as activated adsorption. In this adsorption process, reactant molecules are adsorbed on the surface by free valencies. It is irreversible with the temperature usually occurs at high temperatures.

What are the Characteristics of Chemical adsorption?

1. Chemisorption is highly specific and it will only occur if there is some possibility of chemical bonding between adsorbent and adsorbate. For example, oxygen is adsorbed on metals by virtue of oxide formation, and hydrogen is adsorbed by transition metals due to hydride formation.
2. Chemisorption is also an exothermic process but the process is very slow at low temperatures on account of the high energy of activation. Like most chemical changes, adsorption often increases with rise in temperature.
3. Physisorption of a gas adsorbed at a low temperature may change into chemisorption at a high temperature. Usually, high pressure is also favourable for chemisorption.
4. It is irreversible with temperature, usually occurs at high temperatures.
5. Enthalpy of chemisorption is high (80-240 kJ mol^-1) as it involves chemical bond formation.