When a surface of solid or liquid is exposed to a substance, molecules of that substance concentrate at that surface. The phenomenon of accumulation of a substance on the surface of a solid or liquid is called adsorption. The substance that accumulates at the surface is known as adsorbate while the surface of the material where adsorption occurs is called adsorbent.
The experimental study of adsorption of gases on solids shows that there are two types of adsorption, physisorption (physical adsorption) and chemisorption (chemical adsorption).
There are many differences between these two types of adsorption such as physisorption has weak van der Waals forces and has a multilayer of molecules. However, chemisorption has strong chemical bonds and has a unimolecular layer of molecules that are bound to active sites on the surface of the adsorbent.
Physisorption vs Chemisorption
It is caused by van der Waals forces
It is caused by chemical bonds
Physisorption is a multilayered phenomenon
Chemisorption is a monolayered phenomenon
It is not specific to the adsorbent
It is highly specific to the adsorbent
Physisorption usually occurs at low temperatures
Chemisorption usually occurs at high temperatures
Enthalpy of adsorption is smaller for physisorption
Enthalpy of adsorption is larger for chemisorption
There is no activation energy is involved
Activation energy is significantly involved
It is a reversible kind of adsorption
It is an irreversible adsorption
Physisorption increases with an increase in pressure
It has no such effect of pressure but a very high pressure increases chemisorption
This is a function of adsorbate as compared to an adsorbent
This is characteristic of both adsorbate and adsorbent
Examples of physisorption are: Adsorption of oxygen or hydrogen on the surface of charcoal
Examples of chemisorption are: Adsorption of oxygen on the surface of metals
Physisorption (Physical adsorption)
In physisorption, the molecules of adsorbate are held on the surface of the adsorbent by van der Waals type of interaction. This is the reason why physisorption is also known as van der Waals adsorption. These forces are weak to hold gas molecules on the surface of the adsorbent.
In physical adsorption, the total amount of gas adsorbed on the surface of the adsorbent depends on the nature of the gas. This gas can be adsorbed easily if it is more liquefiable. This means it has a higher critical temperature. For example, 379 ml of sulfur dioxide is adsorbed on one gram of activated charcoal at a critical temperature of 157 °C. However, 16 ml of methane is adsorbed at a critical temperature of -83 °C.
The heat (enthalpy) of adsorption is the energy liberated when one gram mole of a gas is adsorbed on the surface of the adsorbent. The forces between the molecules of adsorbate and adsorbent are very weak. Therefore, the heat of adsorption is very small as compared to chemisorption with a general value of 5 kcal per mol.
This type of adsorption occurs very fast at low temperatures. This is because they have weak forces which work better at low temperatures. At high temperatures, these forces rapidly start to decrease and the process of physisorption changes to chemisorption.
Adsorption isotherms show that at low pressure, the gas is adsorbed on the adsorbent in a single molecular thick layer. At high pressure, the process of adsorption increases. The molecules of gases are in chaos and start to bind on the surface of other molecules. In this way, they form a multimolecular thick layer.
Examples of Physisorption
Adsorption of gases molecules on charcoal such as;
- Oxygen (O2)
- Nitrogen (N2), etc
Chemisorption (Chemical adsorption)
In this type of adsorption, molecules of adsorbate are accumulated on the surface of the adsorbent by chemical bonds. In chemisorption, the forces that are involved are very strong such as ionic bonds, covalent bonds, and certain types of intermolecular forces. Chemisorption is also known as activated adsorption.
This type of adsorption is highly selective such that, only certain types of gas molecules are adsorbed by specific types of solids. It is more specific than physisorption and does not occur when there are chances of chemical reactions between the adsorbed molecules (adsorbate) and surface molecules (adsorbent).
Chemical adsorption has larger heat (enthalpy) of adsorption than physical adsorption which is about 20 to 100 kcal per mol. The larger value of heat of adsorption is a result of the strong forces of attraction in chemisorption.
Most chemical changes basically increase when their temperature increases. Therefore, when there is an increase in the temperature physisorption gets changed into chemisorption. For example, at a temperature of 190 °C nitrogen is physically adsorbed on iron while at 500 °C, it starts chemisorption.
In this type of adsorption, only a unimolecular layer is formed. This is because the gas molecules bind chemically with the sites on the surface of the adsorbent and do not bind with each other.
Examples of Chemisorption
- Adsorption of oxygen on the surface of metals.
- Adsorption of hydrogen on nickel for hydrogenation reactions.
Is physisorption highly specific in nature?
Physisorption is not highly specific because it has weak van der Waals forces. This occurs at any solid surface for a limited time while chemisorption is a type of adsorption which is highly selective.
What is the application of adsorption?
There is the following application of adsorption:
- Removal of heavy metals from water
- Heterogeneous catalysis
- Gas masks
- Ion exchange method
- Dye industry
- Hydrogenation, etc
Why physisorption is reversible whereas chemisorption is not?
It is because physisorption has weak forces of attraction while chemisorption has stronger forces of attraction.
Why is adsorption an exothermic process?
Whenever a bond is formed during the process, it releases energy. As bonds are formed during adsorption, heat is released in the form of energy and thus, adsorption reactions are exothermic.
What is the difference between ion exchange and adsorption?
Ion exchange is a method where ions of substances are exchanged with resin ions while adsorption is a process where molecules of substances accumulate on the surface of a solid.
Why is chemisorption called activated adsorption?
It is because chemisorption requires certain activation energy to start the process. Once it starts, it becomes irreversible.
- Essential of Physical Chemistry: 2nd edition By B.S Bahl (Gurdaspur, India) and Arun Bahl (RSC, UK) and G.D. Tuli (Delhi University, India)
- Principle of Physical Chemistry by Haq Nawaz Bhatti (University of Agriculture, Faisalabad, Pakistan)
- Physisorption (corrosionpedia.com)