Before we delve into the concepts of physisorption and chemisorption, it is important for students to know what adsorption is. It is basically the process of accumulation of molecular species at the surface instead of solid or liquid bulks.
Adsorption is usually a surface phenomenon, and the best example that we can relate to is the air becoming dry in the presence of silica gel. During this process, the molecules of water get adsorbed on the surface of the gel. In essence, during adsorption, the molecules or atoms or ions of a gas or a liquid get attached to the surface.
Depending on the substance being deposited and adsorbed, adsorption is classified into two types:
- Physisorption
- Chemisorption
Physisorption
Physisorption is also known as physical adsorption and it is an exothermic process. Its adsorption enthalpy is low, nearly 20 to 40 kJ/mol. Normally in physisorption, the gas is accumulated on the solid surface due to weak force, known as Van der Waals forces.
Physisorption lacks specificity because the adsorbent (the surface or the material on which the process of adsorption takes place) in the given surface does not show any particular gas. It has a reversible nature, that is physisorption of gas by a solid can be reversed to a solid by gas.
Gas + solid = Solid/Gas + Heat
An example of physisorption is the adsorption of gases like hydrogen, nitrogen, etc at lower temperature on the surface of an adsorbent like charcoal.
Physisorption depends on the surface area of the adsorbent. As the surface area increases, the extent of adsorption also increases. For example, finely divided metals and porous substances have a large surface area. Therefore, they are considered good adsorbents. It also depends on the nature of the adsorbate (the accumulation of molecular species or substance at the surface).
Chemisorption
Chemisorption is also known as chemical adsorption. In chemisorption, adsorption takes place in adsorbed substance that is held by chemical bonds. Chemisorption has high specificity that is it is highly specific, and it takes place only if there is a chemical bonding between adsorbent and adsorbate.
Chemisorption has an irreversible nature and it also favours high pressure. Due to chemical bonding, enthalpy of adsorption of chemisorption is high, nearly 80 to 240 kJ/mol. Physisorption of gas adsorbed at a lower temperature may be converted into chemisorption at a higher temperature.
Chemisorption depends on the surface area. As the surface area of the adsorbent increases, chemisorption also increases. An example of chemisorption is the adsorption of hydrogen, nitrogen, etc on the surface of the adsorbent like ferrous catalyst at a high temperature.
Difference between Physisorption and Chemisorption
| Physisorption | Chemisorption |
| It is due to the formation of van der Waals forces. | It is due to the formation of chemical bonds. |
| It is reversible in nature. | It is irreversible in nature. |
| Physisorption is not specific in nature. | It is very specific in nature. |
| It has low adsorption enthalpy, nearly 20 to 40 kJ/mol. | Chemisorption has high adsorption enthalpy, nearly 80 to 240 kJ/mol. |
| It favours low temperature. | It favours high temperature. |
| Physisorption decreases with increase in temperature. | Chemisorption increases with increase in temperature. |
| It results in a multimolecular layer. | It results in a unimolecular layer. |
| Activation energy is less in physisorption. | Activation energy is high in chemisorption. |
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