What is the Difference Between Chemisorption and Physisorption?

The main difference between chemisorption and physisorption lies in the forces that hold the atoms, molecules, or ions to the surface and the energy required for adsorption. Here are the key differences between the two processes:

1. Forces involved:

• Physisorption involves weak physical forces, such as Van der Waals forces, to hold the substances onto the surface.
• Chemisorption involves strong chemical bonds, such as ionic or covalent bonds, to hold the substances onto the surface.

1. Adsorption Enthalpy:

• The adsorption enthalpy of physisorption is low, typically around 20 to 40 kJ/mol.
• The adsorption enthalpy of chemisorption is high, usually around 80 to 240 kJ/mol.

1. Temperature Dependence:

• Physisorption decreases with an increase in temperature.
• Chemisorption increases with an increase in temperature.

1. Activation Energy:

• Physisorption has a low activation energy, making it a reversible process.
• Chemisorption has a high activation energy, making it irreversible in some cases.

1. Layer Formation:

• Physisorption results in a unimolecular layer, meaning only one molecule is adsorbed on the surface.
• Chemisorption results in a multimolecular layer, meaning several molecules are adsorbed on the surface.

1. Surface Area Dependence:

• Chemisorption depends on the surface area of the adsorbent, meaning that as the surface area increases, chemisorption also increases.

Examples of physisorption include the adsorption of gas on a solid surface at low temperatures due to weak forces, while examples of chemisorption include the adsorption of hydrogen, nitrogen, etc., on the surface of a catalyst like ferrous at high temperatures.

Comparative Table: Chemisorption vs Physisorption

Here is a table comparing the differences between chemisorption and physisorption:

Physisorption is a reversible process with weak interactions, while chemisorption is an irreversible process with strong interactions involving chemical bonding.