What is the primary difference in the forces involved in physisorption and chemisorption?

The fundamental distinction lies in the nature of the attractive forces. Physisorption involves weak intermolecular forces, primarily Van der Waals forces (like London dispersion forces, dipole-dipole interactions), which are physical in nature. Chemisorption, on the other hand, involves the formation of strong chemical bonds (covalent or ionic) between the adsorbate and the adsorbent surface, making it a chemical process. This difference dictates most other contrasting properties.

Why is physisorption reversible, while chemisorption is generally irreversible?

Physisorption is reversible because the weak Van der Waals forces holding the adsorbate to the surface can be easily overcome by increasing the temperature or decreasing the pressure. The adsorbate molecules simply gain enough kinetic energy to escape. Chemisorption, however, involves the formation of strong chemical bonds. Breaking these bonds requires a significant amount of energy, making the process largely irreversible under normal conditions.

How does temperature affect physisorption and chemisorption differently?

Physisorption is an exothermic process with low enthalpy, so it decreases with increasing temperature. Higher temperatures provide more kinetic energy to adsorbate molecules, favoring desorption. Chemisorption is also exothermic but often requires an activation energy. Thus, it typically increases with temperature initially (to overcome the activation barrier), reaches a maximum, and then decreases at very high temperatures as desorption becomes dominant.

Can both physisorption and chemisorption occur simultaneously on a surface?

Yes, it is possible for both types of adsorption to occur on the same surface, though usually under different conditions or at different sites. At very low temperatures, physisorption might dominate. As the temperature is raised, physisorption decreases, and if the conditions are right, chemisorption might begin to occur, especially if an activation energy barrier needs to be overcome. Sometimes, physisorption can even precede chemisorption, acting as a precursor state where molecules are weakly held before forming stronger chemical bonds.

Why is chemisorption highly specific, while physisorption is non-specific?

Chemisorption is highly specific because it requires the formation of chemical bonds between the adsorbate and the adsorbent. This means there must be a chemical affinity, specific orbital overlap, or suitable electronic configuration for bond formation to occur, similar to any chemical reaction. Physisorption, relying on universal Van der Waals forces, is non-specific; any gas can physisorb on any solid surface, provided the temperature is low enough and pressure is high enough.

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Factors Affecting Adsorption

Physisorption and Chemisorption

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Version 1Updated 22 Mar 2026

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Adsorption is a surface phenomenon where molecules of a substance (adsorbate) accumulate on the surface of another substance (adsorbent), forming a layer. This process is distinct from absorption, where molecules penetrate the bulk of the material. Adsorption can be broadly classified into two main types based on the nature of the forces holding the adsorbate to the adsorbent surface: physisorptio…

Quick Summary

Adsorption is a surface phenomenon where molecules (adsorbate) accumulate on a surface (adsorbent). It's distinct from absorption, which involves bulk penetration. There are two main types: physisorption and chemisorption.

Physisorption involves weak Van der Waals forces, is non-specific, reversible, forms multilayers, has a low enthalpy of adsorption ( $20-40 \text{ kJ/mol}$ ), and decreases with increasing temperature. Chemisorption involves strong chemical bonds, is highly specific, generally irreversible, forms a monolayer, has a high enthalpy of adsorption ( $80-240 \text{ kJ/mol}$ ), and typically increases with temperature initially before decreasing.

Both are exothermic processes. Understanding their differences is crucial for applications in catalysis, purification, and separation techniques.

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Key Concepts

Effect of Temperature on Adsorption

Temperature plays a crucial, yet contrasting, role in physisorption and chemisorption. For physisorption,…

Specificity and Nature of Forces

The specificity of adsorption is directly linked to the nature of the forces involved. Physisorption is…

Enthalpy of Adsorption and Reversibility

The magnitude of the enthalpy of adsorption ( $\Delta H_{ads}$ ) is a key indicator of the strength of…

Physisorption: — Weak Van der Waals forces, low

\Delta H_{ads}

(

20-40 \text{ kJ/mol}

), non-specific, reversible, multilayer, decreases with T, negligible activation energy.

Chemisorption: — Strong chemical bonds, high

\Delta H_{ads}

(

80-240 \text{ kJ/mol}

), highly specific, irreversible, monolayer, increases then decreases with T (often needs activation energy).

Both are exothermic processes.

To remember the key differences: Physical is Peaceful, Chemical is Committed.

Physical (Physisorption):

Peaceful forces (Weak Van der Waals)

Paltry energy (Low

\Delta H_{ads}

)

Plethora of layers (Multilayer)

Passive to temperature (Decreases with T)

Partial commitment (Reversible)

Particularity not needed (Non-specific)

Chemical (Chemisorption):

Committed bonds (Strong chemical bonds)

Considerable energy (High

\Delta H_{ads}

)

Coverage is one (Monolayer)

Challenged by temperature (Increases then decreases with T, needs activation)

Complete commitment (Irreversible)

Choosy (Highly specific)

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Factors Affecting Adsorption