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Adsorption: Types

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Adsorption: Types - Lesson Summary

Adsorption of gases on solids can be of two types - physical adsorption and chemical adsorption.

Physical adsorption or Physisorption or van der Waals adsorption:

When gas molecules or atoms are held to the surface of a solid by weak van der Waals' forces, then it is called physical adsorption.

EX: Adsorption of dihydrogen on the surface of finely divided platinum.

Chemical adsorption:

Chemical adsorption or chemisorption is defined as the phenomenon that occurs when gas molecules or atoms are held to the surface of a solid by chemical bonds.

The chemical bond can be ionic or covalent in nature.

Characteristics of Adsorption
→ Specificity of the Adsorbent
→ Nature of Adsorbate
→ Reversibility of the Process
→ Enthalpy of the Process
→ Activation Energy
→ layers of Adsorption
→ Surface Area of Adsorbent

Physical adsorption is not specific in nature, chemical adsorption is highly specific.

Both physical and chemical adsorption depends on the nature of the adsorbate.

Physical adsorption is reversible, but chemical adsorption is not.

Enthalpy change associated with physical and chemical adsorption:

In physical adsorption, since the van der Waals forces of attraction are weak, the heat evolved or the enthalpy of adsorption is very little - around 20 to 40 kilo joules per mole.

Chemical adsorption is also an exothermic process.

However, in chemical adsorption, surface compounds form, and the forces involved are similar to chemical bonds. Thus, a relatively high amount of heat is evolved about 80 to 240 kilo joules per mole,

Enthalpy of physical adsorption is low, whereas the enthalpy of chemical adsorption is very high.

Activation energy required for adsorption:

Very little activation energy is required for physical adsorption, but high activation energy is required for chemisorption

A gas may be physically adsorbed at low temperature, but chemisorbed at high temperature.

An increase in the temperature supplies the necessary activation energy for the formation of surface compounds, and the process is called activated adsorption.

In physical adsorption multi-molecular layers are formed under high pressure.

In chemical adsorption, a chemical bond is formed with the molecules that come in direct contact with the surface of the adsorbent.

Hence, only a unimolecular layer is formed in chemical adsorption.

Physical & chemical adsorption increases with an increase in the surface area of the adsorbent.

 S.No Characteristics of

          Physical Adsorption          
           Chemical Adsorption
1 Specificity Non specific in nature Highly specific in nature 2 Nature of adsorbate Depends upon nature of adsorbate Depends upon nature of adsorbate 3 Reversibility Reversible process Irreversibile in nature 4 Enthalpy Low (20 - 40 KJ / mol) High (80 - 240 KJ / mol) 5 Activation energy Very less activation energy is required High activation energy is required 6 Layers of adsorption Multi Molecular layers are formed under high pressure Unimolecular 7 Surface area of adsorbent Increases with the increase in surface area of adsorbent Increases with the increase in surface area of adsorbent


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