Difference between Adsorption and Absorption

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chemistry 12th class cbse notes

Difference between Adsorption and Absorption

Main points of difference between adsorption and absorption are given below:


Adsorption Absorption
It is surface phenomenon. It concerns with the whole mass of the absorbent.
In it, the substance is only retained on the surface and does not go into the bulk or interior of the solid or liquid. It implies that a substance is uniformly distributed, through the body of the solid or liquid.
In it the concentration of the adsorbed molecules is always greater at the free phase. In it the concentration is low.
It is rapid in the beginning and slows down to attain equilibrium. It occurs at the uniform rate.
Example:
Water vapours are adsorbed by CaCl2.
NH3 is adsorbed by charcoal.
Decolourization of sugar solution by activated or animal charcoal.
Ink is adsorbed by blotting paper.
Example:
Water vapours are absorbed by anhydrous silica gel.
NH3 is absorbed in water forming NH4OH.

Comparison Between Physisorption and Chemisorption


Physisorption Chemisorption
Low heat of adsorption usually in range of 20-40KJ/mol. The high heat of adsorption usually in the range of 50-400KJ/mol.
Force of attraction are Vander Waal’s forces. Force of attraction are Chemical bond forces.
It is reversible. It is irreversible.
It usually takes place at low temperature and decreases with increasing temperature. It takes place at high temperature.
It is related to the case of liquefication of the gas. It is not related.
It forms multimolecular layers. It forms monomolecular layers.
It does not require any activation energy. It requires high activation energy.
High pressure is favourable. The decrease of pressure causes desorption. High pressure is favourable. The decrease of pressure does not cause desorption.
It is not very specific. It is highly specific.

Note:

  • Due to formation of multilayers physical adsorption decreases after some times.
  • Chemisorption and physisorption both are exothermic.

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