24 March 2017

Difference between physical adsorption and chemical adsorption

What is Physical adsorption?

It is one in which the adsorbed molecules are held to the surface of the adsorbent by weak physical or van der Waal's forces.
This types of adsorption are completely reversible.
It is also called physisorption or van der Waal's adsorption.

Example: Adsorption of various gases by charcoal belongs to this type.

What is Chemical adsorption?

It is the force which holds the adsorbed molecules is of valence type.
This type of adsorption is irreversible.
It is also called chemisorption or activated adsorption.

Example: Adsorption of oxygen on tungsten is an example of this type.

Let us have a deep insight into the difference between physical and chemical adsorption. 

Difference :
  • Chemical adsorption heat evolved is considerably higher than evolved in physical adsorption.
  • The heat of adsorption for physical adsorption is 20-40 kcal/mol while 40-400 kcal/mol for chemical adsorption.
  • In physical adsorption, molecules are not tightly retained by the adsorbent while molecules are tightly retained by the adsorbent in case of chemical adsorption.
  • Adsorption is appreciable only at a temperature below the boiling point of the adsorbate in case of physical while in chemical it occurs at high temperature.
  • Multiplayer adsorption occurs in case of physical adsorption while adsorption leads to at most a monolayer in case of chemical adsorption.
  • Forces responsible for physical adsorption are very weak while in the case of chemical adsorption it is quite strong.
  • In physical adsorption, the rate of adsorption increases with the increase of pressure or of the adsorbate while in case of chemical adsorption rate of adsorption decrease with the increase of pressure or concentration of adsorbate.
  • Physical adsorption involves very small or little activation energy while chemical adsorption generally involves appreciable activation energy.
  • The equilibrium is established rapidly in a physical case while the establishment of equilibrium requires time in case of chemical adsorption.
  • No surface compound takes place in case of physical adsorption while actual surface compound formation between the adsorbent and adsorbate takes place in chemical adsorption.
  • Physical adsorption is very specific in nature while chemical adsorption is highly specific in nature.
  • Amount of adsorption on a surfaced is more a function of the adsorbate than the adsorbent in case of physical adsorption while the amount of adsorption is characteristics of both adsorbate and adsorbent in chemical adsorption.
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