JEE Main & Advanced Chemistry Surface & Nuclear Chemistry / भूतल और नाभिकीय रसायन Classification Of Adsorption

Adsorption can be classified into two categories as described below,

(1) Depending upon the concentration :  In adsorption the concentration of one substance is different at the surface of the other substance as compared to adjoining bulk or interior phase.

(i) Positive adsorption : If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk phase then it is called positive adsorption.

Example : When a concentrated solution of KCl is shaken with blood charcoal, it shows positive adsorption.

(ii) Negative adsorption : If the concentration of the adsorbate is less than its concentration in the bulk then it is called negative adsorption.

Example : When a dilute solution of KCl is shaken with blood charcoal, it shows negative adsorption.

(2) Depending upon the nature of force existing between adsorbate molecule and adsorbent

(i) Physical adsorption : If the forces of attraction existing between adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called physical adsorption. This type of adsorption is also known as physisorption or Vander Waal’s adsorption. It can be easily reversed by heating or decreasing the pressure.

(ii) Chemical adsorption : If the forces of attraction existing between adsorbate particles and adsorbent are almost of the same strength as chemical bonds, the adsorption is called chemical adsorption. This type of adsorption is also called as chemisorption or Langmuir adsorption. This type of adsorption cannot be easily reversed.

Comparison between physisorption and chemisorption

Factors which affect the extent of adsorption : The following are the factors which affect the adsorption,

(1) Nature of the adsorbate (gas) and adsorbent (solid)

(i) In general, easily liquefiable gases e.g., CO₂, NH₃, Cl₂ and SO₂ etc. are adsorbed to a greater extent than the elemental gases e.g. H₂, O₂, N₂, He etc. (while chemisorption is specific in nature.)

(ii) Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks.

(2) Surface area of the solid adsorbent

(i) The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption.   

(ii) Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface area.

(3) Effect of pressure on the adsorbate gas

(i) An increase in the pressure of the adsorbate gas increases the extent of adsorption.

(ii) At low temperature, the extent of adsorption increases rapidly with pressure.

(iii) Small range of pressure, the extent of adsorption is found to be directly proportional to the pressure.

(iv) At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting value.

(4) Effect of temperature

(i) As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature.  

(ii) The relationship between the extent of adsorption and temperature at any constant pressure is called adsorption isobar.

(iii) A physical adsorption isobar shows a decrease in x/m (where ‘m’ is the mass of the adsorbent and ‘x’ that of adsorbate) as the temperature rises.

(iv) The isobar of chemisorption show an increase in the beginning and then decrease as the temperature rises.