Types of Adsorption

On the basis of nature of forces which hold molecules of adsorbate to the surface of the adsorbent, adsorption is classified into two types as under:-

            I.          Physical Adsorption

            II.        Chemical Adsorption

I.          PHYSICAL ADSORPTION OR PHYSISORPTION

                        In this type of adsorption, the molecules of adsorbate are held by the weak vanderwaal’s forces of attraction. The heat of adsorption in this type is of order of 20 to 40 KJ/Mol. If the temperature is raised, the kinetic energy of the gas molecules increases and they leave the surface of the adsorbent. Thus, rise in temperature lowers the extent of adsorption. Moreover, in physical adsorption, the equilibrium (between adsorption and desorption) is reversible and is established rapidly. Physical adsorption does not depend upon the chemical nature of substance which is adsorbed. Physical adsorption increases with increase in pressure.

II.        CHEMICAL ADSORPTION OR CHEMISORPTION

                        Unlike physical adsorption chemical adsorption involves formation of surface compound i.e. chemical linkage is formed between adsorbed molecule and the surface of adsorbent. Thus it is highly selective. It is found that only particular type of molecules are adsorbed by a solid in chemisorption. In other words, this type of adsorption depends upon the chemical properties of gas and the adsorbent. Moreover, chemisorption is accompanied by much higher heat changes 40–400KJ Mol^–1.

                        Unlike physical adsorption, it is not reversible. In many cases it is found that physical adsorption takes place at low temperature. But as the temperature is raised, it changes into chemical adsorption. Chemical adsorption is often called as Activated Adsorption.

DIFFERENCE BETWEEN Physical Adsorption  AND Chemical Adsorption

S.No.	PHYSICAL ADSORPTION	S.No.	CHEMICAL ADSORPTION
1.	Molecules are held due to vanderwaal’s forces.	1.	Molecules are held due to chemical bond forces.
2.	Heat of adsorption are in the range of 20 – 40KJ/Mol.	2.	Heat of adsorption are in the range of 40 – 400 KJ/Mol.
3.	Activation energy are small.	3.	Activation energy are appreciable.
4.	Usually occurs rapidly at low temperatures and decreases with increase in temperature.	4.	Can occur at high temperature.
5.	Usually completely reversible.	5.	Often irreversible.
6.	The extent of adsorption is approximately related to the ease of liquification of gas.	6.	No such correlation is there.
7.	Not very specific.	7.	Often highly specific.
8.	Form multilayer on the surface of adsorbent.	8.	Forms unimolecular layer.
9.	No surface compound is formed	9.	Surface compounds are formed.

Applications of adsorption

                        The phenomenon of adsorption finds extensive application in field of Industry, laboratory and various other technical processes. A few important applications of adsorption are as under:-

1.       Manufacture Of Gas Masks

                    Activated charcoal used in gas masks removes all undesirable toxic and poisonous gases while purified air passes through its pores.

2.         Removing Of Colouring Matter From Sugar Juice & Vegetable Oils

                        Activated animal charcoal is used as decolourizer in removing colouring matter of sugar solution and other vegetable materials.

3.         Dehydration & Purification Of Gases

                        Silica gel and Alumina are used as good adsorbents for removing moisture and for controlling humidities in rooms which store delicate articles.

4.         Dyeing of Clothes

                        Modrants used in dyeing adsorb the colouring matter which does not attach to fabric otherwise.

5.         Creation Of High Vacuum In Laboratory

                        Finely divided coconut charcoal adsorbs all gases creating a vacuum as low as 10–4 mm.

6.         Heterogenous Catalysis

                        The action of heterogeneous catalysts like finely divided nickel, finely divided platinum, finely divided iron in hydrogenation of oils, conversion of SO2 to SO3 and manufacture of ammonia respectively is based on the phenomenon of adsorption.    

7.         Ion-Exchange Resins

                         The organic high polymers containing groups such as –COOH, –SO3H or –NH2 possess the property of selective adsorption of ions from solution. These resins are largely used in Industrial softening of water and separation of rare earths from their mixture.

8.         Chromotography

                         There are a number of chromotographic techniques like adsorption chromotography, paper chromotography and vapour phase chromotography which are based on selective adsorption of different substances of the adsorbent.

9.         Qualitative Inorganic Analysis

                        The confirmatory lake test for Al3+ ions is based on the adsorption of colour of litmus by Al(OH)3.

10.       Drugs

                        Various drugs get adsorbed on the tissues which then are heated. The germicidal action of medicines is also based on the property of adsorption.