This tendency of accumulation of molecular species at the surface than in the bulk of a solid (or liquid) is termed adsorption. The molecular species or substance which concentrates or accumulates at the surface is termed adsorbate and the material on whose surface the adsorption has taken place is called adsorbent.
The reverse process i.e. removal of adsorbed substance from the surface is called desorption.
The adsorption of gases on the surface of metals is called occlusion.
The term sorption is employed when adsoption as well as absorption take place simultaneously.
Distinction Between Adsorption And Absorption:
In adsorption the concentration of the adsorbate increases only at the surface of the adsorbent, while in absorption the concentration is uniform throughout the bulk of the solid
Adsorption is due to the fact that the surface particles of the adsorbent are in different state than the particles inside the bulk. Inside the adsorbent all the force acting between the particles are mutually balanced but on the surface the particles are not surrounded by atoms or molecules of their kind on all sides and hence they possess unbalanced or residual attractive forces.
These forces of the adsorbent are responsible for attracting the adsorbate particle on its surface.
Adsorption is a surface phenomenon, whereas absorption is a bulk phenomenon. Adsorption occurs only at the surface of adsorbent, whereas absorption occurs throughout the body of the material.
1. It is a surface phenomenon.
1. It concerns with the whole mass of the a absorbent.
2. In it, the substance is only retained on the surface and the surface and does not go into the bulk or interior of the solid or liquid
2. It implies that a is substance is uniformly distributed , through the body of the solid or liquid
3. In it the concentration of the adsorbed molecules is always greater in the free phase.
3. In it the concentration is low.
4. It is rapid in the beginning and slows down near the equilibrium.
4. It occurs at the uniform rate.
5 Examples -
(a) Water vapours adsorbed by silica gel, NH3 is adsorbed by charcoal.
(a) Water vapours absorbed by anhy. CaCb
(b) N2 is adsorbed on mica, O2 is adsorbed on tungsten surface
(b) NH3 is absorbed in water forming NH4OH
Types of adsorption
(A) Positive and negative adsorption
When the concentration of the adsorbate is more on the surface of the adsorbent than in the bulk, it is called positive adsorption.
When the concentration of the adsorbate is less relative to its concentration in the bulk, it is called negative adsorption.
(B) Physisorption and chemisorption
When a gas is adsorbed at the surface of a solid by week forces (Van der Waal's forces), it is called physical adsorption.
When a gas is held on the surface of a solid by forces similar to those of a chemical bond, it is called chemical adsorption or chemiosorption. The chemical bonds may be covalent or ionic in nature. Chemisorption has a rather high energy of activation and is, therefore, often referred to as activated adsorption.
Sometimes these two processes occur simultaneously and it is not easy to ascertain the type of adsorption. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased. For example, hydrogen is first adsorbed on nickel by van der Walls' force. Molecules of hydrogen then dissociate and hydrogen atoms are held on the surface by chemisorption.
Potential energy map of physi-sorption and chemisorption:
Fig: Potential energy map of physiosorption and chemosorptionComparison of physisorption and chemisorption:
|Physical adsorption||Chemical adsorption|
|1. It is caused by intermolecular van der Walls' forces||It is caused by chemical bond formation.|
|2. It is not specific.||It is highly specific.|
|3. It is reversible.||It is irreversible.|
|4. It depends on the nature of gas. More easily form com liquefiable gases are adsorbed readily.||It depends on the nature of gas. Gases which pounds with the adsorbent exhibit chemisorption.|
|5. Heat of adsorption is low.||Heat of adsorption is high.|
6. Low temperature is favourable. It decreases with increase of temperature
High temperature is favourable. It increases with increase of temperature.
|7. No appreciable activation energy is involved.||High activation energy is involved.|
|8. High pressure is favourable. does pressure causes desorption.||Decrease of High pressure is favourable. Decrease of pressure not cause desorption.|
|9. It depends on the surface area. It increases with increase of surface area.||It also depends on the surface area. It increases with increase of surface area.|
|10. It forms multilayers on adsorbent surface under high pressure.||It forms unimolecular layer.|
Characteristic of Adsorption