Adsorption is a surface phenomenon. It is observed at the surface of the solution. Adsorption is a phenomenon of concentration of substance on the surface of a liquid or solid.
'The condition in which concentration of a substance in the interfacial layers between two phases is greater than in the bulk of either phase, then the substance is said to be adsorbed at the interface and the phenomenon is known as adsorption'.
The process of adsorption of gases by solids is a common phenomenon. The charcoal specially coconut charcoal has a great capacity of the adsorption of gases. Silica gel is also utilised for the adsorption of number of gases. The solid that takes up gas or vapour or solute from a given solution is called adsorbent and the solute or gas which is held to surface of solid is known as adsorbate.
Classification of adsorption of gases on solids
The adsorption of gases on solids has been divided in to two types based on the nature of forces holding the gas molecules to the solids.
i. Physical adsorption (or) Vander waal's adsorption
ii. Chemical adsorption (or) chemisorption.
1. Physical adsorption
This adsorption is due to the operation of forces between solid surface and the adsorbate molecules that are similar to vander waal's forces between molecules. These forces are generally undirected and relatively non specific. Physical adsorption can also be defined as that type of adsorption where physical forces hold the gas molecules to the solids.
2. Chemical adsorption
Chemical adsorption is defined as a type of adsorption in which chemical bonds serve the function of holding gas molecules to the surface. It occurs due to the stronger binding forces, comparable with those leading to formation of chemical compounds. It is generally an irreversible process.
The main differences can be summarised as follows:-
1. It is due to intermolecular Vander waal's force.
2. Depends on the nature of gas. Easily liquefiable gases are adsorbed readily.
3. Heat of adsorption is small.
5. If occurs rapidly at low temperature and decreases with increase of temperature.
6. Increase of pressure increases adsorption.
7. Forms multimolecular layers on adsorbent surface.
1. It is due to chemical bond formation.
2. More specific than the physical adsorption.
3. Heat of adsorption is large.
5. Increases with increase of temperature.
6. Change of pressure has no effect.
7. Forms unimolecular layer
Factors affecting adsorption
The magnitude of gaseous adsorption depends upon the following factors:
3. Nature of the gas and
4. Nature of the adsorbent.
Effect of temperature and pressure
Adsorption is invariably accompanied by evolution of heat. Therefore, in accordance with Le chatelier's principle, the magnitude of adsorption increases with decrease in temperature. Further, since adsorption of a gas leads to decrease of pressure, the magnitude of adsorption increases with increase in pressure. Thus, decrease of temperature and increase of pressure both tend to cause increase in the magnitude of adsorption of a gas on a solid.
Nature of the gas
It is observed that the more readily soluble and easily liquefiable gases such as ammonia, chlorine and sulphur dioxide are adsorbed more than the hydrogen, nitrogen and oxygen. The reason is that Vander waal's or intermolecular forces which are involved in adsorption are more predominant in the former than in the latter.
Nature of the adsorbent
Adsorption is a surface phenomenon. Therefore, the greater the surface area per unit mass of the adsorbent, the greater is its capacity for adsorption under the given conditions of temperature and pressure.