When solids are allowed to remain in contact with a gas. a film of gas molecules accumulates on the surface. The molecules or ions at the surface of a solid have unsaturated or residual forces. Solid and liquid surfaces therefore have a tendency to satisfy their residual forces by attracting onto and retaining on their surfaces, molecules of gases or other substances with which they come in contact. This results in higher concentration of molecules on the surface of a solid or liquid than in the bulk of the medium. The phenomenon is known as adsorption.
The substance on which the adsorption takes place is known as the adsorbent and the substance which adsorbed is known as adsorbate. The phenomenon of adsoiption should not be confused with absorption which refers m one material passing into the bulk of another. For example water is absorbed by a sponge and water vapours arc adsorbed by anhydrous calcium chloride. Similarly, the process in which both absorption and adsorption take place simultaneously is known as sorption. For example: dyeing of cotton fibers.
All solids adsorb gases to some measurable extent. The amount of adsorbed gas as a rule decreases with increase of temperature at a constant pressure and increases with a fhll in temperature. Hence, according to Le Chatelier Braun principle, adsorption of a gas by a solid is accompanied by evolution of heat which is known as the heat of adsorption. The extent of adsorption increases with increase of pressure.
TYPES OF ADSORPTION:
Depending on the type of forces that hold the adsorbed molecules on the surface has a considerable effect on certain characteristics. Thus adsorption can be divided into two types.
(1) PHYSICAL ADSORPTION OR PHYSISORPTION OR VAN DER WAALS ADSORPTION
It is the adsorption in which molecules of the adsorbate are held to the surface of the adsorbent by weak Vander Waal’s forces. Thus physical adsorption has tow heat of adsorption of the order of 5 to 10 kcal per mole of the gas. A rise in temperature will increase the kinetic energies of molecules and the molecules will leave the surface thus lowering the extent of adsorption. Another characteristic of physical adsorption is that the adsorption equilibrium is reversible and is established rapidly. Examples; Adsorption of hydrogen or oxygen on charcoal or adsorption of nitrogen on mica.
(2)CHEMICAL ADSORPTION OR CHEMISORPTION
It is the adsorption in which molecules of the adsorbate are held to the surface of the adsorbent by chemical bond which may be covalent or ionic. Chemisorption is thus highly selective since only certain types of molecules will be adsorbed by a particular solid. This depends on the chemical properties of gas and the adsorbent. The chemical bond produces strong forces hence chemical adsorption has high heat of adsorption from 10 kcals to 100 kcals per mole. Unlike physical adsorption, chemical adsorption is not reversible. Hydrogen is strongly adsorbed by the metals;nickel, iron and platinum.
In many cases, it has been observed that adsorption is neither physical nor chemical but a combination of the two. Some systems show physical adsorption at low temperatures but as the temperature is raised physical adsorption changes into chemical adsorption.