Write short note on adsorption and explain it's mechanism
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Adsorption :
Adsorption is essentially a surface phenomenon. There are several examples, which reveal that the surface of a solid has the tendency to attract and retain the molecules with which itcomes into contact. These molecules remain only at the surface and do not go deeper into thebulk. The accumulation of molecular species at the surface rather than in the bulk of a solid or liquid is termed adsorption.
Explanation:
The molecular species or substance, which concentrates or accumulates at the surface is termed adsorbate and the material on thesurface of which the adsorption takes place is called adsorbent.
The adsorbed molecular species can be removed from the surface of the adsorbent and the process of removing an adsorbed substance from a surface on which it is adsorbed is called desorption.Solids, particularly in finely divided state, have large surface area and therefore, charcoal,silica gel, alumina gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.
Adsorption in action:
(i) If a gas like O2, H2, CO, Cl2, NH3 or SO2 is taken in a closed vessel containing powdered charcoal, it is observed that the pressure of the gas in the enclosed vessel decreases. The gas molecules concentrate at the surface of the charcoal, i.e., gases are adsorbed at the surface.
(ii) In a solution of an organic dye, say methylene blue, when animal charcoal is added and the solution is well shaken and filtered, it is observed that the filtrate turns colorless. The molecules of the dye, accumulate on the surface of charcoal, i.e., are adsorbed and the solution becomes colorless.
(iii) Aqueous solution of raw sugar, when passed over beds of animal charcoal, colorlessness as the coloring substances are adsorbed by the charcoal.
(iv) The air becomes dry in the presence of silica gel because the water molecules get adsorbed on the surface of the gel.
It is clear from the above examples that solid surfaces can hold the gas or liquid molecules by virtue of adsorption. The mask used for protection from air pollution works on the principle of adsorption. It contains finely divided charcoal that adsorbs the pollutant gases and leaves pure air for breathing.
Mechanism of Adsorption:
Adsorption arises due to the fact that the surface particles of the adsorbent are not in the same environment as the particles inside the bulk. Inside the adsorbent all the forces 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. The molecules at the surface have higher energy than those in the bulk. This extra energy per unit surface area is called surface energy. The residual attractive forces of the adsorbent are responsible for attracting the adsorbate particles on itssurface. The extent of adsorption increases with the increase of surface area per unit mass ofthe adsorbent at a given temperature and pressure.
Another important factor featuring adsorption is the enthalpy of adsorption. During adsorption, there is always a decrease in residual forces of the surface, i.e., there is decrease in surface energy which appears as heat. Adsorption, therefore, is invariably an exothermic process. In other words, the enthalpy of adsorption (∆H) is always negative. When a gas is adsorbed, the freedom of movement of its molecules becomes restricted. This amounts to decrease in the entropy of the adsorbate molecules after adsorption, i.e., ∆S is negative.Adsorption is thus accompanied by decrease in enthalpy as well as the entropy of the system.For a process to be spontaneous, the thermodynamic requirement is that, at constant temperature and pressure, ∆G must be negative, i.e., there is a decrease in Gibbs energy. Onthe basis of equation, ∆G = ∆H – T∆S, ∆G can be negative if ∆H has sufficiently highnegative value as – T∆S is positive. Thus, in an adsorption process, which is spontaneous, acombination of these two factors makes ∆G negative. As the adsorption proceeds, ∆H becomes less and less negative ultimately ∆H becomes equal to T∆S and ∆G becomes zero.At this state equilibrium is attained.