Shaillee Kaushal is teaching live on Unacademy Plus
XIl -05 SURFACE CHEMISTRY
The tendency of accumulation of molecular species at the surface ihan in the bulk of a solid (or liquid) is termed adsorption. Adsorbate :_The moecular species or substance that concentrates or accumulates on the surface is termed adsorbate Adsorbent: The surface on which adsorption takes place is called adsorbent Adsorption is essentially a surface phenomenon and solids particularly in finely divided state have a large surface area and therefore show this proprty to a large extent than liquids. Charcoal, silica gel, alumina gel, fuller's earth, colloids, metals in finely divided state act as good adsorbents. The adsorption of gases on metal surfaces is called occulusion.
Examples of adsorption: (i) If a gas like Cl2, NH3 or SO2 is enclosed in presence of powdered charcoal in a closed vessel, it is observed that the pressure of the gas decreases. The gas molecules concentrate at the surface of charcoal i.e, gases are adsorbed at the surface. (ii) Water solution of raw sugar, when passed over beds of animal charcoal, becomes colorless as the coloring substance are adsorbed by animal charcoal (iii) The air becomes dry, i.e loses moisture in presence of silica gel. molecules of water concentrate on the surface of the gel i.e are adsorbed The process of removing an adsorbed substance is called desorption.
Adsorption Absorption 1. The substance is The substance is uniformly concentrated only at thedistributed throughout the bulk of the solid or liquid. surface and does not penetrate through the surface to go inside the bulk. This is a bulk phenomenon 2. This is a surface Absorption occurs at uniform rate phenomenon 3. Adsorption is rapid in the beginning and its rate slowly decreases
Mechanisms of Adsorption Adsorption is due to the fact that the surface particles of the adsorbent are in a different state than the particles inside the bulk. The particles on the surface experience residual attractive forces. These forces of the adsorbent are responsible for attracting the adsorbate particles on its surface. The extent of adsorption increases with the increase of surface area per unit mass of the adsorbent at a given temperature and pressure. This shows that with increase of surface area, the unbalanced attractive forces of the surface are also increased. During adsorption there is always a decrease in residual forces on the surface ie there is decrease in surface energy which appears as heat Adsorption therefore is exothermic process. In other words is always negative. when the gas is adsorbed, the freedom of movement of its molecules becomes restricted this results in decreases in entropy of the gas after adsorption i.e AS in negative Adsorption is thus accompanied by decrease in enthalpy as well as decrease in entropy of the system. For the process to be spontaneous, AG should be negative. G can be negative if has sufficiently high negative value as (-TAS) is positive. becomes less and less negative as adsorption proceeds further and further. Ultimately becomes equal to TAS and G becomes zero. Thus equilibrium is attained.
Ivpes of adsorption (i) Physical adsorption: If accumulation of gas on the surface of the solid occurs on account of weak vander waal's forces, the adsorption is called physical adsorption or physisorption. (ii) Chemical adsorption:_ When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption. -Chemisorption has high energy of activation and is therefore referred to as activated adsorption. These two processes can also occur simultaneously. A physical adsorption at low temperature may pass into chemisorption as the temperature is increased