Friday, October 26, 2012

Factors Affecting The Adsorption of Gases by Solids

Following are the factors which influence the adsorption of gases by solids.
            1.         Surface area
            2.         Nature of gas
            3.         Temperature
            4.         Pressure
1.        Surface Area
                       Adsorption being a surface phenomenon, the extent of adsorption depends upon the surface area. Increase in the surface area of the adsorbent, increases the total amount of gas adsorbed. Thus finely divided metals (nickel, platinum) and porous substances (Charcoal, silica gel) provides large surface area and are best solid adsorbents.
2.        Nature of Gas
                     The amount of gas adsorbed by a solid depends upon the nature of gas. In general, more easily liquefiable a gas is (i.e. higher its critical temperature), the more readily will it be adsorbed. Thus 1gm of activated charcoal adsorbs 380 ml of sulphur dioxide (critical temperature 157°C), 16 ml of methane (critical temperature –83°C) and 4.5 ml of hydrogen (critical temperature  –240°C). This is valid for physical adsorption only.
                      Since Chemical Adsorption is Specific in nature, it occurs only if the gas can form a chemical bond with the solid.
3.        Tempreture
                    The process of Adsorption is an Exothermic Reaction. Thus according to Le-chatlier’s Principle, the magnitude of adsorption should increase with decrease in temperature. Infact it is found to be so in case of physical adsorption because vanderwaal’s forces are strong at low temperatures. However, the chemisorption first increases with rise in temperature and then starts decreasing. The initial increase shows that like chemical reactions, chemisorption also needs activation energy.
                        If a plot is drawn between amount of gas adsorbed (x/m) and temperature at constant equilibrium pressure, then curve obtained for physical adsorption shows there is a regular decrease in adsorption with temperature rise. While for chemisorption it first increases and then shows regular decrease. Such curves are known as Adsorption Isobars.


4.       Effect of Pressure
Effect of Pressure                        In order to understand the effect of pressure on adsorption of gas on some solid we must keep in mind that physical adsorption is reversible in nature and is accompanied by decrease in pressure. Therefore, it is expected that extent of adsorption increases with increase in pressure and decrease in pressure causes desorption.
                        The extent of adsorption is generally expressed as x/m where ‘m’ is mass of adsorbent and ‘x’ is mass of adsorbate when equilibrium has attained. The graph between extent of adsorption (x/m) and the pressure ‘P’ of gas at constant temperature is called Adsorption  Isotherm.
This is a simple type of adsorption isotherm in which at equilibrium pressure Ps,  reaches its maximum value and no more adsorption takes place even if the pressure is further increased. This state is also called SaturationState
and the corresponding pressure (Ps) is called Saturation Pressure. Such isotherms are obtained in cases where adsorbing gases forms unimolecular layers on the surface of adsorbent and adsorbing gas behaves ideally in vapour phase.

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