Qualitatively, the extent of surface adsorption depends on the following factors (i) Nature of adsorbent (ii) Nature of adsorbate (iii) Pressure (iv) Concentration at a given temperature. . Surface area of adsorbent: As the adsorption is a surface phenomenon it depends on the surface area of adsorbent. i.e., higher the surface area, higher is the amount adsorbed.
. Nature of adsorbate The nature of adsorbate can influence the adsorption. Gases like SO ,NH ,HCl and CO are easily liquefiable as they have greater van der waal’s force of attraction. On the other hand, permanent gases like H ,N and O can not be liquefied easily.
These permanent gases are having low critical temperature and adsorbed slowly, while gases with high critical temperature are adsorbed readily. XII U10-Surface XII U10-Surface - - - - . Effect of temperature When temperature is raised chemisorption first increases and then decreases. whereas physisorption decreases with increase in temperature.
. Effect of pressure: chemical adsorption is fast with increase in pressure, it can not alter the amount of adsorption. In Physisorption the extent of adsorption increases with increase in pressure. .
. Adsorption isotherms and isobars. Adsorption isotherms represents the variation of adsorption at constant temperature. When amount of adsorption is plotted versus temperature at constant pressure it is called adsorption isobar.
Adsorption isobars of physisorption and chemisorption are different as represented in the graphs. Figure . (a) Physical Adsorption Figure . (b) Chemical Adsorption Temp m x Temp m x x is the amount of adsorbate adsorbed on ‘m’ g of adsorbent.
In physical adsorption, x m decreases with increase in Temprature, But in chemical adsorption, x m increases with rise in temperature and then decreases. The increase illustrates the requirement of activation of the surface for adsorption is due to fact that formation of activated complex requires certain energy. The decrease at high temperature is due to desorption, as the kinetic energy of the adsorbate increases. .
. . Adsorption isotherms Adsorption isotherm can be studied quantitatively. A plot between the amount of adsorbate adsorbed and pressure (or concentration of