Factor Affecting Adsorption
Factors which affect the extent of adsorption
The following are the factors which affect the adsorption,
(1) Nature of the adsorbate (gas) and adsorbent (solid)
(i) In general, easily liquefiable gases e.g., CO2, NH3, Cl2 and SO2 etc. are adsorbed to a greater extent than the elemental gases e.g. H2, O2, N2, He etc. (while chemisorption is specific in nature.)
(ii) Porous and finely powdered solid e.g. charcoal, fullers earth, adsorb more as compared to the hard non-porous materials. Due to this property powdered charcoal is used in gas masks.
(2) Surface area of the solid adsorbent
(i) The extent of adsorption depends directly upon the surface area of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the extent of adsorption.
(ii) Surface area of a powdered solid adsorbent depends upon its particle size. Smaller the particle size, greater is its surface area.
(3) Effect of pressure on the adsorbate gas
(i) An increase in the pressure of the adsorbate gas increases the extent of adsorption.
(ii) At low temperature, the extent of adsorption increases rapidly with pressure.
(iii) Small range of pressure, the extent of adsorption is found to be directly proportional to the pressure.
(iv) At high pressure (closer to the saturation vapour pressure of the gas), the adsorption tends to achieve a limiting value.
(4) Effect of temperature
(i) As adsorption is accompanied by evolution of heat, so according to the Le-Chatelier’s principle, the magnitude of adsorption should decrease with rise in temperature.
(ii) The relationship between the extent of adsorption and temperature at any constant pressure is called adsorption isobar.
(iii) A physical adsorption isobar shows a decrease in x/m (where ‘m’ is the mass of the adsorbent and ‘x’ that of adsorbate) as the temperature rises.
(iv) The isobar of chemisorption show an increase in the beginning and then decrease as the temperature rises.