Surface Chemistry
3.0 Factors affecting adsorption of gases by solids
3.0 Factors affecting adsorption of gases by solids
1. Nature and surface area of the adsorbent
Under given conditions of temperature and pressure, the greater the surface area (as of finely divided metals) of the adsorbent, greater is the gas adsorbed. Thus, substances like charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface areas.
2. Nature of the adsorbate
Easily liquefiable gases (i.e., gases with higher critical temperature) like $S{O_2}$, $N{H_3}$, $HCl$, $C{O_2}$ etc. are readily adsorbed than gases which do not liquefy easily. Easily liquefiable gases have higher critical temperature and stronger van der Waal's forces of attraction.
3. Temperature
Since adsorption is an exothermic process, increase in temperature will decrease adsorption. This actually happens except in chemisorption where extent of adsorption first increases and then decreases with increase in temperature. The curve showing the effect of temperature on the extent of adsorption at a given pressure is called an adsorption isobar.
4. Pressure
At constant temperature the adsorption of a gas increases with increase of pressure. At low temperature, the adsorption of a gas increases very rapidly as the pressure is increased. The increase in adsorption with increase in pressure is not proportionate at high temperatures. The variation of extent of adsorption with pressure at a given constant temperature is shown in figure. Each curve is called adsorption isotherm for that temperature.
