(i) The nature of the gas (i.e. nature of the adsorbate). The easily liquefiable gases such as `HCl, NH_(3), Cl_(2)` etc. are adsorbed more than the permanent gases such as `H_(2), N_(2)` and `O_(2)`. The ease with which a gas can be liquefied is primarity deyermined by its critical temperature. Higher the critical temperature `(T_(c))` of a gas, the more easily it will be liquefied and, therefore, more readily it will be adsorbed on the solid.
`{:("Gas",SO_(2),CH_(4),H_(2)),(T_(C),330K,190K,33K):}`
(ii) Nature of adsorbent. The extent of adsorption of gas depends upon the nature of adsorbent. Activated charcoal (i.e. activated carbon), metal oxides (silica gel and aluminimum oxide) and clay can adsorb gases which are easily liquified. Gases such as `H_(2), N_(2)` and `O_(2)` are generally adsorbed on finely divided transition metals Ni and Co.
(iii) Activation of adsorbent.
(a) Metallic adsorbents are activated by mechanical rubbing or by subjecting it to some chemical reactions.
(b) To increase the adsorbing power of adsorbents, they are sub-divided into pieces. As a results, the surface area is increased and therefore, the adsorbing power increases.
(iv) Effect to temperature.
Mostly the process of adorption is exothermic and the reverse process or endothermic. If the above equilibrium is subjected to increase in temperature, then according to Le-Chaterlier's principle, with increase in temperature, the desorption will be favoured. Physical adsorption decreases continuously with increase in temperature whereas chemisorption increases initially, shows a mamimum in the curve and then it decreases continuously.
`(##RES_PHY_CHM_XII_V02_C03_E01_023_A01##)`
The initial increase in chemisorption with increase in temperature is because of activation energy required. This is why the chemical adsorption is also known as ''Activated adsorption''.
A graph between degree of adsorption `(x//m)` and temperature 't' at a constant pressure of adsorbate gas is know as adsorption isobar.
(v) Effect of pressure. The extent of adsorption of a gas per unit mass of adsorbent
depends upon the pressure of the gas. The variation of extent of adsorption expressed as x/m (where x is the mole of adsorbate and m is the mass of the adsorbent) and the pressure is given as below. A graph between the amount of adsorption and gas pressure keeping the temperature constant is called an adsorption isotherm. It is clear from the figure that extent of adsorption (x/m) increases with pressure and becomes maximum corresponding to pressure `P_(s)` called equilibrium pressure. Since adsorption is a reversible process, the desorption also takes place simultaneously. At this pressure `(P_(s))` the amount of gas adsorbed becomes equal to the amount of gas desorbed.
`(##RES_PHY_CHM_XII_V02_C03_E01_023_A02##)`
