The extent of adsorption of a gas on the surface of a solid depends on the following factors:
(a) Nature of gas
(b) Nature of solid
(c) Specific area of solid
(d) Pressure of gas
(f) Activation of solid
(i) Nature of gas:
Since physical adsorption is non-specific in nature, any gas will be adsorbed on the surface of a solid to some extent or other. However, under any given conditions of temperature and pressure, easily liquefiable gases such as NH3, CH4HCI, CI2, SO2, CO etc. are adsorbed more than permanent gases like H2, O2, N2 etc. Chemisorption is specific in nature. Therefore, only those gases will be adsorbed which form chemical bonds with it.
(ii) Nature of solid:
Activated charcoal is the most common adsorbent for easily liquefiable gases. Poisonous gases such as CH4 and CO fall in this group. Therefore, it is used in gas masks. Other gases such as O2, H2 and N2 adsorb more on metals such as Ni, Pt and Pd.
(iii) Specific area of solid:
Specific area of an adsorbent is the surface area available for adsorption per gm of adsorbent. Greater the specific area of an adsorbent, greater will be the adsorption. The specific area of an adsorbent can be increased by making the surface rough. The pores must be large enough to allow penetrations of gas molecules.
(iv) Pressure of a gas:
As physical adsorption is reversible, it is accompanied by decrease in pressure. Therefore, it is expected that at a given temperature the extent of adsorption will increase with the increase of pressure of the gas. The extent of adsorption is measured as x/m where m is the mass of adsorbent and x that of adsorbate. If the physical adsorption is limited to unimolecular layer, the plot of x/m vs. equilibrium pressure at a constant temperature is as shown.
It is evident from the graph that at a certain pressure the adsorption reaches a maximum value i.e. the adsorption becomes saturated and the corresponding pressure is called saturation pressure (Ps). Beyond this pressure the adsorption remains constant.
At low pressures, x/m varies linearly with p
\ x/mµ p1 or x/m = kp1
At high pressures, x/m is independent of p
\ x/m µp1 or x/m = kp0
At intermediate pressures, the variation of x/m vs p can be expressed as x/m µ
n > 1.
or x/m = k p(1/n)
or logx/m = logk + 1/nlog p
This is called Freundlich adsorption isotherm.
As adsorption is accompanied by release of heat energy, so in accordance with Le-Chatelier’s principle, the increase of temperature should decrease the extent of adsorption. This has indeed been found to be so. A plot of x/m vs. temperature at constant pressure is called adsorption isobar. In the case of physical adsorption x/m decreases with increase of temperature. However, in the case of chemisorption x/m initially increases with temperature and then decreases as shown below. The initial increase is due to the fact that chemisorptions require activation energy.
(vi) Activation of solid:
Activation of adsorbent means increasing its adsorbing power. This is increased by increasing specific area either by making the surface rough or by breaking the solid into smaller particles. But care must be taken so that particles do not become very small, otherwise the inter-particle spaces will be too small to allow penetration of gas molecules.
There is always a competition between different adsorbates to adsorb on the adsorbent. A strongly adsorbable substance can easily displace a weakly adsorbed substance. For example, on the surface of activated charcoal, gases such as O2, N2 etc. are already adsorbed. But when charcoal comes in contact with poisonous gases such as CH4, CO which are strongly adsorbable, O2 and N2 get displaced. If a mix of gases is allowed to adsorb on a particular adsorbent, the strongly adsorbable adsorbate adsorbs to a greater extent than its partial pressure indicates. For example, moisture though present in small proportion in air is strongly adsorbed by silica gel. Charcoal adsorbs poisonous gases even though they are present in small concentration in air.