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.