Adsorption depends on the following factors;
- Temperature
- Pressure
- Nature of the gas and
- Nature of the adsorbent
Effect of Temperature and Pressure
The evolution of heat invariably accompanies adsorption; therefore, according to Le-Chatelier’s principle, adsorption’s magnitude should increase with a temperature fall. Adsorption of a gas leads to a decrease in pressure, and the importance of adsorption increase with the increase in pressure. Thus, a drop in temperature and an increase in pressure both tend to cause an increase in the magnitude of the adsorption of a gas or a solid.
Nature of the gas and nature of the adsorbent
It has been found that more readily soluble and easily liquefiable gases like ammonia, hydrochloric acid, chlorine, and sulphur dioxide are adsorbed more than permanent gases such as hydrogen, nitrogen, and oxygen.
The reason is that van der Waals or intermolecular forces involved in adsorption are more predominant in the former list than in the latter list of gas.
Adsorption is a surface phenomenon; the greater the surface area per unit of the adsorbent, the greater it’s capacity for adsorption under the given condition of temperature and pressure.
Adsorption Isobar and Adsorption Isostere
As shown in the figure, expressed graphically, the effect of temperature on the extent of adsorption at a given pressure of the adsorbate.
The curve showing the effect of temperature on the extent of adsorption at a given pressure is called an adsorption isobar.
Adsorption isobar
The amount of adsorption decreases with an increase in temperature; according to Le Chatelier’s principle, the adsorption process is exothermic.
Adsorption isostere
A temperature rise tends to lower the extent of adsorption; to get the same amount of adsorption at a higher temperature, we shall also have to increase the system’s pressure. The straight-line relationship between temperature and pressure is generally obtained. As shown in the figure, the curve showing the variation of pressure with temperature for a given amount of adsorption is called an isostere.
