Factors Affecting Adsorption of Gases by Solids:
Almost all the solids adsorb gases to some extent. However, the exact amount of a gas adsorbed depends upon a number of factors, as briefly explained below:
1. Nature of surface area of the adsorbent:
It is observed that the same gas is adsorbed to different extents by different solids at the same temperature. Further, as may be expected, the greater the surface area of the adsorbent, greater is the volume of the gas adsorbed. It is for this reason that substances like charcoal and silica gel are excellent adsorbents because they have highly porous structures and hence large surface area.
The surface area per gram of the adsorbent is called specific area.
2. Nature of the gas being adsorbed:
Different gases are adsorbed to different extents by the same adsorbent at the same temperature. Higher the critical temperature of a gas, greater is the amount of that gas adsorbed. In other words, a gas which is more easily liquefiable or is more soluble in water is more readily adsorbed.
3. Temperature:
We know that adsorption decreases with increase of temperature and vice versa. For example, one gram of charcoal adsorbs about 10 cm3 of N2 at 273 K, 20 cm3 at 244 K and 45 cm3 at 195 K. the decrease of adsorption with increase of temperature is explained below:
Like any other equilibrium, adsorption is a process involving a true equilibrium. The two opposing processes involved are condensation i.e. adsorption of the gas molecules on the surface of the solid and evaporation i.e. desorption of the gas molecules from the surface of the solid into the gaseous phase.
The amount of heat evolved when one mole of the gas is adsorbed on the adsorbent is called the heat of adsorption.
4. Pressure:
At constant temperature, the adsorption of a gas increases with increase of pressure. It is observed that at low temperature, the adsorption of a gas increases very rapidly as the pressure is increased from small values.
5. Activation of the solid adsorbent:
It means increasing the adsorbing power of an adsorbent. This is usually done by increasing the surface are of the adsorbent which can be achieved by any of the following ways:
a) By making the surface of the adsorbent rough:
For example: by mechanical rubbing or by chemical action or by depositing finely dispersed metals on the surface of the adsorbent by electroplating.
b) By substituting the adsorbent into smaller pieces or grains:
No doubt this method increases the surface area but it has a limitation that is if the adsorbent is broken into too fine particles that it becomes almost powder, then the penetration of the gas becomes difficult and this will obstruct adsorption.
c) By removing the gases already adsorbed:
For example: charcoal is activated by heating in superheated steam or in vacuum at a temperature ranges from 623 K to 1273 K.
