Characteristics of Adsorption
1.
Adsorption is a spontaneous process.
2.
Adsorption is a selective process.
3.
The rate of adsorption depends on temperature.
4.
Adsorption is a physical phenomenon, but accompanied by a chemical change.
5.
It depend of the nature of adsorbent and adsorbate.
Adsorption of Gases on Solids
All solids absorb gases to some measurable extent. The magnitude of adsorption of gases by solids depends upon the a.
Nature of the absorbent b.
Nature of gas adsorbed c.
Area of the adsorbent d.
Types of adsorbent e.
Temperature f.
Pressure of the gas.
FACTORS INFLUENCING ADSORPTION OF GASES ON SOLIDS
1.
Nature of the absorbent and adsorbate
The amount of the gas adsorbed depends upon the nature of the absorbent and the gas
(adsorbate). Gases like SO
2
, NH
3
, HCl and CO
2
which liquefy more easily are absorbed more readily than the permanent gases like H
2
, N
2
and O
2
which do not liquefy easily. Liquefiable gases have greater van der Waal’s or the molecular forces of attraction or cohesive forces.
2.
Surface area of the adsorbent
The adsorption of gases by solids depends upon the exposed surface area of the adsorbent. The larger will be the adsorption under given conditions of temperature and pressure. For this reason that silica gel and charcoal obtained from different sources becomes activated.
3.
The partial pressure of the gas in the phase
In accordance with Le Chatelier’s principle, the magnitude of adsorption decreases with the decreases in pressure and vice versa. The variation of adsorption with pressure at constant temperature is expressed graphically by a curve known as adsorption isotherm.
4.
Effect of temperature
For a given adsorbate and an adsorbent, the adsorption depends upon the temperature of the experiment. According to the Le Chatelier’s principle, the decrease in temperature with increase the adsorption and vice versa.

5.
Effect of Pressure
The relationship between the amount of adsorption and pressure can be mathematically expressed using an empirical relation called Freundlich adsorption isotherm.
X / m = KP 1/n
6.
Activation of adsorbent a.
Mechanical rubbing of metallic adsorbent. b.
Sub-dividing the solid adsorbent into fine particles and thus increasing the surface area. c.
Strong heating in superheated steam of some adsorbents so that pores of the adsorbents are opened and adsorption activity increases.
7.
Reversible character
Physical adsorption
It is a reversible process.
Gas ↔ Gas/Solid + Heat
Chemical adsorption
It is not a reversible process, because a strong chemical bond is formed between adsorbent and adsorbate.