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The substance which has different concentration at the surface than in the bulk of the adjoining phases is said to be adsorbed. The phenomenon of existence of a substance in different concentrations at the surface of another substance than in the bulk phases is called adsorption.
The substance whose surface adsorbs molecular species of another substance is called adsorbent, and the substance which gets adsorbed is called adsorbate.
If the concentration of a substance at the surface of another substance is more than in the bulk of the adjoining phases, it is called positive adsorption. If the concentration of a substance at the surface of another substance is less than in the bulk of the adjoining phases, it is called negative adsorption.

Types of adsorption

Physisorption (physical adsorption) Chemisorption (chemical adsorption)
1. Caused by intermolecular van der Waal’s forces. Caused by chemical bond formation.
2. Depends on the nature of gas. Easily liquefiable gases are adsorbed readily. Much more specific and depends on the nature of both adsorbate and adsorbent.
3. Heat of adsorption is small (about 5 kcal/mol) Very large (20–100 kcal/mol)
4. Reversible Irreversible
5. Forms multimolecular layers on adsorbent surface. Forms unimolecular layer.
6. Occurs at low temperature; decreases with increase in temperature. Increases with increase in temperature.
7. Increase in pressure increases adsorption. High pressure is favorable. Decrease in pressure does not cause desorption.
8. Equilibrium is attained readily, and it is reversible. Equilibrium is attained slowly and mostly not reversible.

Adsorption of gases on solids

The extent of adsorption of a gas on the surface of a solid depends on the following factors: (i) nature of gas, (ii) nature of solid, (iii) specific area of solid, (iv) pressure of gas, (e) temperature, and (f) activation of solid.
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, CH4, HCl, Cl2, SO2, and CO are adsorbed more than permanent gases such as H2, O2, and N2. Chemisorption is specific in nature. Therefore, only those gases will be adsorbed which form chemical bonds with it.
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.
Pressure of gas: If the physical adsorption is limited to unimolecular layer, the plot of x/m vs equilibrium pressure at a constant temperature is as shown in Fig. (12.1).
Description: Description: 38900.png
Fig. 12.1
At low pressures, x/m varies linearly with p. Therefore,
Description: 37832.png
At high pressures, x/m is independent of p. Therefore,
Description: 37847.png
At intermediate pressures, the variation of x/m vs p can be expressed as Description: 37864.png, where n > 1.
or Description: 37872.png
or Description: 37879.png
This is called Freundlich adsorption isotherm.
Temperature: 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 in temperature, whereas in the case of chemisorption, x/m initially increases with temperature and then decreases as shown in Fig. 12.2. The initial increase is due to the fact that chemisorption requires activation energy.
Description: 37933.png
Fig. 12.2
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.

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