Friday, November 13, 2009

ADSORPTION OF GASES ON SOLIDS

ADSORPTION OF GASES ON SOLIDS
The magnitude of adsorption of gases by solids depends on the following factors.
1. Nature of the adsorbent
2. The surface area of the adsorbent
3. Nature of the gases
4. Pressure of the gas
5. Temperature of the gas
6. Reversible character of the adsorbed gases
7. Activation of the adsorbent
8. Enthalpy of adsorption

Factors infleunce adsorption of gases on solids
1. Nature of the adsorbent
The adsorption depends upon the type of adsorbent used. The excellent adsorbents are highly porous in nature. The larger the pores, the greater is the adsorption.
Ex : porous substances are silica gel, aluminia and charcoal.
The adsorption power of these substances can further be enhanced by a pores called activation. During activation, the adsorbent is heated in steam to about 1500°c. Heating drives out all impurities and leads to a lager free surface for adsorption.
Ex. Charcoal adsorbs 0.011 gms of CCl4 at 24°C.
Activated charcoal adsorbs 1.48 gm of at 24°C.


The surface area of adsorbent
The extent of adsorption depends on the surface area.
1. The greater the surface area, the greater is the adsorption.
2. Larger pores on the adsorbent, larger is the adsorption.
Ex. Charcoal and silica gel ( excellent adsorbents).

Nature of gases
The amount of gas adsorbed by a solid depends on the nature of the gas. Easily liquefiable gases like Hl, NH3, Cl2, SO2 etc., are adsorbed more easily then the permanent gases like H2, N2, O2 etc.,
This is due to the following reasons :

i. Critical temperature
The ease of liquifications depends on its critical temperature (i.e, the maximum temperature above which a gas cannot be liquefied). Thus if the critical temperature of the gas is more, it will be liquefied and adsorbed more readily.
Ex. Adsorption of various gases on 1g of activated charcoal.

ii. Vander waal’s forces
Easily liquefiable gases possess greater vander waal’s forces than permanent gases, so they are adsorbed more readily.

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