Thursday, January 31, 2008

IIT JEE Revision Elementary concepts of Adsorption

Elementary concepts of adsorption (excluding adsorption isotherms);

The term adsorption implies the presence of excess concentration of any particular component in one of the three phases of matter (known as adsorbate) at the surface of liquid or solid phase (known as adsorbent) as compared to that present in the bulk of the material.

On the basis of the forces of attraction between adsorbent and adsorbate, two types of adsorption, namely, physisorption (i.e. physical adsorption) and chemisorption, may be identified.

In physisorption only van der waals forces attration are present between molecules of absorbent and absorbate.

In chemisorption covalent bonding takes place between molecules of absorbent and absorbate. Ex: Iron nitride.

The characteristics of physisorption are;

1. the forces of attraction are of van der waals type (weak forces).
2. Predominates at low temperature.
3. All gases show this adsorption at low temperatures.
4. Heat of adsorption is low, about 40 kJ/mol.
5. Reversible in nature.
6. Low activation energy (appx. 5 kJ)
7. Adsorption is multilayer.

The characteristics of chemisorption are:

1. the forces of attraction are of a chemical nature (strong forces)
2. Predominates at high temperature.
3. This is highly specific in nature.
4. Heat of adsorption is large 9appx. 80 to 420 kJ/mol)
5. Usually irreversible.
6. Large activation energy
7. Adsorption is monolayer.

The extent of adsorption of gases increases with increase in the pressure of the gases and it decreases with increase in temperature of the gas.

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