Distinguish Between Physical and Chemical Adsorption
Physical Adsorption
When the particles of the adsorbate are held to the surface of the adsorbent by the van der Waals forces, the adsorption is called physical adsorption or Physisorption.For example- H2 and N2 gases adsorb on coconut charcoal.
The attractive forces are weak so, these forces can be easily overcome either by increasing the temperature or by decreasing the pressure. Physical adsorption can be easily reversed or decreased.
For example- Formation of iron nitride on the surface when the iron is heated in N2 gas at 623 K.>
Characteristics of physical adsorption or Physisorption
1. Physisorption is caused by physical forces.2. Physisorption is a weak surface phenomenon.
3. Physisorption is a multi-layered process.
4. Physisorption is not specific and takes place all over the adsorbant.
5. Surface area, temperature, pressure, nature of adsorbate effects physisorption.
6. Energy for activation is low (20 – 40 kg/mol).
Chemical adsorption
When the molecules of the adsorbate are held to the surface of the adsorbent by the chemical forces or chemical bonds the adsorption is called chemical adsorption or chemisorption.Characteristics of chemical adsorption or chemisorption
1. chemisorption is caused by chemical forces.2. chemisorption is a very strong process.
3. chemisorption is almost a single-layered phenomenon.
4. Chemisorption is highly specific and takes place at reaction centres on the adsorbant.
5. Surface area, temperature, nature of adsorbate effects chemisorption.
6. Energy of activation is very high 40 – 400 kJ/mol.
| Physical Adsorption | Chemical Adsorption |
|---|---|
| It is due to weak Van der Waals forces between adsorbate and adsorbent. | It is due to strong chemical forces of bonding type between adsorbate and adsorbent. |
| It is a weak surface phenomenon. | It is a very strong surface phenomenon. |
| It is reversible in nature. | It is irreversible in nature. |
| Adsorption will decrease by increasing temperature. | Adsorption increases by an increase in temperature. |
| Increase in pressure will increase adsorption. | Pressure has no effect. |
| It is not specific and takes place all over the adsorbant. | It is highly specific and takes place at reaction centres on the adsorbant. |
| This adsorption is a multi-layered process. | This type of adsorption is almost a single-layered phenomenon. |
| Energy for activation is low (20 – 40 kg/mol). | Energy of activation is very high 40 – 400 kJ/mol. |
| Examples: H2 and N2 gases adsorb on coconut charcoal. | Examples: Charcoal, Silica gel, Alumina. |
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