Factors influencing adsorption process

Specific area of adsorbent affects adsorption of gases on solids

The surface area of an adsorbent available for adsorption is known as specific area of adsorbent. Depending on the nature of surface, impurity at the surface etc, specific area of adsorbent changes. Rough surface can adsorb more, due to greater available surface area. Similarly solids can adsorb more when powdered. When powdered, surface area increases. Hence porous and finely divide forms of adsorbent have greater adsorption power.

Activation of adsorbent

We can increase the activity of an adsorbent using different methods. Cleaning surface, making rough surface, powdering etc are some of these method.

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Factors affecting adsorption of gases on solids

Important Factors affecting adsorption of gases are

1. Nature of gas.
2. Nature of adsorbent.
3. Specific area of adsorbent.
4. Activation of the adsorbent.
5. Pressure of the gas.
6. Temperature

1. Nature of gas

Adsorption can be either physisorption or chemisorption. Chemical adsorption is highly specific, hence only a particular adsorbent can adsorb a gas. For example, Nitrogen is adsorbed by Iron. Adsorption of hydrogen by nickel or platinum.

But physisorption is not specific. In such cases it is observed that easily liquefiable gases are more adsorbed than permanent gases like He, N₂, O₂, H₂ etc. It is due to the reason that HCl, SO₂, CH₄, NH₃, SO₃ etc are more adsorbed. In case of easily liquefiable gases, van der Waals  force or molecular forces are more predominant, and hence physisorption becomes more significant.


2. Nature of adsorbent

The extent of adsorption depends on nature of the adsorbent. Charcoal and silica gel are good adsorbent for gases and moisture respectively. Similarly H₂ is strongly adsorbed by Ni.


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Physisorption and Chemisorption

Physisorption and Chemisorption definition

In adsorbed state the adsorbate is held on the surface of adsorbent by attractive forces (bond). Depending on the nature of attractive forces, adsorption can be of two types - physical adsorption (Physisorption) and chemical adsorption (Chemisorption). In chemisorption there is a strong chemical bond.

During adsorption, a new bond is formed between adsorbent and adsorbate. Therefore adsorptions are generally exothermic (▵H = -ve). But entropy and free energy decreases during adsorption.

Enthalpy change during adsorption process are called enthalpy of adsorption. It is defined as the heat evolved at constant pressure, when one mole of an adsorbate is adsorbed on the surface of adsorbent. For physical adsorption and chemical adsorption, its values ranges in the order of -20KJ/mol and 200kjmol^-1. Difference between physisorption and chemisorption are given below.

Comparison Between Physisorption and Chemisorption

1. In physisorption adsorbate is held on the surface of  adsorbent by van der Waals force.

In chemisorption molecules of adsorbate and adsorbent are held by chemical bonds.

2. In Physisorption enthalpy of adsorption is comparatively low. ie, in the order of 20 kjmol^-1.

 In Chemisorption enthalpy of adsorption is high. ie, in the order of 200 kjmol^-1.

3.  Physisorption is reversible.

Chemisorption is irreversible.

4. Physisorption is not specific. ie, all gases are adsorbed by an adsorbent.

Chemisorption is highly specific.

5. In physisorption multi molecular layer of adsorbate occurs at the surface.

Chemisorption forms only unimolecular layer.

6. Physisorption usually takes place at low temperature and the extent of adsorption decreases with increase of temperature.

Chemisorption takes place at relatively higher temperature

7. In physisorption easily liquefiable gases are more adsorbed. For example NH₃, HCl etc are more adsorbed than permanent gases like He, O₂, N₂ etc.

 In Chemisorption there is no such correlation.