What is adsorption?
Adsorption can be best described as a prodigy of attracting and accumulation of matter in high concentration on the surface of a solid or a liquid. The phenomenon of accumulation on the surface is due to the free valence electrons or uneven attractive forces which is a part of solid or liquid covering. These surfaces have the capability to hold the molecules of a gas or any matter when the molecules come in contact with the surface. A substance that occupies the surface of a solid or a liquid is called as the adsorbate. The surface at which the accumulation happens is known as the adsorbent.
This process of collective aggregation on the surface and absorption are the characteristics of surface chemistry. Absorption is a phenomenon in which the substance is absorbed by a surface. Adsorption and absorption are different in many ways. The phenomena of accumulation involve the unequal distribution of matter on the outer surface of the substance which is fast at first whereas absorption involves the homogeneous distribution of the molecules throughout the substance at a constant rate. The process in which both adsorption and absorption are involved is called as sorption.
This aspect is classified into physisorption and chemisorption based upon the attributes of force existing between the adsorbate and adsorbent.
If Vander Waal’s forces exist between the adsorbent and adsorbate, it is called physical adsorption or physisorption. This type can easily be reverted by lowering the pressure or by heating.
Characteristics of physisorption
- The range of heat required for accumulation is low. It ranges between 20-40 kJ/mol.
- Vander Waal’s force act as an attracting force between the adsorbent and adsorbate.
- Physisorption is reversible.
- This process can occur at low temperatures.
- The molecules accumulated form multi-layers.
- Activation energy is not required.
If the attracting forces between the adsorbate and adsorbent are approximately equal to the strength of the chemical bonds then this phenomenon is called chemical adsorption or chemisorption.
Characteristics of chemisorption are:
- The range of heat required for accumulation is high. It ranges between 40-500 kJ/mol.
- Chemical bond acts as an attracting force between the adsorbent and adsorbate.
- Chemisorption is not reversible.
- This process requires high temperatures.
- The molecules accumulated, form a monolayer.
- High activation energy is required.
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