Types of Adsorptlon Chemistry Notes
Types of Adsorption :
→ On the basis of nature of bonding or forces between the molecules of adsorbate and adsorbent, the adsorption is of two types:
- Physical adsorption
- Chemical adsorption
→ Physical Adsorption or van der Waal’s adsorption When the particles or molecules of the adsorbate are held to the surface of adsorbent by some physical forces like van der Waal’s forces then such type of adsorption is termed as physical adsorption or physisorption or van der Waal’s adsorption. van der Waal’s interactions are weak in nature hence such forces can easily be overcome either by increasing the temperature or by decreasing the pressure.
→ Therefore, physical adsorption can be easily reversed or decreased. It has low heat of adsorption. It’s value is the range of 20-40 kJ mol-1. It’s nature is not specific ie, nature of physical adsorption is non-specific. Physical adsorption takes place rapidly in all stages.
→ Chemical Adsorption or Langmuir Adsorption When the forces of attraction existing between adsorbate and adsorbent are strong chemical bonds, the adsorption is called chemical adsorption or chemisorption or Langmuir adsorption. In chemical adsorption, the adsorbate forms product by reaction at the surface of adsorbent.
→ This product is known as surface compound. Example : The desorption of tungsten oxide and Carbon dioxide or carbon monoxide takes place after adsorption of oxygen at the surface of tungsten or carbon. The value of heat of adsorption in chemical adsorption lies between 80-240 kJ/mole Chemical adsorption is an irreversible and specific process.
→ Differences between Physisorption and Chemisorption
|Nature of bonding||It involves weak van der Waal’s forces.||It involves the formation of chemical bonds.|
|Specificity||Due to the presence of van der Waal’s interaction it is not specific in nature.||Due to the formation of chemical bonds, it is highly specific in nature.|
|Reversibility||It is reversible in nature.||It is irreversible in nature.|
|Heat of adsorption||Low heat of adsorption (20-40 kJmol-1).||High heat of adsorption (80-240 kJ mol-1)|
|Temperature||Usually occurs at low temperature and decreases with increase in temperature.||It occurs at high temperature and increases on increasing temperature.|
|Nature of molecular layers||It forms multimExtent of adsorption of gas is related to the ease of liquification of the gas.olecular layers at high pressure.||It forms unimolecular layer.|
|Activation enecgy Extent of adsorption||It does not require any activation energy.||It requires activation energy.
There is no such relation.
|Formation of ComPounds||No compound formation takes place.||Surface compounds are formed.|
|State of adsorbate||Same state as in the bulk.||The state may be quite different from that in the bulk.|
|Surface area||It increases on increasing surface area.||
It also increases on increasing surface area.
|Pressure||It increases on increasing pressure.||It also increases with increasingpressure.|