Adsorption
Adsorption is a surface phenomenon where particles (i.e. molecules, atoms or even ions) of a gas, liquid or solid in a dissolved state are attached(accumulate) at the top layer(surface) of solid material.
The substance whose molecules get adsorbed at the surface is called the adsorbate. while the substance on whose surface the process takes place is called the adsorbent.
Types of Adsorption
Depending upon the nature of forces which hold the molecules of the adsorbate on the surface of the adsorbent, the adsorption is classified into two types.
1. Physical adsorption
2. Chemical adsorption
1. 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).
2. 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.
Applications of Adsorption
The process of adsorption is very important in domestic as well as in industrial processes. Some important applications are given below-
1. Heterogeneous catalysis: Surface active material are widely used as catalyst mostly due to adsorption processes.
2. Removal of colouring material: Many coloured materials or impurities are removed through adsorption by suitable surface active materials like charcoal. Activated charcoal has been extensively used for this purpose.
3. Ion exchange resins: Several polymeric materials are used for the separation of ionic substances in chromatography through ion-exchange.
4. Adsorption indicators: Several dyes like eosin and fluoresein are used as indicators in the titrations of Cl-, Br-etc. against Ag+ (Fajan's method).
5. Gas masks: Activated charcoal is used to remove toxic gases in gas masks.
6. Dyeing of cloth: Many substances work as mordants for dyeing of cloths. Several metal cyanogen complexes, alums work as efficient mordants in dyeing cloths.
7. Control of humidity: Silica and aluminium gels are used as adsorbents for removing moisture and controlling humidity.
8. Separation of inert gases: Due to the difference in degree of adsorption
of gases by charcoal, a mixture of noble gases can be separated by adsorption on coconut charcoal at different temperatures.
9. Curing diseases: A number of drugs are used to kill germs by getting adsorbed on them.
10. Froth floatation process: A low grade sulphide ore is concentrated by separating it from silica and other earthy matter by this method using pine oil and frothing agent.
