What is Adsorption ?
Adsorption: - The accumulation of molecular species at the surface rather than bulk of solid or liquid is called Adsorption.
Mechanism of Adsorption
Inside the Adsorbent (in bulk) the force acting between the particles are mutually balanced but on the surface, the particles are not surrounded by atoms or molecules of their kind on all sides and hence they posses attraction force so particle stick on the surface of the Adsorbent.
The extent of adsorption increases with increase in surface area per unit mass of the adsorbent at a given temperature and pressure.
Heat of adsorption: - With increase in heat Adsorption process decreases.
Adsorption equilibrium: -
As the molecules of the adsorb ate are held on the surface of the solid adsorbent.
Entropy decreases, i.e. DS is negative
For the process of adsorption to occur, DG must be negative which is possible only when, DS keeps on decreasing and TDS keeps on increasing till ultimately DH becomes equal.
To TDS so that DG = 0, this state is called adsorption equilibrium.
Types of adsorption
There are two types of adsorption
i. Physical Adsorption or physisorption: -
If accumulation of gas on the surface of solid occurs on account of weak vanderwalls forces is called physical Adsorption.
ii. Chemical Adsorption or chemosorption: -
When gas molecules or atoms are held to the surface (solid) by chemical bonds, the Adsorption is called Chemical Adsorption.
1) Lack of specificity: - A given surface of an Adsorbent does not show any preference for a particular gas as the vanderwalls forces are universal.
2) Nature of Adsorbate: - The amount of gas Adsorbed by a solid depends on the nature of the gas.
3) Reversible nature: - Physisorption is reversible because adsorbate may be removed by decreasing pressure.
4) Surface area of Adsorbent: - Physisorption increases with increase in surface area.
5) Enthalpy of Adsorption: - Physical Adsorption is exothermic process but its enthalpy of adsorption is low (20-40 KJ mol-1).
1) High specificity: - It is high specific because it occurs if there is some possibility of chemical bonding.
2) Irreversibility: - As chemisorptions involve compound formation, so it is usually irreversible process.
3) Temperature: - Chemisorptions increases with increase in temperature after saturation starts decreasing.
4) Pressure: - it is also increases with increase in pressure.
5) Surface area: - chemisorptions increases with increase in surface area.
6) Enthalpy of Adsorption: - Enthalpy of chemisorptions is high (80-240 KJ mol-1) as it involves chemical bond formation.
The variation in the amount of gas Adsorbed by the adsorbent with pressure at constant temperature can be expressed by means of a curve termed as Adsorption isotherm.