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.
