CHE 101 Chapter Notes - Chapter 11: Physisorption, Chemisorption, Adsorption

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3 Feb 2021
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Revision Notes on Surface Chemistry:
Adsorption
Reversible and irreversible adsorption
The adsorption is reversible, if the adsorbent can be easily removed from the surface of the
adsorbent by physical methods. It is called irreversible adsorption, if the adsorbate can not be
removed from the surface of the adsorbent.
A gas adsorbed on a solid surface can be completely removed in vacuum. It is, therefore, reversible
adsorption. Examples of irreversible adsorption are adsorption of oxygen on tungsten adsorbate and
adsorption of CO on tungsten surface
Adsorbent, Adsorbate and Interface
The substances upon whose surface the change of concentration occurs, is called
absorbent
.
The substance taken up on the surface is call adsorbate
.
The common surface between the two phases where the adsorbed molecules
concentrate is called the interface.
Physisorption and Chemosorption:
Physisorption
Chemisorption
Only van der Waals force are present between
adsorbate and surface of adsorbent
Chemical bonds are formed between
adsorbate and surface of adsorbent
Low enthalpy of adsorption ie, in the order of 20
kjmol-1.
High enthalpy of adsorption i.e, order of 200
kjmol-1.
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Langmular Isotherm:
If A, B & AB represents the adsorbed, absorbent and the absorbed
– adsorbent complex then,
A + B ↔AB
k
a
= Equilibrium constant for adsorption = [AB]/[A][B]
k
d
= Equilibrium constant for desorption = [A][B]/[AB]
K
= Distribution coefficient = k
a
/k
b
Θ
= Fraction of the surface of adsorbent available for adsorption.
P
= pressure
So,
Θ= KP
/(1+KP
) (Langmular Equation)
Freundlich Isotherm:
Reversible
Irreversible
It is usually takes place at low temperature and
does not require any activation energy.
It takes place at high temperature and
require activation energy..
Multi molecular layer of adsorbate are formed on
the surface
Only monomolecular layers are formed.
Not specific.
Highly specific.
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