Adsorption Isotherms

1. ADSORPTION
ISOTHERMS
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2. Table of contents
1.Adsorption
2.Types of adsorption
3.Adsorption isotherms
4.Applications of adsorption

3. ADSORPTION
 The accumulation of molecular species at the surface rather than in the bulk of a solid
or liquid is termed adsorption.
 The material on the surface of which the adsorption takes place is called adsorbent.
Eg: Charcoal, Silica gel, Alumina
 The molecular species or substance, which concentrates
or accumulates at the surface is termed as adsorbate
Eg: H2, N2 and O2 gases
Examples of adsorption:
1.silica gel adsorbs water vapour
2.Adsorption of a dye(methylene blue) by charcoal
The process of adsorption is an exothermic process and
is invariably accompanied by evolution of heat

4. •
∆Hadsorption is always –ve
∆Sadsorption is –ve
For process of adsorption to occur i.e., ∆G to be –ve, ∆H must be > T∆S.
i.e., ∆H should have sufficiently high value (>T∆S) so that ∆G becomes negative.
Types of Adsorption
1.Physisorption
accumulation of gases on the surface of a solid occurs on account of weak van der waal’s forces
Eg: H2 and N2 gases adsorb on coconut charcoal.
2.Chemisorption
accumulation of gases on the surface of a solid on account of chemical bond formation
Eg: formation of iron nitride on the surface when the iron is heated in N2 gas at 623 K

5. Adsorption Isotherm
 An adsorption isotherm is a graph that represents the variation in the amount of adsorbate(x)
adsorbed on the surface of the adsorbent with the change in pressure at a constant temperature.
FREUNDLICH ADSORPTION ISOTHERM
LANGMUIR ADSORPTION ISOTHERM
BET ADSORPTION ISOTHERM
DUBININ RADUHKEVITCH (D-R) ADSORPTION ISOTHERM:
TEMKIN ADSORPTION ISOTHERM

6. Freundlich Adsorption Isotherm
 The variation of extent of adsorption (x/m) with pressure (P)
at a particular temperature was given mathematically by
Freundlich in 1909.
 From the adsorption isotherm, following observations can be
easily made
(i) At low pressure
(ii) At high pressure
(iii) Thus, in the intermediate range of pressure
The above equation is known as Freundlich adsorption equation where K
and n are the constants which depend on the nature of the gas and
adsorbent
The value of ‘n’ is generally greater than one and thus the reciprocal, 1/n,
can have values between 0-1.

7. Taking log both sides of equation, we get,
The equation above equation is comparable with comparable with equation
of straight line, y = m x + c where,
m represents slope of the line
c represents intercept on y axis
•Freundlich adsorption isotherm is only applicable at low pressures.
Experimentally it was determined that extent of adsorption varies directly
with pressure till saturation pressure Ps is reached. Beyond that point rate
of adsorption saturates even after applying higher pressure. Thus
Freundlich Adsorption Isotherm failed at higher pressure.
Limitations of Freundlich adsorption isotherm

8. Langmuir Adsorption isotherm
 Surface of the solid is made up of elementary sites, each of which can
adsorb one gas molecule
 surface of the solid is homogeneous (same affinity)
 Mono-layer adsorption only
 Adsorbed gases behave ideally (No lateral interactions between adsorbate
molecules)
 Heat of adsorption is constant and equivalent for all sites
 Adsorbate gas molecules are localized (do not move around on the
surface)
assumptions of Langmuir adsorption isotherm

9. P is the pressure of the gas.
Below given expression is known as Langmuir Adsorption Equation.
Where
•θ is the fraction of the surface covered by the adsorbed molecule.
•K1 is an equilibrium constant known as the adsorption coefficient.
•{𝑲𝟏 = (𝑲𝒂/𝑲𝒅) = rate constant for adsorption/ rate constant for desorption}