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Adsorption
y The term adsorption was introduced by
Scheele during the discovery of uptake Definitions
of gases by charcoal.
y It was discovered by Raymonds and The phenomenon of adherence
Kayser. of liquid or gaseous molecules
y The substance on which the other on the surface of a solid by
substance is adsorbed is called an physical or chemical processes
adsorbent or interface, for example, is termed as adsorption.
charcoal, solid Al2O3, silica gel, metals,
colloids and clay in finely divided states.
y The substance which gets adsorbed on
the adsorbent is called adsorbate.
Concept Ladder
Air becomes dry
Surface Chemistry
Features of Adsorption (moisture free) when
y Adsorption is highly specific and silica gel as silica gel
selective in nature. adsorbs H2O molecules.
1.
,Surface Chemistry
2.
,y It is a spontaneous process, so ∆H, ∆S, ∆G
are negative.
y When absorption and adsorption occur Concept Ladder
simultaneously it is called sorption
y Adsorption is fast and exothermic. For In positive adsorption,
example, inert gases, SO2, Cl2, CO2, NH3 get the concentration of the
adsorbed on charcoal and silica gel adsorbs adsorbate is more on the
H2O. surface of the adsorbent
y Dyes can be adsorbed on animal charcoal. than in bulk.
In negative adsorption,
Absorption the concentration of the
It occurs throughout the body of absorbing adsorbate is less relative
material and hence it is a slower process. to its concentration in
It is an endothermic process (∆H is positive).
bulk.
For example, anhydrous CaCl 2 absorbs water,
followed by a decrease in temperature.
Occlusion
Adsorption of gases on surface of metal is
known as occlusion. For example, H2 gas on Pd
(palladium) surface. Definitions
Types of Adsorption Desorption
Physical adsorption It is reverse of adsorption,
y Here molecules are attracted to the surface that is, removal of adsorbed
by weak van der Waals forces. substance from the surface of
y e.g., Charcoal adsorbs SO2. the adsorbent.
y It is less specific as all gases are adsorbed on
metals to some extent.
y It is reversible in nature.
Solid + Gas
Gas / Solid+ Heat
y Adsorption ∝ Liquification of gases.
y It is exothermic in nature. So observed at low
temperatures. Rack your Brain
y Here, adsorption energy is low. (20–40) kJ/
Mole. What is relation between
y It decreases with temperature. Adsorption and liquification of
gas?
Surface Chemistry
3.
, y Here, multiple layers are formed.
y Here, adsorption ∝ surface area
Concept Ladder
y Here, activation energy barrier is absent. e.g.,
Adsorption of N2 on Mica. That is why finely divided
metals and porous
substances are good
adsorbents due to larger
surface area
Chemical Adsorption/ Activated Adsorption
y In this process,the molecules are held on the
surface by strong chemical bonds. e.g., H2
adsorbed on metals due to hydride formation. Rack your Brain
y It is very specific in nature.
y It is irreversible in nature. Where does chemical adsorption
occur?
y Adsorption energy is high in this case. (80–
240 kJ/Mole)
y It is an endothermic process.
y Initially, it increases with increase in
temperature then decreases.
Previous Year’s Question
Which one of the following
characteristics is associated with
adsorption? [NEET]
(1) DG and DH are negative but DS
y Adsorption ∝ Surface area is positive.
y In this, a unimolecular layer is formed. (2) DG and DS are negative but DH
y For this process to take place, Activation is positive.
Surface Chemistry
(3) DG is negative DH and DS are
energy is needed to be overcome for this
positive.
process to occur. Adsorption of O2 on
(4) DG, DH and DS all are negative.
Tungsten, H2 on Nickel.
4.
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