it's all about science yo it's science

1. Shree Saurabh Science College-
Gadu
Created By: Chetana Zankat
Toll Free:1800-121-2524
Chapter: Adsorption

2. Introduction :
 It is a surface phenomenon and refers to the uniform distribution of a substance through
another at the surface.
Example : solution of water in pb
. The word adsorption was first used by scientist kayser in 1881.
It is the phenomenon in which a layer of ions , molecules or aggregates of molecules
condense upon the surface with which they come in contact.
It is defined as the concentration of a substance at the interface or boundary between
heterogeneous phase. Eg. Solid or gas or two immiscible liquids.

3. Adsorption :
 Addition of the molecule of a substance on the surface of a liquid or a solid resulting into a
higher concentration of the molecules on the surface is called adsorption.. Example : colour
adsorb by charcoal.
Adsorbent :
It is the substance which is adsorbs the other substance at it’s surface.Example : activated charcoal
, bone charcoal, silica gel.

4. Adsorbate :
 It is the substance which is adsorbed at the surface of another substance. Example : gases, dyes,
water vapors etc.
Absorption :
the material diffuses into the bulk of another in other words “absorption of a substance by a
substance B means that the molecules of A diffuses into the bulk of substance .Example : water is
absorbed by a sponge.

5. Sorption :
 The term sorption is describe both the adsorption and absorption.
Example : water vapour is absorbed by anhydrous calcium chloride while it is absorbed on silica gel.
Desorption :
the process of removal of an adsorbed substance from the substance on which it is adsorbed is called
desorption.

6. Types of adsorption :
(1) physical adsorption (physisorption )
Gas is adsorbed on the surface of a solid by vander waal’s Forces without formation of any
chemical bond between physisorption.
Example : the adsorption of oxygen & hydrogen by charcoal.
(2) chemical adsorption ( chemiosorption )
The chemical bond may appear between the molecule of adsorbate and the surface of adsorbent
with the formation of a new surface chemical compound.
Chemical force are responsible for chemiosorption.
It is a irreversible.

7. Difference between Adsorption &
Absorption
Adsorption
 It is a surface phenomenon.
 In Adsorption concenration increase
On the surface of adsorbent.
 Graph of x/m p get this type.
 E.g. Adsorption of ammonia by
charcoal Adsorption phenomenon.
Absorption
 Absorption is bulk phenomenon.
 In Absorption adsorbate diffuse in to
the bulk of adsorbent.
 Graph of x/m p get this type.
 E.g. solubility of ammonia in water is
Adsorption phenomenon.

8. Enthalpy of Adsorption :
 In an Adsorption process the changes in enthalpy for one mole of an adsorbate on
an adsorbent surface is called the enthalpy of Adsorption.
 The enthalpy of chemisorption is of the order of 200 KJ/ mol while enthalpy of
physorption is of the order of only 20 KJ / mol.

9. Difference between physisorption and
chemisorption :
Physisorption
 Molecules of adsorbate and adaobent
are held by van def waal’s interaction.
 It is takes place at low temp.
 Adsorption decrease with increase in
temperature.
 Enthalpy is 20KJ/ mol.
 It is not very specific.
 It is reversible in nature.
 E.g. Adsorption of ammonia chlorine.
Chemisorption
 Molecules of adsorbate and adsorbent
are held by chemcal Bond.
 It takes place at relatively high temp.
 Adsorption increase With increase
temperature.
 Enthalpy is 200KJ/mol.
 It is highly specific.
 It is irreversible in nature.
 E.g. Adsorption of oxygen by tungsten.

10. Factor affecting Adsorption of gases on
solid :
(1) nature of the adsorbate :
Physical Adsorption depend upons on the surface of gas.the gas molecules to
condense into liquids by van def waal’s forces.higher the critical temperature of a gas
or more easily liquifiable a gas , it will be ready adsorbed.
(2) Nature of the adsorbent :
It is depend on property of adaorbent. Most common adaorbent are activated
charcoal, metal oxide etc.
(3) Surface of area of the adsorbent :
Surface area are available for Adsorption per gram of the adsorbent. Physically
Adsorption increase with the increase in surface area of the solid adsorbent.

11. (4) Effect of pressure :
Since increase of pressure helps condensation of a gas, physical Adsorption with pressure. As the
pressure approaches critical pressure,multilayer Adsorption takes place. As a result of this effect the
Adsorption tends to increase very rapidly.
(5) Effect of temperature :
since van def waal’s forces are stronger near the critical temperature and below it, the physical
Adsorption is strong at low temperatures. Alternative one can say that, the rate of reaction remains
almost unaffected at very low temperature.

14. Limitations :
(1) the equation is emperical i.e. It has no theoritical base.
(2) this equation is useful only for a certain period of pressure and
concentration.
(3) value Of constant k and n varies with temperature.

15. Langmuir Adsorption isotherm :
 Langmuir derived on Adsorption on theoritical based on kinetic theory of gases.
This is named as the Langmuir Adsorption isotherm.
 While fredndlich Adsorption isotherm is only a special case, Langmuir isotherm
for intermediate pressure. This isotherm is based on the assumption.
 Adsorption could only take place until the adsorbing surface is completely covered
with a unimolecular layer of adsorbed gas.
 Let n be the number of molecules of the gas which strike on surface.
 Let the fraction of molecules that is condensed on the remaining surface.

16. Postulates of Langmuir’s theory
(1) the gas adsorbed on a solid surface can not form a layer more than a single
molecule in thickness.
(2) Adsorption consists of two opposing actions.
(a) condensation of the gas molecule on the solid surface.
 (b) Evaporation of the gas molecules from the surface.
(3) the rate of condensation equal to the rate of Evaporation and on equilibrium is
said to have been attained.
(4) The rate of condensation also depends upon the pressure if gas P.

17. Derivation of Langmuir Adsorption
isotherm :

22. Limitations of Langmuir theory :
(1) This theory could not explain all types of experimental Adsorption isotherm.
(2) It assumes that Adsorption is unilayer in nature .how ever in actual practice
multilayer Adsorption have been reported.
(3) According to Langmuir view Adsorption is independent from temperature on
the saturation position but actual practice this not possible.

23. Application or uses of
Adsorption :
(1) aremoval of colour :
Coloured impurities are removed by animal
charcoal. E.g. light yellow colour in sugar is
decolorized by treating sugar solution with animal
charcoal powder.
(2) In gas maskes :
All gas masks contain adsorbent by
Adsorption and purify the air for breathing.

24. (3) creation of high vaccum :
In Dewar flask, activated charcoal or silica gel is placed between the walls of two flask for creating
high vaccum.
(4) Heterogeneous catalysis :
As a catalyst the gaseous reactant are absorbed on the Surface of the solid catalyst. The
concentration of the reaction increase in the surface thUs the rate of reaction increases.
(5) Adsorption indicators :
they are extremely useful for the titration of dilute solution and give accurate e.g. in titration of
“Cl “ by silver nitrate is used as Adsorption indicator.

25. (6). In separation and purification technique :
Chromatography is best technique for separation and purification .
In which silica gel,Alumina, magnesium etc.
(7) Removal of Disorder and Airfreshner :
Adsorbent used in packing material for long time storage or free from
bed smell. e.g. packing of home appliances , eater bag etc.

26. (8) Adsorption of sound :
It is also used for Adsorption of sound in theatre for clear
sound it adsorbed sound . It is placed on in side wall of talkies.
(9) In Agriculture :
Soul contain small amounts of colloidal fractions in the form
of very fine particulars of clay. So it can retain and absorb
moisture in which nutrients such as compounds of nitrogen,
phosphorous and moisture in which nutrients such as compounds
of nitrogen , phosphorous and potassium can dissolve and pass
up to the plant through roots.

27. (10) Refining petroleum,vegetable oils etc. :
Vegetable oils and petroleum are refined by using silica gel and Fuller’s earth which act as
adsorbents.
(11). Silica and Alumina. :
Adsorbent for removing moisture and controlling huminity in rooms, desicaster etc .

28. Thank you