These are Online Notes PART 1 on Chapter => 5 (Surface Chemistry) For Practice of CBSE Board, CBSE NEET, CSIR NET etc.
Concepts:
1. Adsorption: The accumulation of molecular species at the surface
rather than in the bulk of a solid or liquid is termed as Adsorption.
2. Adsorbate: The substance which is being adsorbed on the surface of
another substance is called Adsorbate.
3. Adsorbent: The substance present in bulk, on the surface of which
adsorption is taking place is called Adsorbent.
4. Desorption: The process of removing an adsorbed substance from a
surface on which it is adsorbed is called Desorption.
5. Absorption: It is the phenomenon in which a substance is uniformly
distributed all over the surface.
6. Difference between Adsorption and Absorption:
Absorption
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Adsorption
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(i) It is the
phenomenon in which a
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(i) The accumulation
of molecular
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substance is uniformly distributed
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species at the
surface rather than
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in the bulk of a solid or liquid is
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throughout the bulk of the solid.
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termed as adsorption.
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(ii) It is a bulk phenomenon.
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(ii) It is a surface
phenomenon.
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(iii) The
concentration is uniform
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(iii) The
concentration of
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throughout the bulk
of solid.
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adsorbate increases
only at the
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surface of the adsorbent.
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7. Sorption: When adsorption and absorption take place simultaneously,
it is called Sorption.
8. Enthalpy or Heat of adsorption: Adsorption generally occurs with
release in energy, i.e., it is exothermic in nature. The enthalpy change
for the adsorption of one mole of an adsorbate on the surface of
adsorbent is called Enthalpy or Heat of adsorption.
9. Types of Adsorption:
a. Physical adsorption or physisorption: If the adsorbate is held on
a surface of adsorbent by weak van der Waals’ forces, the
adsorption is called Physical Adsorption or Physisorption.
b. Chemical adsorption or chemisorption: If the forces holding the
adsorbate are as strong as in chemical bonds, the adsorption
process is known as Chemical Adsorption of Chemisorption.
10. Difference between Physical and Chemical Adsorption:
Physical
adsorption
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Chemical
adsorption
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(Physisorption)
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(Chemisorption)
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(i) It is
non-specific
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(i) It is highly
specific
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(ii) It is reversible
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(ii) It is
irreversible
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(iii) The amount of
gas depends upon
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(iii) The amount of
gas adsorbed is not
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nature of gas, i.e., easily liquefiable
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related to critical temperature of the
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gases like NH3, CO2, gas adsorbed to
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gas.
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greater extent than H2 and He.
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Higher the critical
temperature of
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gas, more will be the extent of
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adsorption.
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(iv) The extent of
adsorption
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(iv) It also
increases with increase in
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increases with increase in surface
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surface area.
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area, e.g. porous and
finely divided
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metals are good adsorbents.
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(v) There are weak
van der Waals’
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(v) There is strong
force of attraction
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forces of attraction between
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similar to chemical bond.
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adsorbate and
adsorbent.
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(vi) It has low
enthalpy of adsorption
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(vi) It has enthalpy
heat of adsorption
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(20 – 40 kJ mol-1)
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(180 – 240 kJ mol-1)
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(vii) Low temperature
is favourable.
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(vii) High
temperature is favourable.
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(viii) No appreciable activation
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(viii) High
activation energy is
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energy is needed.
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sometimes needed.
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(ix) It forms
multimolecular layers.
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(ix) It forms
unimolecuar layers.
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11. Factors Affecting Adsorption of Gases on Solids:
a. Nature of Adsorbate: Physical adsorption is non-specific in nature and therefore every gas gets adsorbed on the surface of any solid to a lesser or greater extent. However, easily liquefiable gases like NH3. HCl, CO2, etc. which have higher critical temperatures are absorbed to greater extent whereas H2, O2, N2 etc. are adsorbed to lesser extent. The chemical adsorption being highly specific, therefore, a gas gets adsorbed on specific solid only if it enters into chemical combination with it.
b. Nature of adsorbent: Activated carbon, metal oxides like aluminum oxide, silica gel and clay are commonly used adsorbents. They have their specific adsorption properties depending upon pores.
c. Specific area of the adsorbent: The greater the specific area, more will be the extent of adsorption. That is why porous or finely divided forms of adsorbents adsorb larger quantities of adsorbate. The pores should be large enough to allow the gas molecules to enter.
d. Pressure of the gas: Physical adsorption increases with increase
in pressure.
12. Adsorption isotherm: The variation in the amount of gas adsorbed by
the adsorbent with pressure at constant temperature can be expressed
by means of a curve is termed as adsorption isotherm.
13. Freundlich Adsorption isotherm: The relationship between
mx and pressure of the gas at constant temperature is called adsorption
isotherm and is given by
mx = (kP) 1 /n (n > 1)
Where x- mass of the
gas adsorbed on mass m of the adsorbent and the gas at a particular temperature
k and n depends upon the nature of gas and the
solid
mx first increases with
increase in pressure at low pressure but becomes independent of pressure at
high pressure.
14. Catalyst: Substances which alter the rate of a chemical reaction and
themselves remain chemically and quantitatively unchanged after the
reaction are known as catalysts and the phenomenon is known as
catalysis.
15. Promoters: Those substances which increase the activity of catalyst
are called promoters. Example: Mo is promoter whereas Fe is catalyst
in Haber’s Process.
N2 (g) + 3H2 (g) ==Fe, Mo==> 2NH3(g)
16. Catalytic poisons (Inhibitors): The substances which decrease the
activity of catalyst are called catalytic poisons or inhibitors e.g.,
arsenic acts as catalytic poison in the manufacture of sulphuric acid by
‘
contact process.’
17. Types of catalysis:
a. Homogeneous Catalysis: When the catalyst and the reactants
are in the same phase, this kind of catalytic process is known as
homogeneous catalysis.
b. Heterogeneous Catalysis: When the catalyst and the reactants
are in different phases, the catalytic process is said to be
heterogeneous catalysis.
18. Activity of catalyst: The ability of a catalyst to increase the rate
of a chemical reaction is called activity of a catalyst.
19. Selectivity of catalyst: It is the ability of catalyst to direct a
reaction to yield a particular product (excluding others).
For example: CO and H2 react to form different products in presence of
different catalysts as follows:
a) CO(g) + 3 H2(g) == Ni ==> CH4(g) + H2O(g)
b) CO(g) + 2H2(g) == Cu/ZnO-Cr2O3 ==> CH3OH(g)
c) CO(g) + H2(g) ==Cu ==> HCHO (g)
20. Shape – selective catalysis: The catalysis which depends upon
the pore structure of the catalyst and molecular size of reactant and
product molecules is called shape- selective catalysis. E.g. Zeolites are
shape – selective catalysts due to their honey- comb structure.
Click Link for "Surface Chemistry" Notes "PART 2"
Click Link for "Surface Chemistry" Notes "PART 2"
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