Surface chemistry involves absorption and adsorption. Absorption occurs when particles accumulate uniformly throughout a bulk solid, while adsorption occurs when particles accumulate only on the surface. Adsorption involves the formation of new interactions as molecules bind to a surface. It is a spontaneous process characterized by negative enthalpy and entropy. The factors that affect adsorption include surface area, temperature, pressure, and the nature of the gas. Adsorption can be physical (physisorption) or chemical (chemisorption) in nature. Applications of adsorption include producing high vacuums, controlling humidity, removing color from solutions, and enabling heterogeneous catalysis.

1. Surface Chemistry
Absorption Adsorption
Bulk phenomenon Surface phenomenon
Accumulation of the particle in the bulk is known as
absorption
Accumulation of the particle on the surface is
known as absorption
The substance is uniformly distributed throughout
the bulk of the solid
The substance is concentrated only at the surface
does not penetrates the solid
Examples:
1. Anhydrous CaCl2,
2. Charcoal absorbs water,
3. Dye absorbs by the cotton fibres
Example:
1. Silica gel adsorbs water vapours,
2. Charcoal adsorbs NH3 gas,
3. Dye adsorbed on the cotton fibres

2. Basic Terms
1. Adsorbate: The particles being adsorbed on the surface are known as adsorbate.
2. Adsorbent: The surface which provides a surface for the adsorption is known as adsorbent.
3. Desorption: Opposite of the adsorption is desorption.
4. Sorption: Adsorption and absorption takes place simultaneously.
5. Occlusion: The adsorption of a gas on the surface of a metal is called occlusion.
C2H2 + H2 C2H6
Ni (s)

3. Thermodynamics of adsorption
1. Enthalpy (ΔH)
When a bond is formed the energy is released.
ΔH = -ve
When a bond is braked the energy get absorbed.
ΔH = +ve
A + B C + heat … ΔH = -ve
A + B + heat C … ΔH = +ve
 Molecules are adsorbed on the surface
 New interactions formed in the process of
adsorption.
So, ΔH = -ve

4. 2. Entropy (ΔS) : It is a measure of randomness
Molecules are adsorbed on the surface in the,
randomness decreases
So, ΔS = -ve
3. Gibbs free energy (ΔG):
 ΔG = ΔH – T ΔS (Gibbs Helmholtz equation)
 ΔG = +ve (Non-spontaneous process)
 ΔG = -ve (Spontaneous process)

5. For adsorption,
ΔH = -ve , ΔS = -ve , and so
– T ΔS = + ve
And as, ΔG = ΔH – T ΔS
 Therefore , ΔG = -ve, if ΔH > T ΔS
Spontaneous at low temperatures
 Therefore , ΔG = +ve, if ΔH < T ΔS
Non-spontaneous at high temperatures
3. Gibbs free energy (ΔG):

6. Factors Affecting Adsorption Of The Gases On Solids
1. Surface area : More surface area, more adsorption takes place.
Surface area ∝ Adsorption
2. Temperature : At high temperature molecules get off from the surface
i.e. desorption happens
Adsorption ∝ (Temperature)-1
3. Pressure : Pressure ∝ Adsorption
If we increase the pressure then the more numbers of molecules will adsorbed.

7. 4. Nature of the gas : If any gas is easily liquify, means it has more force of attraction.
“a” is also high
so, adsorption is also high
Tc =
a = ↑ , Tc = ↑ , Adsorption also ↑
Tc = Critical temperature
8a
27Rb

8. Adsorption
Physisorption Chemisorption
It arises due to van der waal forces It is caused by the chemical bond
formation
It is not specific in nature It is highly specific in nature
It is reversible It is irreversible
It depends on the nature of gas, More
easily liquefiable gases get adsorbed
easily
It is also depends on nature of the gas ,
Gases which react with adsorbent show
chemisorption
Enthalpy of adsorption is low (20-40 KJ
/mol)
Enthalpy of adsorption is high (80-240 KJ
/mol)
Low temperature is favourable for
adsorption, Adsorption decrease with
increase in the temperature.
High temperature is favourable for
adsorption, Adsorption increase with
increase in the temperature.
x/m
T
x/m
T

9. Physisorption Chemisorption
No appreciable activation energy is
needed
High activation energy is sometimes
needed
It depends on the surface area, it
increase with increase in surface area.
It also depends on the surface area, it too
increase with increase in surface area.
It results into multimolecular layers on
adsorbate surface under high pressure.
It results into unimolecular layer.

10. Applications of adsorption
1. Production of high vacuum :
Air can be adsorbed by the charcoal from the vessel evacuated by a vacuum pump to give very high vacuum
2. Control of humidity :
Silica and aluminium gels are used as adsorbent for removing moisture and controlling humidity.
3. Removal of colouring matter from solutions :
Animal charcoal removes colour of solution by adsorbing coloured impurities.
4. Heterogeneous catalysis :
Adsorption of reactants on the solid surface of the catalyst increase the rate of reaction. There are many gaseous
reaction of industrial importance involving solid catalysts.
Manufacture of NH3 using Fe as a catalyst.
Manufacture of H2SO4 by contact process and use of finely divided Nickel in the hydrogenation of it.
Are some excellent examples of Heterogenous catalysis

11. Shape selective catalysis by Zeolites :
 Zeolites have honeycomb like structure.
 They are microporous aluminosilicates with three dimensional network of silicates in which some silicon atoms
are replaced by aluminium atoms giving Al-O-Si framework
 The catalytic reactions that depends upon the porous structure of a catalyst and the size of reactant and product
molecule.
 Important zeolite catalyst used in petroleum industry is ZSM-5 . It converts alcohols directly into gasoline by
dehydrating them to give a mixture of hydrocarbons.