UNIT-I
SURFACE CHEMISTRY
Introduction – Adsorption- difference between adsorption and absorption - types of adsorption -
Factors influencing adsorption - Adsorption from solutions- Types of adsorption isotherms -
Freundlich adsorption isotherm - Langmuir adsorption isotherm - Industrial applications of
adsorption – Adsorption Chromatography - Role of adsorption in Catalysis - Enzyme catalysis-
Michael’s Menten equation.
1.1 INTRODUCTION
Surface chemistry is the branch of chemistry which deals with study of the phenomena
occurring at the surface or interface, i.e., at the boundary separating two bulk phases.
The two bulk phases can be pure compounds or solutions. The interface is represented by
putting a hyphen or a slash between the two bulk phases involved, e.g., solid- liquid solid-gas.
Many important phenomena, noticeable amongst these being corrosion, electrode processes,
heterogeneous catalysis, dissolution and crystallisation occur at interfaces. The subject of surface
chemistry finds many applications in industry, analytical work and daily life situations.
1.1.1 DEFINITIONS
1. Adsorption
When a solid surface is exposed to a gas or a liquid, molecules from the gas or the solution
phase accumulate (or) concentrate at the surface.
The phenomena of concentration or assimilation of a gas or liquid at a solid surface is called
adsorption. This is accompanied by decrease in residual forces.
E.g:
When a dilute solution of litmus is shaken with animal charcoal the surface of latter carries
away some of the litmus.
Atmospheric moisture gets adsorbed over the surface of silica.
2. Adsorbent
The material surface on which adsorption occurs is known as adsorbent.
Example: Charcoal, Silica gel, clay, Fuller’s earth, alumina gel, Ni. Pt, Pd etc…
3. Adsorbate
The substances that get adsorbed or attached on the surface of the solid is known as
adsorbate.
E.g : Gas (H2, N2), Liquid (NH3) etc...
4. Interface
The interface is the boundary between two spatial regions occupied by different matter, or
by matter in different physical states.
5. Absorption
Absorption is a bulk phenomenon in which the substance assimilated is uniformly
distributed throughout the body of the solid or liquid.
Adsorption is a fast process when compared to absorption, which is slow due to
diffusion of adsorbate into the interior of the adsorbent.
E.g: When a blotting paper comes in contact with ink, the latter is absorbed and
penetrates uniformly into the adsorbent
, Atmospheric moisture is absorbed by CaCl2.
Lump of sugar dipped in water.
6. Sorption
It is the phenomenon in which both adsorption and absorption takes place simultaneously.
E.g
o Uptake of gases by zeolites.
o When ammonia is passed through water in contact with charcoal sorption takes place.
o Water dissolves ammonia (absorption) and then it is absorbed by charcoal crystals.
Fig: 1.1 Illustration of adsorption, absorption and sorption
7. Desorption
The removal of the adsorbed substance from the surface is called desorption.
Table 1.1 Difference between Adsorption and Absorption
Adsorption Absorption
It is the phenomena of assimilation of a It is phenomena in which the substance
gas or liquid at the solid surface. assimilated is uniformly throughout the body
of the solid or liquid.
It is a surface phenomena It is a bulk phenomena
It is not a diffusion process Diffusion processes
Equilibrium is attained easily Equilibrium is attained slowly
It is a fast process It is a slow process
It depends on the surface area of the It requires porous structure in a substance that
adsorbent absorbs simple molecules.
Example: Adsorption of atmospheric Example: Absorption of atmospheric moisture
moisture on silica gel. by CaCl2
1.1.2 CHARACTERISTICS OF ADSORPTION
Important characteristics of adsorption phenomenon are
1. Adsorption on surface of a solid is always spontaneous because more number of new
surfaces are created by ordinary breaking of the lump. Thereby an unbalanced or
residual force generated holds any molecules with its immediate proximity.
, 2. Adsorption is always accompanied by evolution of heat because adsorption of gases
decreases the residual forces, thereby it results decrease in surface energy which in turn
appears in the form of heat.
3. Adsorption is always accompanied by decrease in enthalpy and entropy of the system
because gas adsorbed on solid adsorption surface has restricted movement, which
decreases entropy of the system. Since adsorption is always a spontaneous process, the
free energy change must be negative.
4. Adsorption is a selective process. i.e., A given adsorbent always show preference for
one adsorbate over another.
E.g: Gases like NH3, Cl2, HCl, SO2, etc., which are liquefiable, get readily adsorbed,
whereas N2, H2, O2, etc. are less readily adsorbed. It is observed that gases with low
volatility are adsorbed readily than others.
5. Adsorption is specific and its extent under a given set of conditions depends both on
the nature of adsorbent and adsorbate.
6. Increase of partial pressure of gas being adsorbed increases adsorption permit mass
of the adsorbent till a saturation point is reached.
7. The rate of adsorption depends on temperature because adsorption is an exothermic
process.
1.2 TYPES OF ADSORPTION
The adsorption of a gas on a solid surface is of two types, which is based on the nature and
affinity between the adsorbate and adsorbent.
1. Physical adsorption (or) physisorption (or) Vander Waals adsorption
2. Chemical adsorption (or) Chemisorption (or) Activated adsorption
1.2.1 Physical adsorption (or) Physisorption (or) Vander Waal’s adsorption
When a gas is adsorbed on the surface of a solid by vander waal’s forces without resulting
in the formation of any chemical bond between the adsorbate and adsorbent, it is known as physical
adsorption or vander waal’s adsorption.
E.g: Adsorption of H2 (or) O2 on charcoal.
Fig 1.2 Adsorption of H2 (or) O2 on charcoal
Characteristics of physical adsorption
The adsorbate is held on adsorbent by vander vaals forces (inter molecular).
The heat evolved during physisorption is low i.e. 20- 40 kJ/mole.
This is reversible.
Increase of pressure causes more gas to be adsorbed and the release of pressure frees the
adsorbed gas.
SURFACE CHEMISTRY
Introduction – Adsorption- difference between adsorption and absorption - types of adsorption -
Factors influencing adsorption - Adsorption from solutions- Types of adsorption isotherms -
Freundlich adsorption isotherm - Langmuir adsorption isotherm - Industrial applications of
adsorption – Adsorption Chromatography - Role of adsorption in Catalysis - Enzyme catalysis-
Michael’s Menten equation.
1.1 INTRODUCTION
Surface chemistry is the branch of chemistry which deals with study of the phenomena
occurring at the surface or interface, i.e., at the boundary separating two bulk phases.
The two bulk phases can be pure compounds or solutions. The interface is represented by
putting a hyphen or a slash between the two bulk phases involved, e.g., solid- liquid solid-gas.
Many important phenomena, noticeable amongst these being corrosion, electrode processes,
heterogeneous catalysis, dissolution and crystallisation occur at interfaces. The subject of surface
chemistry finds many applications in industry, analytical work and daily life situations.
1.1.1 DEFINITIONS
1. Adsorption
When a solid surface is exposed to a gas or a liquid, molecules from the gas or the solution
phase accumulate (or) concentrate at the surface.
The phenomena of concentration or assimilation of a gas or liquid at a solid surface is called
adsorption. This is accompanied by decrease in residual forces.
E.g:
When a dilute solution of litmus is shaken with animal charcoal the surface of latter carries
away some of the litmus.
Atmospheric moisture gets adsorbed over the surface of silica.
2. Adsorbent
The material surface on which adsorption occurs is known as adsorbent.
Example: Charcoal, Silica gel, clay, Fuller’s earth, alumina gel, Ni. Pt, Pd etc…
3. Adsorbate
The substances that get adsorbed or attached on the surface of the solid is known as
adsorbate.
E.g : Gas (H2, N2), Liquid (NH3) etc...
4. Interface
The interface is the boundary between two spatial regions occupied by different matter, or
by matter in different physical states.
5. Absorption
Absorption is a bulk phenomenon in which the substance assimilated is uniformly
distributed throughout the body of the solid or liquid.
Adsorption is a fast process when compared to absorption, which is slow due to
diffusion of adsorbate into the interior of the adsorbent.
E.g: When a blotting paper comes in contact with ink, the latter is absorbed and
penetrates uniformly into the adsorbent
, Atmospheric moisture is absorbed by CaCl2.
Lump of sugar dipped in water.
6. Sorption
It is the phenomenon in which both adsorption and absorption takes place simultaneously.
E.g
o Uptake of gases by zeolites.
o When ammonia is passed through water in contact with charcoal sorption takes place.
o Water dissolves ammonia (absorption) and then it is absorbed by charcoal crystals.
Fig: 1.1 Illustration of adsorption, absorption and sorption
7. Desorption
The removal of the adsorbed substance from the surface is called desorption.
Table 1.1 Difference between Adsorption and Absorption
Adsorption Absorption
It is the phenomena of assimilation of a It is phenomena in which the substance
gas or liquid at the solid surface. assimilated is uniformly throughout the body
of the solid or liquid.
It is a surface phenomena It is a bulk phenomena
It is not a diffusion process Diffusion processes
Equilibrium is attained easily Equilibrium is attained slowly
It is a fast process It is a slow process
It depends on the surface area of the It requires porous structure in a substance that
adsorbent absorbs simple molecules.
Example: Adsorption of atmospheric Example: Absorption of atmospheric moisture
moisture on silica gel. by CaCl2
1.1.2 CHARACTERISTICS OF ADSORPTION
Important characteristics of adsorption phenomenon are
1. Adsorption on surface of a solid is always spontaneous because more number of new
surfaces are created by ordinary breaking of the lump. Thereby an unbalanced or
residual force generated holds any molecules with its immediate proximity.
, 2. Adsorption is always accompanied by evolution of heat because adsorption of gases
decreases the residual forces, thereby it results decrease in surface energy which in turn
appears in the form of heat.
3. Adsorption is always accompanied by decrease in enthalpy and entropy of the system
because gas adsorbed on solid adsorption surface has restricted movement, which
decreases entropy of the system. Since adsorption is always a spontaneous process, the
free energy change must be negative.
4. Adsorption is a selective process. i.e., A given adsorbent always show preference for
one adsorbate over another.
E.g: Gases like NH3, Cl2, HCl, SO2, etc., which are liquefiable, get readily adsorbed,
whereas N2, H2, O2, etc. are less readily adsorbed. It is observed that gases with low
volatility are adsorbed readily than others.
5. Adsorption is specific and its extent under a given set of conditions depends both on
the nature of adsorbent and adsorbate.
6. Increase of partial pressure of gas being adsorbed increases adsorption permit mass
of the adsorbent till a saturation point is reached.
7. The rate of adsorption depends on temperature because adsorption is an exothermic
process.
1.2 TYPES OF ADSORPTION
The adsorption of a gas on a solid surface is of two types, which is based on the nature and
affinity between the adsorbate and adsorbent.
1. Physical adsorption (or) physisorption (or) Vander Waals adsorption
2. Chemical adsorption (or) Chemisorption (or) Activated adsorption
1.2.1 Physical adsorption (or) Physisorption (or) Vander Waal’s adsorption
When a gas is adsorbed on the surface of a solid by vander waal’s forces without resulting
in the formation of any chemical bond between the adsorbate and adsorbent, it is known as physical
adsorption or vander waal’s adsorption.
E.g: Adsorption of H2 (or) O2 on charcoal.
Fig 1.2 Adsorption of H2 (or) O2 on charcoal
Characteristics of physical adsorption
The adsorbate is held on adsorbent by vander vaals forces (inter molecular).
The heat evolved during physisorption is low i.e. 20- 40 kJ/mole.
This is reversible.
Increase of pressure causes more gas to be adsorbed and the release of pressure frees the
adsorbed gas.
