SURFACE CHEMISTRY
Adsorption
Adsorption is due to the presence of unbalanced forces, believed to have developed either during
crystallisation of solids or due to presence of unpaired, electrons or free valencies in solids having d–orbital.
In liquids it is due to surface tension.
Difference between adsorption and absorption
Sr. No. Adsorption Absorption
1 Adsorption is a surface phenomenon. The adsorbing It is a bulk phenomenon. In
substance is called adsorbate and is only concentrated absorption the substance
on the surface of adsorbent. penetrates into the bulk of the
other substance.
2 The rate of adsorption is rapid to start with and its rate Absorption occurs at a uniform
slowly decreases. rate
The amount of heat evolved when one mole of an adsorbate gets adsorbed on the surface of an adsorbate is
called molar heat (enthalpy) of adsorption.
Adsorption in Terms of Gibb’s Helmholtz Equation
Adsorption is an exothermic reaction. Therefore, adsorption is accompanied by release of energy or H is
always negative and favours the process. Also absorbate molecules get lesser opportunity to move about
on the surface of adsorbent. Thus, entropy factor opposes the process. According to Gibb’s Helmholtz
equation,
G = H – T S
Since adsorption does actually take place, H is greater than T S G is negative. As the adsorption
continues, the difference between the two opposing tendencies becomes lesser and lesser till they are
equal i.e., H = TS or G = 0. At this stage, equilibrium called adsorption equilibrium gets established
and there is no net adsorption taking place at this stage.
Types of Adsorption
There are two types of adsorption.
a) Physical adsorption (or Vander waal’s adsorption) or physiosorption.
b) Chemisorption
Characteristics of Physical and Chemisorption
Physio–sorption Chemisorption
1. Adsorption by weak vander waal’s forces. By chemical force (covalent or ionic bond)
2. Multimolecular layer may be formed Unimolecular layer will be formed
3. Low heat of adsorption viz. about 20–40 High heat of adsorption viz. about (200–400
kJ/mol kJ/mol)
4. Easily reversed Not reversed
5. Molecular state of adsorbate on Molecular state may be different. Surface
adsorbent is same. No surface compounds are found.
compounds are found.
6. Usually occurs rapidly at low It occurs at high temperature initially but then
temperature and decreases with increase decreases.
in temperature.
7. It increases with pressure Change in pressure will have less effect on
chemisorptions
8. Not specific Highly specific
9. Extent of adsorption depends on ease of There is no relative between extent of
, Finally divided coconut charcoal is used as gas masks for absorbing toxic gases like CH4, CO, COCl2.
It is usually used for breathing in coal mines to adsorb poisonous gases.
2. In preserving Vacuum:
In Dewar flasks, activated charcoal is placed between the walls of the flask so that any gas which
enters into annular space either due to glass imperfection of diffusion through glass is adsorbed.
3. As dehumidizer e.g silica gel
4. Fe(OH)3 antidote adsorbent in Arsenic poisoning.
5. In clarification of sugar: Sugar decolorized by treating sugar solution with charcoal powder. The
latter adsorbs the undesirable colours present.
6. Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases
the rate of reaction. There are many gaseous reactions of industrial importance involving solid
catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H 2SO4 by contact
process and use of finely divided nickel in the hydrogenation of oils are excellent examples of
heterogeneous catalysis.
7. In Chromatography: The different chromatographic techniques such as adsorption, paper or
column chromatography which are used for the purification and the separation of the substances
available in small amounts, are based upon the theory of selective adsorption.
Catalysis
Catalyst is a substance which changes the speed of a reaction, and usually, can be recovered completely
unchanged at the end of a reaction. However it may take part in a reaction consumed in one step and
regenerated in another. This phenomenon is known as Catalysis.
A substance is termed a positive catalyst or simply as catalyst if it accelerates the rate of chemical
reaction. On the other hand, the added substance is termed as negative catalyst if it retards the rate of a
chemical reaction.
Example of Negative Catalysts:
i) H3PO4 or acetanilide in the decomposition of H2O2.
ii) Alcohol in the oxidation of chloroform leading to the formation of phosgene.
iii) Tertraethyl lead or nickel carbonyl acting as antiknock material in internal combustion engines.
iv) Antifreezes like glycerol which retard the rusting of the machines.
Catalytic Promoters
There are certain substances which when added only in small quantity to a catalyst enhance its activity.
This substance itself may not be catalyst. Such substances which enhance the activity of a catalyst are
called catalytic promoters. E.g. Molybdenum is used as Promoter for Fe catalyst in Haber’s process.
Catalytic Inhibitors or poisons
The rates of some reactions are reduced considerably by the presence of small amounts of other
substances called inhibitors. The substance whose presence decreases or destroy the activity of a
catalyst are called poison. CO or H2S in hydrogen gas acts as a poison for Fe catalyst in Haber process.
Types of Catalysis
Broadly, two types of catalysis are known:
(a) Homogeneous catalysis (b) Heterogeneous catalysis
(a) Homogeneous catalysis
If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst
and this type of catalysis is called homogeneous catalysis.
Ex:
C12 H 22O11 (aq ) H 2O(l) H2SO4 C6 H12O 6 (aq ) C6 H12O6 (aq )
Sucrose Glu cos e Fructose
2SO 2 (g) O 2 (g) 2SO 3 (g)
NO( g )
CH 3COOCH 3 () H 2O() HCl
()
CH 3COOH (aq ) CH 3OH(aq )
b) Heterogeneous catalysis:
In this type of catalysis the catalyst is present in a difference phase than that of the reactants. In
heterogeneous catalysis, catalyst is generally a solid and the reactants are generally gases but
sometimes liquid reactants are also used. It is also known as surface catalysis.
Ex:
i) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of copper, ZnO and Cr2O3 as
Adsorption
Adsorption is due to the presence of unbalanced forces, believed to have developed either during
crystallisation of solids or due to presence of unpaired, electrons or free valencies in solids having d–orbital.
In liquids it is due to surface tension.
Difference between adsorption and absorption
Sr. No. Adsorption Absorption
1 Adsorption is a surface phenomenon. The adsorbing It is a bulk phenomenon. In
substance is called adsorbate and is only concentrated absorption the substance
on the surface of adsorbent. penetrates into the bulk of the
other substance.
2 The rate of adsorption is rapid to start with and its rate Absorption occurs at a uniform
slowly decreases. rate
The amount of heat evolved when one mole of an adsorbate gets adsorbed on the surface of an adsorbate is
called molar heat (enthalpy) of adsorption.
Adsorption in Terms of Gibb’s Helmholtz Equation
Adsorption is an exothermic reaction. Therefore, adsorption is accompanied by release of energy or H is
always negative and favours the process. Also absorbate molecules get lesser opportunity to move about
on the surface of adsorbent. Thus, entropy factor opposes the process. According to Gibb’s Helmholtz
equation,
G = H – T S
Since adsorption does actually take place, H is greater than T S G is negative. As the adsorption
continues, the difference between the two opposing tendencies becomes lesser and lesser till they are
equal i.e., H = TS or G = 0. At this stage, equilibrium called adsorption equilibrium gets established
and there is no net adsorption taking place at this stage.
Types of Adsorption
There are two types of adsorption.
a) Physical adsorption (or Vander waal’s adsorption) or physiosorption.
b) Chemisorption
Characteristics of Physical and Chemisorption
Physio–sorption Chemisorption
1. Adsorption by weak vander waal’s forces. By chemical force (covalent or ionic bond)
2. Multimolecular layer may be formed Unimolecular layer will be formed
3. Low heat of adsorption viz. about 20–40 High heat of adsorption viz. about (200–400
kJ/mol kJ/mol)
4. Easily reversed Not reversed
5. Molecular state of adsorbate on Molecular state may be different. Surface
adsorbent is same. No surface compounds are found.
compounds are found.
6. Usually occurs rapidly at low It occurs at high temperature initially but then
temperature and decreases with increase decreases.
in temperature.
7. It increases with pressure Change in pressure will have less effect on
chemisorptions
8. Not specific Highly specific
9. Extent of adsorption depends on ease of There is no relative between extent of
, Finally divided coconut charcoal is used as gas masks for absorbing toxic gases like CH4, CO, COCl2.
It is usually used for breathing in coal mines to adsorb poisonous gases.
2. In preserving Vacuum:
In Dewar flasks, activated charcoal is placed between the walls of the flask so that any gas which
enters into annular space either due to glass imperfection of diffusion through glass is adsorbed.
3. As dehumidizer e.g silica gel
4. Fe(OH)3 antidote adsorbent in Arsenic poisoning.
5. In clarification of sugar: Sugar decolorized by treating sugar solution with charcoal powder. The
latter adsorbs the undesirable colours present.
6. Heterogeneous catalysis: Adsorption of reactants on the solid surface of the catalysts increases
the rate of reaction. There are many gaseous reactions of industrial importance involving solid
catalysts. Manufacture of ammonia using iron as a catalyst, manufacture of H 2SO4 by contact
process and use of finely divided nickel in the hydrogenation of oils are excellent examples of
heterogeneous catalysis.
7. In Chromatography: The different chromatographic techniques such as adsorption, paper or
column chromatography which are used for the purification and the separation of the substances
available in small amounts, are based upon the theory of selective adsorption.
Catalysis
Catalyst is a substance which changes the speed of a reaction, and usually, can be recovered completely
unchanged at the end of a reaction. However it may take part in a reaction consumed in one step and
regenerated in another. This phenomenon is known as Catalysis.
A substance is termed a positive catalyst or simply as catalyst if it accelerates the rate of chemical
reaction. On the other hand, the added substance is termed as negative catalyst if it retards the rate of a
chemical reaction.
Example of Negative Catalysts:
i) H3PO4 or acetanilide in the decomposition of H2O2.
ii) Alcohol in the oxidation of chloroform leading to the formation of phosgene.
iii) Tertraethyl lead or nickel carbonyl acting as antiknock material in internal combustion engines.
iv) Antifreezes like glycerol which retard the rusting of the machines.
Catalytic Promoters
There are certain substances which when added only in small quantity to a catalyst enhance its activity.
This substance itself may not be catalyst. Such substances which enhance the activity of a catalyst are
called catalytic promoters. E.g. Molybdenum is used as Promoter for Fe catalyst in Haber’s process.
Catalytic Inhibitors or poisons
The rates of some reactions are reduced considerably by the presence of small amounts of other
substances called inhibitors. The substance whose presence decreases or destroy the activity of a
catalyst are called poison. CO or H2S in hydrogen gas acts as a poison for Fe catalyst in Haber process.
Types of Catalysis
Broadly, two types of catalysis are known:
(a) Homogeneous catalysis (b) Heterogeneous catalysis
(a) Homogeneous catalysis
If the catalyst is present in the same phase as the reactants, it is called a homogeneous catalyst
and this type of catalysis is called homogeneous catalysis.
Ex:
C12 H 22O11 (aq ) H 2O(l) H2SO4 C6 H12O 6 (aq ) C6 H12O6 (aq )
Sucrose Glu cos e Fructose
2SO 2 (g) O 2 (g) 2SO 3 (g)
NO( g )
CH 3COOCH 3 () H 2O() HCl
()
CH 3COOH (aq ) CH 3OH(aq )
b) Heterogeneous catalysis:
In this type of catalysis the catalyst is present in a difference phase than that of the reactants. In
heterogeneous catalysis, catalyst is generally a solid and the reactants are generally gases but
sometimes liquid reactants are also used. It is also known as surface catalysis.
Ex:
i) Synthesis of methyl alcohol (CH3OH) from CO and H2 using a mixture of copper, ZnO and Cr2O3 as
