Chapter – 5 Surface Chemistry
Introduction
Surface chemistry is a branch of chemistry which deals with phenomena that
occur at the surfaces or interfaces. between solid- solid, solid – liquid, liquid-
liquid, liquid – gas.
Example: The interface between a solid and a gas may be represented by solid-
gas or solid / gas. Due to complete miscibility, there is no interface between the
gases.
Many important phenomena, noticeable amongst these being corrosion,
electrode processes, heterogeneous catalysis, dissolution and crystallisation
occur at interfaces.
Adsorption: It is phenomena in which one of the substance accumates overthe
surface of another rather than in the bulk of a solid or liquid is termed
adsorption.
Adsorbate: The molecular species or substance, which adsorbs or accumulates
at the surface is termed adsorbate.
Adsorbent : The substance at the surface of which the adsorption takes place is
called adsorbent.
Adsorption is essentially a surface phenomenon. Charcoal, silica gel, alumina
gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.
Examples for adsorption
(i) Example – gas adsorbed over solid: O2, H2, NH3, CO, CO2, SO2,SO3 can be
adsorbed over activated charcoal.
(ii) Liquid adsorbed over solid – moisture adsorbed over silica gel or
alumina(Al2O3).
(iii) Solid adsorbed over solid- Organicdyes pigments (from aqueous solution of
raw sugar) adsorbed over animal charcoal.
Desorption:- The process of removal of adsorbed substance from a surface on
which it is adsorbed is called desorption.
Distinction between adsorption and Absorption
Adsorption Absorption
In adsorption, the substance is While in absorption the substance
concentrated only at the surface and is uniformly distributed throughout
does not penetrate through the the bulk of the solid.
surface to the bulk of the adsorbent. It is bulk phenomenon.
it is surface / phenomenon Concentration per unit volume
Adsorption increases with surface remains the same
area.
, Eg: Water vapour adsorbed on silica Water vapours are absorbed by
gel. anhydrous calcium chloride.
Both adsorption and absorption can take place
simultaneously also. The term sorption is used to describe
both the processes.
In the figure, m is the amount of the substance that has moved
across the interface. In adsorption, the process is rapid in the
beginning and slows down near equilibrium while absorption
takes place at uniform rate.
Mechanism of Adsorption(in point)
(i)Adsorption arises due to the fact that the surface particles of the adsorbent
are not in the same environment as the particles inside the bulk.
Types
of sorption separations
(ii) Inside the absorbent all the forces acting between the particles are mutually
balanced but on the surface the particles are no surrounded by atoms or
molecules of their kind on all sides, and hence they possess unbalanced or
residual attractive forces.
(iii)These forces of the adsorbent are responsible for attracting the
adsorbate particles on its surface.
(iv)The extent of adsorption increases with the increase of surface area per unit
mass of the adsorbent at a given temperature and pressure.
(v)During adsorption, decrease in residual forces of the surface, i.e., there is
decrease in surface energy therefore, is invariably an exothermic
process.
(vi)The energy of the surface of adsorbent is called surface energy.
Thermodynamics of adsorption
(i)H of adsorption is always negative, because it is exothermic in nature.
(ii)When a gas is adsorbed, there is a decrease in the entropy of the gas after
adsorption, i.e., S is negative.
(iii)At constant temperature and pressure, G must be negative. i.e.. there is a
decrease in Gibbs energy.
(iv)G = H TS, G can be negative if H has sufficiently high negative value
as TS is positive. Thus, in an adsorption process, which is spontaneous, a
combination of these two factors makes G negative.
(v)As the adsorption proceeds, H becomes less and less negative ultimately H
becomes equal to TS and G becomes zero. At this state equilibrium is
attained.
Types of Adsorption
(1)There are mainly two types of adsorption of gases on solids.
,(i)Physical adsorption or physisorption
(ii)Chemical adsorption or chemisorptions
Physisorption Chemisorption
1. It arises because of van der Waals’ 1. It is caused by chemical bond
forces. formation.
2. It is not specific in nature. 2. It is highly specific in nature.
Lack of specificity: A given surface High specificity: Chemisorption is
of an adsorbent does not show any highly specific and it will only occur if
preference for a particular gas as there is some possibility of chemical
the van der Waals forces are bonding between adsorbent and
universal. adsorbent. For example, oxygen is
adsorbed on metals by virtue of oxide
formation.
3. It is reversible in nature. 3. It is Irreversible.
Reversible nature: Physical Irreversibility: As chemisorption
adsorption of a gas by a solid is involves compound formation. it is
generally reversible. Thus, usually irreversible in nature.
Solid + Gas Gas / Chemisorption is also an exothermic
Solid + Heat process but the process is very slow
More gas is absorbed when at low temperatures on account of
pressure is increased as the volume high energy of activation like most
of the gas decreases (Le-Chatelier’s chemical adsorption often increases
principle. Physical adsorption with rise of temperature. High
occurs readily at low temperature pressure is also favourable for
and decreases with increasing chemisorption.
temperature (Le-Chatelier’s
principle).
4. It depends on the nature of gas. 4. It also depends on the nature of gas.
More easily liquefiable gases are Gases which can react with the
adsorbed readily, adsorbent show chemisorption.
Nature of adsorbate: The amount of
gas adsorbed by a solid depends on
the nature of gas. In general, easily
liquefiable gases (i.e., with higher
critical temperatures are readily
adsorbed as van der Waals’ forces
are stronger near the critical
temperatures. Thus, 1g of activated
charcoal adsorbs more sulphur
dioxide (critical temperature
630 K), than methane (critical
temperature 190 K) which is still
more than 4.5 mL of dihydrogen
(critical temperature 33K)
, 5. Enthalpy of adsorption is low (20-40 5. Enthalpy of adsorption is high
kJ mol1) in this case. (80-240 kJ mol1) in this case.
6. Low temperature is favourable for 6. High temperature is favourable for
adsorption. It decreases with adsorption. It increases with the
increase of temperature. increase of temperature.
7. No appreciable activation energy is 7. High activation energy is sometimes
needed. needed.
8. It depends on the surface area. It 8. It also depends on the surface area. It
increases with an increase of too increases with an increase of
surface area. surface area.
Finely divided metals, porous
substances good absorbents.
9. It results into multimolecular layers 9. It results into unimolecular layer.
on absorbent surface under high
pressure.
Ex: H2 molecules held on Ramey Ex: H atom held by catalytic Ni by
nickel surface by van der Waal’s chemical bonds.
forces.
Extent of gas adsorbed during physisorption:-
Adsorption Isotherms
The variation in the amount of gas adsorbed by the adsorbent
with pressure at constant temperature can be expressed by
means of a curve termed as adsorption isotherm.
Freundlich adsorption isotherm: Freundlich, in 1909, gaves an
empirical relationship between the quantity of gas adsorbed
by unit mass of solid adsorbent and pressure at a particular temperature.
x
k.P l / n (n 1) - (1)
m
Where x is the mass of adsorbate is m of the absorbent at pressure P,x/m =
extent of adsorption P= Pressure of gas during adsorption
k and n are constants which depend on the nature of the adsorbent and the gas
at a particular temperature.
Curve where mass of the gas adsorbed per gram of the adsorbent is plotted
against pressure. There is a decrease in physical adsorption with increase in
temperature. These curves always seem to approach saturation at high
pressure.
Taking logarithm, log
x
log k
1
log P -(2)
m n
by plotting log
x on y-axis and log P on x – axis it comes to be
m
a straight line.
intercept gives the value of log K slope is 1
n
Introduction
Surface chemistry is a branch of chemistry which deals with phenomena that
occur at the surfaces or interfaces. between solid- solid, solid – liquid, liquid-
liquid, liquid – gas.
Example: The interface between a solid and a gas may be represented by solid-
gas or solid / gas. Due to complete miscibility, there is no interface between the
gases.
Many important phenomena, noticeable amongst these being corrosion,
electrode processes, heterogeneous catalysis, dissolution and crystallisation
occur at interfaces.
Adsorption: It is phenomena in which one of the substance accumates overthe
surface of another rather than in the bulk of a solid or liquid is termed
adsorption.
Adsorbate: The molecular species or substance, which adsorbs or accumulates
at the surface is termed adsorbate.
Adsorbent : The substance at the surface of which the adsorption takes place is
called adsorbent.
Adsorption is essentially a surface phenomenon. Charcoal, silica gel, alumina
gel, clay, colloids, metals in finely divided state, etc. act as good adsorbents.
Examples for adsorption
(i) Example – gas adsorbed over solid: O2, H2, NH3, CO, CO2, SO2,SO3 can be
adsorbed over activated charcoal.
(ii) Liquid adsorbed over solid – moisture adsorbed over silica gel or
alumina(Al2O3).
(iii) Solid adsorbed over solid- Organicdyes pigments (from aqueous solution of
raw sugar) adsorbed over animal charcoal.
Desorption:- The process of removal of adsorbed substance from a surface on
which it is adsorbed is called desorption.
Distinction between adsorption and Absorption
Adsorption Absorption
In adsorption, the substance is While in absorption the substance
concentrated only at the surface and is uniformly distributed throughout
does not penetrate through the the bulk of the solid.
surface to the bulk of the adsorbent. It is bulk phenomenon.
it is surface / phenomenon Concentration per unit volume
Adsorption increases with surface remains the same
area.
, Eg: Water vapour adsorbed on silica Water vapours are absorbed by
gel. anhydrous calcium chloride.
Both adsorption and absorption can take place
simultaneously also. The term sorption is used to describe
both the processes.
In the figure, m is the amount of the substance that has moved
across the interface. In adsorption, the process is rapid in the
beginning and slows down near equilibrium while absorption
takes place at uniform rate.
Mechanism of Adsorption(in point)
(i)Adsorption arises due to the fact that the surface particles of the adsorbent
are not in the same environment as the particles inside the bulk.
Types
of sorption separations
(ii) Inside the absorbent all the forces acting between the particles are mutually
balanced but on the surface the particles are no surrounded by atoms or
molecules of their kind on all sides, and hence they possess unbalanced or
residual attractive forces.
(iii)These forces of the adsorbent are responsible for attracting the
adsorbate particles on its surface.
(iv)The extent of adsorption increases with the increase of surface area per unit
mass of the adsorbent at a given temperature and pressure.
(v)During adsorption, decrease in residual forces of the surface, i.e., there is
decrease in surface energy therefore, is invariably an exothermic
process.
(vi)The energy of the surface of adsorbent is called surface energy.
Thermodynamics of adsorption
(i)H of adsorption is always negative, because it is exothermic in nature.
(ii)When a gas is adsorbed, there is a decrease in the entropy of the gas after
adsorption, i.e., S is negative.
(iii)At constant temperature and pressure, G must be negative. i.e.. there is a
decrease in Gibbs energy.
(iv)G = H TS, G can be negative if H has sufficiently high negative value
as TS is positive. Thus, in an adsorption process, which is spontaneous, a
combination of these two factors makes G negative.
(v)As the adsorption proceeds, H becomes less and less negative ultimately H
becomes equal to TS and G becomes zero. At this state equilibrium is
attained.
Types of Adsorption
(1)There are mainly two types of adsorption of gases on solids.
,(i)Physical adsorption or physisorption
(ii)Chemical adsorption or chemisorptions
Physisorption Chemisorption
1. It arises because of van der Waals’ 1. It is caused by chemical bond
forces. formation.
2. It is not specific in nature. 2. It is highly specific in nature.
Lack of specificity: A given surface High specificity: Chemisorption is
of an adsorbent does not show any highly specific and it will only occur if
preference for a particular gas as there is some possibility of chemical
the van der Waals forces are bonding between adsorbent and
universal. adsorbent. For example, oxygen is
adsorbed on metals by virtue of oxide
formation.
3. It is reversible in nature. 3. It is Irreversible.
Reversible nature: Physical Irreversibility: As chemisorption
adsorption of a gas by a solid is involves compound formation. it is
generally reversible. Thus, usually irreversible in nature.
Solid + Gas Gas / Chemisorption is also an exothermic
Solid + Heat process but the process is very slow
More gas is absorbed when at low temperatures on account of
pressure is increased as the volume high energy of activation like most
of the gas decreases (Le-Chatelier’s chemical adsorption often increases
principle. Physical adsorption with rise of temperature. High
occurs readily at low temperature pressure is also favourable for
and decreases with increasing chemisorption.
temperature (Le-Chatelier’s
principle).
4. It depends on the nature of gas. 4. It also depends on the nature of gas.
More easily liquefiable gases are Gases which can react with the
adsorbed readily, adsorbent show chemisorption.
Nature of adsorbate: The amount of
gas adsorbed by a solid depends on
the nature of gas. In general, easily
liquefiable gases (i.e., with higher
critical temperatures are readily
adsorbed as van der Waals’ forces
are stronger near the critical
temperatures. Thus, 1g of activated
charcoal adsorbs more sulphur
dioxide (critical temperature
630 K), than methane (critical
temperature 190 K) which is still
more than 4.5 mL of dihydrogen
(critical temperature 33K)
, 5. Enthalpy of adsorption is low (20-40 5. Enthalpy of adsorption is high
kJ mol1) in this case. (80-240 kJ mol1) in this case.
6. Low temperature is favourable for 6. High temperature is favourable for
adsorption. It decreases with adsorption. It increases with the
increase of temperature. increase of temperature.
7. No appreciable activation energy is 7. High activation energy is sometimes
needed. needed.
8. It depends on the surface area. It 8. It also depends on the surface area. It
increases with an increase of too increases with an increase of
surface area. surface area.
Finely divided metals, porous
substances good absorbents.
9. It results into multimolecular layers 9. It results into unimolecular layer.
on absorbent surface under high
pressure.
Ex: H2 molecules held on Ramey Ex: H atom held by catalytic Ni by
nickel surface by van der Waal’s chemical bonds.
forces.
Extent of gas adsorbed during physisorption:-
Adsorption Isotherms
The variation in the amount of gas adsorbed by the adsorbent
with pressure at constant temperature can be expressed by
means of a curve termed as adsorption isotherm.
Freundlich adsorption isotherm: Freundlich, in 1909, gaves an
empirical relationship between the quantity of gas adsorbed
by unit mass of solid adsorbent and pressure at a particular temperature.
x
k.P l / n (n 1) - (1)
m
Where x is the mass of adsorbate is m of the absorbent at pressure P,x/m =
extent of adsorption P= Pressure of gas during adsorption
k and n are constants which depend on the nature of the adsorbent and the gas
at a particular temperature.
Curve where mass of the gas adsorbed per gram of the adsorbent is plotted
against pressure. There is a decrease in physical adsorption with increase in
temperature. These curves always seem to approach saturation at high
pressure.
Taking logarithm, log
x
log k
1
log P -(2)
m n
by plotting log
x on y-axis and log P on x – axis it comes to be
m
a straight line.
intercept gives the value of log K slope is 1
n
