Surface Chemistry 573
Chapter
14
Surface Chemistry
“The branch of physical chemistry, which deals the nature of
surfaces and also with the chemical and physical processes which takes (3) Difference between adsorption and absorption
place on the surfaces, is called surface chemistry”. Adsorption Absorption
In surface chemistry, we study the phenomenon of adsorption, It is a surface phenomenon. It concerns with the whole mass of the
catalysis and colloidal properties. absorbent.
In it, the substance is only retained on It implies that a substance is uniformly
Adsorption the surface and does not go into the distributed, through the body of the
(1) Definition : The phenomenon of attracting and retaining the bulk or interior of the solid or liquid. solid or liquid.
molecules of a substance on the surface of a liquid or solid resulting in to In it the concentration of the adsorbed In it the concentration is low.
higher concentration of the molecules on the surface is called adsorption. molecules is always greater at the free
phase.
(2) Causes of adsorption : Unbalanced forces of attraction or free
It is rapid in the beginning and slows It occurs at the uniform rate.
valencies which is present at the solid or liquid surface, have the property to down near the equilibrium.
attract and retain the molecules of a gas or a dissolved substance on to Examples : (i) Water vapours adsorbed Examples : (i) Water vapours absorbed
their surfaces with which they come in contact. by silica gel. by anhydrous CaCl2
Example : Ammonia gas placed in contact with charcoal gets (ii) NH3 is adsorbed by charcoal. (ii) NH3 is absorbed in water forming
adsorbed on the charcoal whereas ammonia gas placed in contact with NH4OH
water gets absorbed into water,
(4) Surface forces : Only the surface atoms of an adsorbent play an
Table : 14.1 Some basic terms used in adsorption
active role in adsorption. These atoms posses unbalanced forces of various
Interface : Any surface is a plane which Adsorbate and Adsorbent : The types such as, Vander Waal’s forces and chemical bond forces.
separates any two phases in contact substance which gets adsorbed on any
with each other. The plane which surface is called adsorbate for example, Thus, the residual force-field on a free surface which is responsible
separates any two phase is generally if a gas gets adsorbed on to the surface for adsorption is produced. For example, when a solid substance is broken
called an interface between the two of a solid, then the gas is termed as into two pieces, two new surfaces are formed and therefore, the number of
phases. the adsorbate. unbalanced forces becomes more. As a result the tendency for adsorption
The substance on the surface of which become large.
adsorption takes place is called (5) Reversible and Irreversible adsorption : The adsorption is
adsorbent.
reversible, if the adsorbate can be easily removed from the surface of the
adsorbent by physical methods.If the adsorbate can not be easily removed
Desorption : The removal of the Absorption : When the molecules of a from the surface of the adsorbent is called irreversible adsorption.
adsorbed substance from a surface is substance are uniformly distributed
called desorption. throughout the body of a solid or Example for reversible adsorption: A gas adsorbed on a solid surface can
liquid. This phenomenon is called be completely removed in vacuum.
absorption.
Example for irreversible adsorption: Adsorption of O on tungusten
Sorption : The phenomenon in which Occlusion : When adsorption of gases 2
adsorbent.
adsorption and absorption occur occur on the surface of metals this is
simultaneously is called sorption. called occlusion. (6) Characteristics of adsorption
Mc. Bain introduced a general term (i) Adsorption refers to the existence of a higher concentration of
sorption describeing both the
processes, however adsorption is
any particular component at the surface of a liquid or a solid phase.
instantaneous i.e. a fast process while (ii) Adsorption is accompanied by decrease in the G (free energy
absorption is a slow process.
change) of the system when G 0 , adsorption equilibrium is said to be
established.
, 574 Surface Chemistry
(iii) Adsorption is invariably accompanied by evolution of heat, i.e. it (i) In general, easily liquefiable gases e.g., CO , NH , Cl and SO etc.
2 3 2 2
is an exothermic process. In other words, H of adsorption is always are adsorbed to a greater extent than the elemental gases e.g. H , O , N , He 2 2 2
negative. etc. (while chemisorption is specific in nature.)
(iv) When a gas is adsorbed, the freedom of movement of its (ii) Porous and finely powdered solid e.g. charcoal, fullers earth,
molecules becomes restricted. On account of it decrease in the entropy of adsorb more as compared to the hard non-porous materials. Due to this
property powdered charcoal is used in gas masks.
the gas after adsorption, i.e. S is negative.
(2) Surface area of the solid adsorbent
(v) For a process to be spontaneous, the thermodynamic (i) The extent of adsorption depends directly upon the surface area
requirement is that G must be negative, i.e. there is decrease in free of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the
energy. On the basis of Gibb’s Helmholtz equation, G H TS , extent of adsorption.
G can be negative if H has sufficiently high negative value and TS (ii) Surface area of a powdered solid adsorbent depends upon its
has positive value. particle size. Smaller the particle size, greater is its surface area.
(3) Effect of pressure on the adsorbate gas
Classification of adsorption
(i) An increase in the pressure of the adsorbate gas increases the
Adsorption can be classified into two categories as described below, extent of adsorption. Adsorption tends to
(1) Depending upon the concentration : In adsorption the (ii) At low temperature, the reach limiting value
Extent of adsorption
concentration of one substance is different at the surface of the other extent of adsorption increases rapidly
substance as compared to adjoining bulk or interior phase. with pressure.
(iii) Small range of pressure, the
(i) Positive adsorption : If the concentration of adsorbate is more on extent of adsorption is found to be
the surface as compared to its concentration in the bulk phase then it is directly proportional to the pressure. Adsorption increases
called positive adsorption. rapidly at the beginning
(iv) At high pressure (closer Pressure
Example : When a concentrated solution of KCl is shaken with blood to the saturation vapour pressure of
charcoal, it shows positive adsorption. the gas), the adsorption tends to achieve a limiting value.
(ii) Negative adsorption : If the concentration of the adsorbate is less (4) Effect of temperature
than its concentration in the bulk then it is called negative adsorption.
(i) As adsorption is accompanied by evolution of heat, so according
Example : When a dilute solution of KCl is shaken with blood to the Le-Chatelier’s principle, the magnitude of adsorption should decrease
charcoal, it shows negative adsorption. with rise in temperature.
(2) Depending upon the nature of force existing between adsorbate
molecule and adsorbent P - Constant
P - Constant
(i) Physical adsorption : If the forces of attraction existing between
adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called x/m x/m
physical adsorption. This type of adsorption is also known as physisorption
or Vander Waal’s adsorption. It can be easily reversed by heating or
decreasing the pressure. Temperature Temperature
(ii) Chemical adsorption : If the forces of attraction existing between Physical adsorption Chemical adsorption
adsorbate particles and adsorbent are almost of the same strength as (ii) The relationship between the extent of adsorption and
chemical bonds, the adsorption is called chemical adsorption. This type of temperature at any constant pressure is called adsorption isobar.
adsorption is also called as chemisorption or Langmuir adsorption. This type (iii) A physical adsorption isobar shows a decrease in x/m (where ‘m’
of adsorption cannot be easily reversed. is the mass of the adsorbent and ‘x’ that of adsorbate) as the temperature
Comparison between physisorption and chemisorption rises.
Physisorption Chemisorption (iv) The isobar of chemisorption show an increase in the beginning
(Vander Waal's adsorption) (Langmuir adsorption) and then decrease as the temperature rises.
Low heat of adsorption usually in High heat of adsorption in the range of Adsorption isotherms
range of 20-40 kJ/mol 50-400 kJ/mol
Force of attraction are Vander Waal's Forces of attraction are chemical bond A mathematical equation, which describes the relationship between
forces. forces. pressure (p) of the gaseous adsorbate and the extent of adsorption at any
It is reversible It is irreversible fixed temperature, is called adsorption isotherms.
It is usually takes place at low It takes place at high temperature. The extent of adsorption is expressed as mass of the adsorbate
temperature and decreases with adsorbed on one unit mass of the adsorbent.
increasing temperature.
Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent,
It is related to the case of liquefication It is not related.
of the gas. then
It forms multimolecular layers. It forms monomolecular layers. x
Extent of adsorption
It does not require any activation It requires high activation energy. m
energy.
Various adsorption isotherms are commonly employed in describing
High pressure is favourable. Decrease High pressure is favourable. Decrease
the adsorption data.
of pressure causes desorption of pressure does not cause desorption.
It is not very specific. It is highly specific. (1) Freundlich adsorption isotherm
(i) Freundlich adsorption isotherm is obeyed by the adsorptions
Factors which affect the extent of adsorption : The following are the where the adsorbate forms a monomolecular layer on the surface of the
factors which affect the adsorption, adsorbent.
(1) Nature of the adsorbate (gas) and adsorbent (solid)
Chapter
14
Surface Chemistry
“The branch of physical chemistry, which deals the nature of
surfaces and also with the chemical and physical processes which takes (3) Difference between adsorption and absorption
place on the surfaces, is called surface chemistry”. Adsorption Absorption
In surface chemistry, we study the phenomenon of adsorption, It is a surface phenomenon. It concerns with the whole mass of the
catalysis and colloidal properties. absorbent.
In it, the substance is only retained on It implies that a substance is uniformly
Adsorption the surface and does not go into the distributed, through the body of the
(1) Definition : The phenomenon of attracting and retaining the bulk or interior of the solid or liquid. solid or liquid.
molecules of a substance on the surface of a liquid or solid resulting in to In it the concentration of the adsorbed In it the concentration is low.
higher concentration of the molecules on the surface is called adsorption. molecules is always greater at the free
phase.
(2) Causes of adsorption : Unbalanced forces of attraction or free
It is rapid in the beginning and slows It occurs at the uniform rate.
valencies which is present at the solid or liquid surface, have the property to down near the equilibrium.
attract and retain the molecules of a gas or a dissolved substance on to Examples : (i) Water vapours adsorbed Examples : (i) Water vapours absorbed
their surfaces with which they come in contact. by silica gel. by anhydrous CaCl2
Example : Ammonia gas placed in contact with charcoal gets (ii) NH3 is adsorbed by charcoal. (ii) NH3 is absorbed in water forming
adsorbed on the charcoal whereas ammonia gas placed in contact with NH4OH
water gets absorbed into water,
(4) Surface forces : Only the surface atoms of an adsorbent play an
Table : 14.1 Some basic terms used in adsorption
active role in adsorption. These atoms posses unbalanced forces of various
Interface : Any surface is a plane which Adsorbate and Adsorbent : The types such as, Vander Waal’s forces and chemical bond forces.
separates any two phases in contact substance which gets adsorbed on any
with each other. The plane which surface is called adsorbate for example, Thus, the residual force-field on a free surface which is responsible
separates any two phase is generally if a gas gets adsorbed on to the surface for adsorption is produced. For example, when a solid substance is broken
called an interface between the two of a solid, then the gas is termed as into two pieces, two new surfaces are formed and therefore, the number of
phases. the adsorbate. unbalanced forces becomes more. As a result the tendency for adsorption
The substance on the surface of which become large.
adsorption takes place is called (5) Reversible and Irreversible adsorption : The adsorption is
adsorbent.
reversible, if the adsorbate can be easily removed from the surface of the
adsorbent by physical methods.If the adsorbate can not be easily removed
Desorption : The removal of the Absorption : When the molecules of a from the surface of the adsorbent is called irreversible adsorption.
adsorbed substance from a surface is substance are uniformly distributed
called desorption. throughout the body of a solid or Example for reversible adsorption: A gas adsorbed on a solid surface can
liquid. This phenomenon is called be completely removed in vacuum.
absorption.
Example for irreversible adsorption: Adsorption of O on tungusten
Sorption : The phenomenon in which Occlusion : When adsorption of gases 2
adsorbent.
adsorption and absorption occur occur on the surface of metals this is
simultaneously is called sorption. called occlusion. (6) Characteristics of adsorption
Mc. Bain introduced a general term (i) Adsorption refers to the existence of a higher concentration of
sorption describeing both the
processes, however adsorption is
any particular component at the surface of a liquid or a solid phase.
instantaneous i.e. a fast process while (ii) Adsorption is accompanied by decrease in the G (free energy
absorption is a slow process.
change) of the system when G 0 , adsorption equilibrium is said to be
established.
, 574 Surface Chemistry
(iii) Adsorption is invariably accompanied by evolution of heat, i.e. it (i) In general, easily liquefiable gases e.g., CO , NH , Cl and SO etc.
2 3 2 2
is an exothermic process. In other words, H of adsorption is always are adsorbed to a greater extent than the elemental gases e.g. H , O , N , He 2 2 2
negative. etc. (while chemisorption is specific in nature.)
(iv) When a gas is adsorbed, the freedom of movement of its (ii) Porous and finely powdered solid e.g. charcoal, fullers earth,
molecules becomes restricted. On account of it decrease in the entropy of adsorb more as compared to the hard non-porous materials. Due to this
property powdered charcoal is used in gas masks.
the gas after adsorption, i.e. S is negative.
(2) Surface area of the solid adsorbent
(v) For a process to be spontaneous, the thermodynamic (i) The extent of adsorption depends directly upon the surface area
requirement is that G must be negative, i.e. there is decrease in free of the adsorbent, i.e. larger the surface area of the adsorbent, greater is the
energy. On the basis of Gibb’s Helmholtz equation, G H TS , extent of adsorption.
G can be negative if H has sufficiently high negative value and TS (ii) Surface area of a powdered solid adsorbent depends upon its
has positive value. particle size. Smaller the particle size, greater is its surface area.
(3) Effect of pressure on the adsorbate gas
Classification of adsorption
(i) An increase in the pressure of the adsorbate gas increases the
Adsorption can be classified into two categories as described below, extent of adsorption. Adsorption tends to
(1) Depending upon the concentration : In adsorption the (ii) At low temperature, the reach limiting value
Extent of adsorption
concentration of one substance is different at the surface of the other extent of adsorption increases rapidly
substance as compared to adjoining bulk or interior phase. with pressure.
(iii) Small range of pressure, the
(i) Positive adsorption : If the concentration of adsorbate is more on extent of adsorption is found to be
the surface as compared to its concentration in the bulk phase then it is directly proportional to the pressure. Adsorption increases
called positive adsorption. rapidly at the beginning
(iv) At high pressure (closer Pressure
Example : When a concentrated solution of KCl is shaken with blood to the saturation vapour pressure of
charcoal, it shows positive adsorption. the gas), the adsorption tends to achieve a limiting value.
(ii) Negative adsorption : If the concentration of the adsorbate is less (4) Effect of temperature
than its concentration in the bulk then it is called negative adsorption.
(i) As adsorption is accompanied by evolution of heat, so according
Example : When a dilute solution of KCl is shaken with blood to the Le-Chatelier’s principle, the magnitude of adsorption should decrease
charcoal, it shows negative adsorption. with rise in temperature.
(2) Depending upon the nature of force existing between adsorbate
molecule and adsorbent P - Constant
P - Constant
(i) Physical adsorption : If the forces of attraction existing between
adsorbate and adsorbent are Vander Waal’s forces, the adsorption is called x/m x/m
physical adsorption. This type of adsorption is also known as physisorption
or Vander Waal’s adsorption. It can be easily reversed by heating or
decreasing the pressure. Temperature Temperature
(ii) Chemical adsorption : If the forces of attraction existing between Physical adsorption Chemical adsorption
adsorbate particles and adsorbent are almost of the same strength as (ii) The relationship between the extent of adsorption and
chemical bonds, the adsorption is called chemical adsorption. This type of temperature at any constant pressure is called adsorption isobar.
adsorption is also called as chemisorption or Langmuir adsorption. This type (iii) A physical adsorption isobar shows a decrease in x/m (where ‘m’
of adsorption cannot be easily reversed. is the mass of the adsorbent and ‘x’ that of adsorbate) as the temperature
Comparison between physisorption and chemisorption rises.
Physisorption Chemisorption (iv) The isobar of chemisorption show an increase in the beginning
(Vander Waal's adsorption) (Langmuir adsorption) and then decrease as the temperature rises.
Low heat of adsorption usually in High heat of adsorption in the range of Adsorption isotherms
range of 20-40 kJ/mol 50-400 kJ/mol
Force of attraction are Vander Waal's Forces of attraction are chemical bond A mathematical equation, which describes the relationship between
forces. forces. pressure (p) of the gaseous adsorbate and the extent of adsorption at any
It is reversible It is irreversible fixed temperature, is called adsorption isotherms.
It is usually takes place at low It takes place at high temperature. The extent of adsorption is expressed as mass of the adsorbate
temperature and decreases with adsorbed on one unit mass of the adsorbent.
increasing temperature.
Thus, if x g of an adsorbate is adsorbed on m g of the adsorbent,
It is related to the case of liquefication It is not related.
of the gas. then
It forms multimolecular layers. It forms monomolecular layers. x
Extent of adsorption
It does not require any activation It requires high activation energy. m
energy.
Various adsorption isotherms are commonly employed in describing
High pressure is favourable. Decrease High pressure is favourable. Decrease
the adsorption data.
of pressure causes desorption of pressure does not cause desorption.
It is not very specific. It is highly specific. (1) Freundlich adsorption isotherm
(i) Freundlich adsorption isotherm is obeyed by the adsorptions
Factors which affect the extent of adsorption : The following are the where the adsorbate forms a monomolecular layer on the surface of the
factors which affect the adsorption, adsorbent.
(1) Nature of the adsorbate (gas) and adsorbent (solid)
