Concept & Key Learning Points - Surface Chemistry Class 12 Notes | EduRev

Created by: Ashutosh Sharma

Class 12 : Concept & Key Learning Points - Surface Chemistry Class 12 Notes | EduRev

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Class XII: Chemistry 
Chapter 5: Surface Chemistry 
Top Concepts / Key Learnings 
1. Adsorption: The accumulation of molecular species at the surface rather 
than in the bulk of a solid or liquid is termed as adsorption.  
 
2. Adsorbate: The substance which is being adsorbed on the surface of 
another substance is called adsorbate.  
 
3. Adsorbent: The substance present in bulk, on the surface of which 
adsorption is taking place is called adsorbent. 
 
4. Desorption: The process of removing an adsorbed substance from a 
surface on which it is adsorbed is called desorption. 
 
5. Absorption: It is the phenomenon in which a substance is uniformly 
distributed all over the surface. 
 
6. Difference between adsorption and absorption: 
Absorption Adsorption 
(i) It is the phenomenon in which a 
substance is uniformly 
distributed throughout the 
bulk of the solid. 
(i) The accumulation of molecular 
species at the surface rather 
than in the bulk of a solid or 
liquid is termed as 
adsorption. 
(ii) It is a bulk phenomenon. 
(ii) It is a surface phenomenon. 
(iii) The concentration is uniform 
throughout the bulk of solid. 
(iii) The concentration of 
adsorbate increases only at 
the surface of the adsorbent. 
 
7. Sorption: When adsorption and absorption take place simultaneously, it 
is called sorption.  
 
8. Enthalpy or heat of adsorption: Adsorption generally occurs with release 
in energy, i.e., it is exothermic in nature. The enthalpy change for the 
adsorption of one mole of an adsorbate on the surface of adsorbent is 
called enthalpy or heat of adsorption. 
 
9. Types of adsorption:  
a. Physical adsorption or physisorption: If the adsorbate is held on a 
surface of adsorbent by weak van der Waals’ forces, the 
adsorption is called physical adsorption or physisorption. 
b. Chemical adsorption or chemisorption: If the forces holding the 
adsorbate are as strong as in chemical bonds, the adsorption 
process is known as chemical adsorption of chemisorption.  
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Class XII: Chemistry 
Chapter 5: Surface Chemistry 
Top Concepts / Key Learnings 
1. Adsorption: The accumulation of molecular species at the surface rather 
than in the bulk of a solid or liquid is termed as adsorption.  
 
2. Adsorbate: The substance which is being adsorbed on the surface of 
another substance is called adsorbate.  
 
3. Adsorbent: The substance present in bulk, on the surface of which 
adsorption is taking place is called adsorbent. 
 
4. Desorption: The process of removing an adsorbed substance from a 
surface on which it is adsorbed is called desorption. 
 
5. Absorption: It is the phenomenon in which a substance is uniformly 
distributed all over the surface. 
 
6. Difference between adsorption and absorption: 
Absorption Adsorption 
(i) It is the phenomenon in which a 
substance is uniformly 
distributed throughout the 
bulk of the solid. 
(i) The accumulation of molecular 
species at the surface rather 
than in the bulk of a solid or 
liquid is termed as 
adsorption. 
(ii) It is a bulk phenomenon. 
(ii) It is a surface phenomenon. 
(iii) The concentration is uniform 
throughout the bulk of solid. 
(iii) The concentration of 
adsorbate increases only at 
the surface of the adsorbent. 
 
7. Sorption: When adsorption and absorption take place simultaneously, it 
is called sorption.  
 
8. Enthalpy or heat of adsorption: Adsorption generally occurs with release 
in energy, i.e., it is exothermic in nature. The enthalpy change for the 
adsorption of one mole of an adsorbate on the surface of adsorbent is 
called enthalpy or heat of adsorption. 
 
9. Types of adsorption:  
a. Physical adsorption or physisorption: If the adsorbate is held on a 
surface of adsorbent by weak van der Waals’ forces, the 
adsorption is called physical adsorption or physisorption. 
b. Chemical adsorption or chemisorption: If the forces holding the 
adsorbate are as strong as in chemical bonds, the adsorption 
process is known as chemical adsorption of chemisorption.  
 
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10. Difference between Physical and chemical adsorption: 
Physical adsorption 
(Physisorption) 
Chemical adsorption 
(Chemisorption) 
(i) It is non-specific (i) It is highly specific 
(ii) It is reversible (ii) It is irreversible  
(iii) The amount of gas depends upon 
nature of gas, i.e., easily 
liquefiable gases like NH
3
, CO
2
, 
gas adsorbed to greater extent 
than H
2
 and He. Higher the critical 
temperature of gas, more will be 
the extent of adsorption. 
(iii) The amount of gas adsorbed is not 
related to critical temperature of 
the gas. 
(iv) The extent of adsorption increases 
with increase in surface area, e.g. 
porous and finely divided metals 
are good adsorbents. 
(iv) It also increases with increase in 
surface area. 
(v) There are weak van der Waals’ 
forces of attraction between 
adsorbate and adsorbent. 
(v) There is strong force of attraction 
similar to chemical bond. 
(vi) It has low enthalpy of adsorption 
(20 – 40 kJ mol
-1
) 
(vi) It has enthalpy heat of adsorption 
(180 – 240 kJ mol
-1
) 
(vii) Low temperature is favourable. (vii) High temperature is favourable. 
(viii)  No appreciable activation energy 
is needed.  
(viii) High activation energy is 
sometimes needed.  
(ix) It forms multimolecular layers. (ix) It forms unimolecuar layers. 
 
11. Factors affecting adsorption of gases on solids: 
a. Nature of adsorbate: Physical adsorption is non-specific in nature and 
therefore every gas gets adsorbed on the surface of any solid to a 
lesser or greater extent. However, easily liquefiable gases like NH
3
. 
HCl, CO
2
, etc. which have higher critical temperatures are absorbed 
to greater extent whereas H
2
, O
2
, N
2
 etc. are adsorbed to lesser 
extent. The chemical adsorption being highly specific, therefore, a 
gas gets adsorbed on specific solid only if it enters into chemical 
combination with it. 
b. Nature of adsorbent: Activated carbon, metal oxides like aluminum 
oxide, silica gel and clay are commonly used adsorbents. They have 
their specific adsorption properties depending upon pores. 
c. Specific area of the adsorbent: The greater the specific area, more 
will be the extent of adsorption. That is why porous or finely divided 
forms of adsorbents adsorb larger quantities of adsorbate. The pores 
should be large enough to allow the gas molecules to enter. 
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1 
Class XII: Chemistry 
Chapter 5: Surface Chemistry 
Top Concepts / Key Learnings 
1. Adsorption: The accumulation of molecular species at the surface rather 
than in the bulk of a solid or liquid is termed as adsorption.  
 
2. Adsorbate: The substance which is being adsorbed on the surface of 
another substance is called adsorbate.  
 
3. Adsorbent: The substance present in bulk, on the surface of which 
adsorption is taking place is called adsorbent. 
 
4. Desorption: The process of removing an adsorbed substance from a 
surface on which it is adsorbed is called desorption. 
 
5. Absorption: It is the phenomenon in which a substance is uniformly 
distributed all over the surface. 
 
6. Difference between adsorption and absorption: 
Absorption Adsorption 
(i) It is the phenomenon in which a 
substance is uniformly 
distributed throughout the 
bulk of the solid. 
(i) The accumulation of molecular 
species at the surface rather 
than in the bulk of a solid or 
liquid is termed as 
adsorption. 
(ii) It is a bulk phenomenon. 
(ii) It is a surface phenomenon. 
(iii) The concentration is uniform 
throughout the bulk of solid. 
(iii) The concentration of 
adsorbate increases only at 
the surface of the adsorbent. 
 
7. Sorption: When adsorption and absorption take place simultaneously, it 
is called sorption.  
 
8. Enthalpy or heat of adsorption: Adsorption generally occurs with release 
in energy, i.e., it is exothermic in nature. The enthalpy change for the 
adsorption of one mole of an adsorbate on the surface of adsorbent is 
called enthalpy or heat of adsorption. 
 
9. Types of adsorption:  
a. Physical adsorption or physisorption: If the adsorbate is held on a 
surface of adsorbent by weak van der Waals’ forces, the 
adsorption is called physical adsorption or physisorption. 
b. Chemical adsorption or chemisorption: If the forces holding the 
adsorbate are as strong as in chemical bonds, the adsorption 
process is known as chemical adsorption of chemisorption.  
 
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2 
 
10. Difference between Physical and chemical adsorption: 
Physical adsorption 
(Physisorption) 
Chemical adsorption 
(Chemisorption) 
(i) It is non-specific (i) It is highly specific 
(ii) It is reversible (ii) It is irreversible  
(iii) The amount of gas depends upon 
nature of gas, i.e., easily 
liquefiable gases like NH
3
, CO
2
, 
gas adsorbed to greater extent 
than H
2
 and He. Higher the critical 
temperature of gas, more will be 
the extent of adsorption. 
(iii) The amount of gas adsorbed is not 
related to critical temperature of 
the gas. 
(iv) The extent of adsorption increases 
with increase in surface area, e.g. 
porous and finely divided metals 
are good adsorbents. 
(iv) It also increases with increase in 
surface area. 
(v) There are weak van der Waals’ 
forces of attraction between 
adsorbate and adsorbent. 
(v) There is strong force of attraction 
similar to chemical bond. 
(vi) It has low enthalpy of adsorption 
(20 – 40 kJ mol
-1
) 
(vi) It has enthalpy heat of adsorption 
(180 – 240 kJ mol
-1
) 
(vii) Low temperature is favourable. (vii) High temperature is favourable. 
(viii)  No appreciable activation energy 
is needed.  
(viii) High activation energy is 
sometimes needed.  
(ix) It forms multimolecular layers. (ix) It forms unimolecuar layers. 
 
11. Factors affecting adsorption of gases on solids: 
a. Nature of adsorbate: Physical adsorption is non-specific in nature and 
therefore every gas gets adsorbed on the surface of any solid to a 
lesser or greater extent. However, easily liquefiable gases like NH
3
. 
HCl, CO
2
, etc. which have higher critical temperatures are absorbed 
to greater extent whereas H
2
, O
2
, N
2
 etc. are adsorbed to lesser 
extent. The chemical adsorption being highly specific, therefore, a 
gas gets adsorbed on specific solid only if it enters into chemical 
combination with it. 
b. Nature of adsorbent: Activated carbon, metal oxides like aluminum 
oxide, silica gel and clay are commonly used adsorbents. They have 
their specific adsorption properties depending upon pores. 
c. Specific area of the adsorbent: The greater the specific area, more 
will be the extent of adsorption. That is why porous or finely divided 
forms of adsorbents adsorb larger quantities of adsorbate. The pores 
should be large enough to allow the gas molecules to enter. 
 
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d. Pressure of the gas: Physical adsorption increases with increase in 
pressure.  
 
12. Adsorption isotherm: The variation in the amount of gas adsorbed by the 
adsorbent with pressure at constant temperature can be expressed by 
means of a curve is termed as adsorption isotherm.  
 
13. Freundlich Adsorption isotherm: The relationship between 
x
m
 and 
pressure of the gas at constant temperature is called adsorption isotherm 
and is given by  
1/n
x
kP (n 1)
m
= > 
 Where x- mass of the gas adsorbed on mass m of the adsorbent and the 
gas at a particular temperature 
          k and n depends upon the nature of gas and the solid 
        
x
m
first increases with increase in pressure at low pressure but becomes 
independent of pressure at high pressure. 
 
 
 Taking logarithm on both sides, we get, 
 log
x
m
 = log k + 
1
n
logP 
 If we plot a graph between log 
x
m
 and log P, we get a straight line.  
 
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1 
Class XII: Chemistry 
Chapter 5: Surface Chemistry 
Top Concepts / Key Learnings 
1. Adsorption: The accumulation of molecular species at the surface rather 
than in the bulk of a solid or liquid is termed as adsorption.  
 
2. Adsorbate: The substance which is being adsorbed on the surface of 
another substance is called adsorbate.  
 
3. Adsorbent: The substance present in bulk, on the surface of which 
adsorption is taking place is called adsorbent. 
 
4. Desorption: The process of removing an adsorbed substance from a 
surface on which it is adsorbed is called desorption. 
 
5. Absorption: It is the phenomenon in which a substance is uniformly 
distributed all over the surface. 
 
6. Difference between adsorption and absorption: 
Absorption Adsorption 
(i) It is the phenomenon in which a 
substance is uniformly 
distributed throughout the 
bulk of the solid. 
(i) The accumulation of molecular 
species at the surface rather 
than in the bulk of a solid or 
liquid is termed as 
adsorption. 
(ii) It is a bulk phenomenon. 
(ii) It is a surface phenomenon. 
(iii) The concentration is uniform 
throughout the bulk of solid. 
(iii) The concentration of 
adsorbate increases only at 
the surface of the adsorbent. 
 
7. Sorption: When adsorption and absorption take place simultaneously, it 
is called sorption.  
 
8. Enthalpy or heat of adsorption: Adsorption generally occurs with release 
in energy, i.e., it is exothermic in nature. The enthalpy change for the 
adsorption of one mole of an adsorbate on the surface of adsorbent is 
called enthalpy or heat of adsorption. 
 
9. Types of adsorption:  
a. Physical adsorption or physisorption: If the adsorbate is held on a 
surface of adsorbent by weak van der Waals’ forces, the 
adsorption is called physical adsorption or physisorption. 
b. Chemical adsorption or chemisorption: If the forces holding the 
adsorbate are as strong as in chemical bonds, the adsorption 
process is known as chemical adsorption of chemisorption.  
 
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2 
 
10. Difference between Physical and chemical adsorption: 
Physical adsorption 
(Physisorption) 
Chemical adsorption 
(Chemisorption) 
(i) It is non-specific (i) It is highly specific 
(ii) It is reversible (ii) It is irreversible  
(iii) The amount of gas depends upon 
nature of gas, i.e., easily 
liquefiable gases like NH
3
, CO
2
, 
gas adsorbed to greater extent 
than H
2
 and He. Higher the critical 
temperature of gas, more will be 
the extent of adsorption. 
(iii) The amount of gas adsorbed is not 
related to critical temperature of 
the gas. 
(iv) The extent of adsorption increases 
with increase in surface area, e.g. 
porous and finely divided metals 
are good adsorbents. 
(iv) It also increases with increase in 
surface area. 
(v) There are weak van der Waals’ 
forces of attraction between 
adsorbate and adsorbent. 
(v) There is strong force of attraction 
similar to chemical bond. 
(vi) It has low enthalpy of adsorption 
(20 – 40 kJ mol
-1
) 
(vi) It has enthalpy heat of adsorption 
(180 – 240 kJ mol
-1
) 
(vii) Low temperature is favourable. (vii) High temperature is favourable. 
(viii)  No appreciable activation energy 
is needed.  
(viii) High activation energy is 
sometimes needed.  
(ix) It forms multimolecular layers. (ix) It forms unimolecuar layers. 
 
11. Factors affecting adsorption of gases on solids: 
a. Nature of adsorbate: Physical adsorption is non-specific in nature and 
therefore every gas gets adsorbed on the surface of any solid to a 
lesser or greater extent. However, easily liquefiable gases like NH
3
. 
HCl, CO
2
, etc. which have higher critical temperatures are absorbed 
to greater extent whereas H
2
, O
2
, N
2
 etc. are adsorbed to lesser 
extent. The chemical adsorption being highly specific, therefore, a 
gas gets adsorbed on specific solid only if it enters into chemical 
combination with it. 
b. Nature of adsorbent: Activated carbon, metal oxides like aluminum 
oxide, silica gel and clay are commonly used adsorbents. They have 
their specific adsorption properties depending upon pores. 
c. Specific area of the adsorbent: The greater the specific area, more 
will be the extent of adsorption. That is why porous or finely divided 
forms of adsorbents adsorb larger quantities of adsorbate. The pores 
should be large enough to allow the gas molecules to enter. 
 
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d. Pressure of the gas: Physical adsorption increases with increase in 
pressure.  
 
12. Adsorption isotherm: The variation in the amount of gas adsorbed by the 
adsorbent with pressure at constant temperature can be expressed by 
means of a curve is termed as adsorption isotherm.  
 
13. Freundlich Adsorption isotherm: The relationship between 
x
m
 and 
pressure of the gas at constant temperature is called adsorption isotherm 
and is given by  
1/n
x
kP (n 1)
m
= > 
 Where x- mass of the gas adsorbed on mass m of the adsorbent and the 
gas at a particular temperature 
          k and n depends upon the nature of gas and the solid 
        
x
m
first increases with increase in pressure at low pressure but becomes 
independent of pressure at high pressure. 
 
 
 Taking logarithm on both sides, we get, 
 log
x
m
 = log k + 
1
n
logP 
 If we plot a graph between log 
x
m
 and log P, we get a straight line.  
 
 
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          The slope of the line is 
1
n
 and intercept will be equal to log k.  
 
14. Catalyst: Substances which alter the rate of a chemical reaction and 
themselves remain chemically and quantitatively unchanged after the 
reaction are known as catalysts and the phenomenon is known as 
catalysis.  
 
15. Promoters: Those substances which increase the activity of catalyst are 
called promoters. Example: Mo is promoter whereas Fe is catalyst in 
Haber’s Process. 
N
2
 (g) + 3H
2
 (g)
Fe(s)
Mo(s)


 2NH
3
 (g) 
 
16. Catalytic poisons (Inhibitors): The substances which decrease the activity 
of catalyst are called catalytic poisons or inhibitors e.g., arsenic acts as 
catalytic poison in the manufacture of sulphuric acid by ‘contact process’.  
 
17. Types of catalysis: 
a. Homogeneous catalysis: When the catalyst and the reactants are in 
the same phase, this kind of catalytic process is known as 
homogeneous catalysis. 
b. Heterogeneous catalysis: When the catalyst and the reactants are in 
different phases, the catalytic process is said to be heterogeneous 
catalysis. 
 
18. Activity of catalyst: The ability of a catalyst to increase the rate of a 
chemical reaction is called activity of a catalyst. 
 
19. Selectivity of catalyst: It is the ability of catalyst to direct a reaction to 
yield a particular product (excluding others). For example: 
CO and H
2
 react to form different products in presence of different 
catalysts as follows: 
 a) 
Ni
2 4 2
CO(g) 3H (g) CH (g) H O(g) + ??? ? + 
 b) 
Cu/ZnO Cr O
2 3
2 3
CO(g) 2H (g) CH OH(g)
-
+ ???????? ? 
 c) 
Cu
2
CO(g) H (g) HCHO(g) + ???? 
 
20. Shape–selective catalysis: The catalysis which depends upon the pore 
structure of the catalyst and molecular size of reactant and product 
molecules is called shape- selective catalysis. E.g. Zeolites are shape–
selective catalysts due to their honey- comb structure.  
 
21. Enzymes: Enzymes are complex nitrogenous organic compounds which 
are produced by living plants and animals. They are actually protein 
molecules of high molecular mass. They are biochemical catalysts. 
 
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Class XII: Chemistry 
Chapter 5: Surface Chemistry 
Top Concepts / Key Learnings 
1. Adsorption: The accumulation of molecular species at the surface rather 
than in the bulk of a solid or liquid is termed as adsorption.  
 
2. Adsorbate: The substance which is being adsorbed on the surface of 
another substance is called adsorbate.  
 
3. Adsorbent: The substance present in bulk, on the surface of which 
adsorption is taking place is called adsorbent. 
 
4. Desorption: The process of removing an adsorbed substance from a 
surface on which it is adsorbed is called desorption. 
 
5. Absorption: It is the phenomenon in which a substance is uniformly 
distributed all over the surface. 
 
6. Difference between adsorption and absorption: 
Absorption Adsorption 
(i) It is the phenomenon in which a 
substance is uniformly 
distributed throughout the 
bulk of the solid. 
(i) The accumulation of molecular 
species at the surface rather 
than in the bulk of a solid or 
liquid is termed as 
adsorption. 
(ii) It is a bulk phenomenon. 
(ii) It is a surface phenomenon. 
(iii) The concentration is uniform 
throughout the bulk of solid. 
(iii) The concentration of 
adsorbate increases only at 
the surface of the adsorbent. 
 
7. Sorption: When adsorption and absorption take place simultaneously, it 
is called sorption.  
 
8. Enthalpy or heat of adsorption: Adsorption generally occurs with release 
in energy, i.e., it is exothermic in nature. The enthalpy change for the 
adsorption of one mole of an adsorbate on the surface of adsorbent is 
called enthalpy or heat of adsorption. 
 
9. Types of adsorption:  
a. Physical adsorption or physisorption: If the adsorbate is held on a 
surface of adsorbent by weak van der Waals’ forces, the 
adsorption is called physical adsorption or physisorption. 
b. Chemical adsorption or chemisorption: If the forces holding the 
adsorbate are as strong as in chemical bonds, the adsorption 
process is known as chemical adsorption of chemisorption.  
 
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10. Difference between Physical and chemical adsorption: 
Physical adsorption 
(Physisorption) 
Chemical adsorption 
(Chemisorption) 
(i) It is non-specific (i) It is highly specific 
(ii) It is reversible (ii) It is irreversible  
(iii) The amount of gas depends upon 
nature of gas, i.e., easily 
liquefiable gases like NH
3
, CO
2
, 
gas adsorbed to greater extent 
than H
2
 and He. Higher the critical 
temperature of gas, more will be 
the extent of adsorption. 
(iii) The amount of gas adsorbed is not 
related to critical temperature of 
the gas. 
(iv) The extent of adsorption increases 
with increase in surface area, e.g. 
porous and finely divided metals 
are good adsorbents. 
(iv) It also increases with increase in 
surface area. 
(v) There are weak van der Waals’ 
forces of attraction between 
adsorbate and adsorbent. 
(v) There is strong force of attraction 
similar to chemical bond. 
(vi) It has low enthalpy of adsorption 
(20 – 40 kJ mol
-1
) 
(vi) It has enthalpy heat of adsorption 
(180 – 240 kJ mol
-1
) 
(vii) Low temperature is favourable. (vii) High temperature is favourable. 
(viii)  No appreciable activation energy 
is needed.  
(viii) High activation energy is 
sometimes needed.  
(ix) It forms multimolecular layers. (ix) It forms unimolecuar layers. 
 
11. Factors affecting adsorption of gases on solids: 
a. Nature of adsorbate: Physical adsorption is non-specific in nature and 
therefore every gas gets adsorbed on the surface of any solid to a 
lesser or greater extent. However, easily liquefiable gases like NH
3
. 
HCl, CO
2
, etc. which have higher critical temperatures are absorbed 
to greater extent whereas H
2
, O
2
, N
2
 etc. are adsorbed to lesser 
extent. The chemical adsorption being highly specific, therefore, a 
gas gets adsorbed on specific solid only if it enters into chemical 
combination with it. 
b. Nature of adsorbent: Activated carbon, metal oxides like aluminum 
oxide, silica gel and clay are commonly used adsorbents. They have 
their specific adsorption properties depending upon pores. 
c. Specific area of the adsorbent: The greater the specific area, more 
will be the extent of adsorption. That is why porous or finely divided 
forms of adsorbents adsorb larger quantities of adsorbate. The pores 
should be large enough to allow the gas molecules to enter. 
 
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d. Pressure of the gas: Physical adsorption increases with increase in 
pressure.  
 
12. Adsorption isotherm: The variation in the amount of gas adsorbed by the 
adsorbent with pressure at constant temperature can be expressed by 
means of a curve is termed as adsorption isotherm.  
 
13. Freundlich Adsorption isotherm: The relationship between 
x
m
 and 
pressure of the gas at constant temperature is called adsorption isotherm 
and is given by  
1/n
x
kP (n 1)
m
= > 
 Where x- mass of the gas adsorbed on mass m of the adsorbent and the 
gas at a particular temperature 
          k and n depends upon the nature of gas and the solid 
        
x
m
first increases with increase in pressure at low pressure but becomes 
independent of pressure at high pressure. 
 
 
 Taking logarithm on both sides, we get, 
 log
x
m
 = log k + 
1
n
logP 
 If we plot a graph between log 
x
m
 and log P, we get a straight line.  
 
 
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          The slope of the line is 
1
n
 and intercept will be equal to log k.  
 
14. Catalyst: Substances which alter the rate of a chemical reaction and 
themselves remain chemically and quantitatively unchanged after the 
reaction are known as catalysts and the phenomenon is known as 
catalysis.  
 
15. Promoters: Those substances which increase the activity of catalyst are 
called promoters. Example: Mo is promoter whereas Fe is catalyst in 
Haber’s Process. 
N
2
 (g) + 3H
2
 (g)
Fe(s)
Mo(s)


 2NH
3
 (g) 
 
16. Catalytic poisons (Inhibitors): The substances which decrease the activity 
of catalyst are called catalytic poisons or inhibitors e.g., arsenic acts as 
catalytic poison in the manufacture of sulphuric acid by ‘contact process’.  
 
17. Types of catalysis: 
a. Homogeneous catalysis: When the catalyst and the reactants are in 
the same phase, this kind of catalytic process is known as 
homogeneous catalysis. 
b. Heterogeneous catalysis: When the catalyst and the reactants are in 
different phases, the catalytic process is said to be heterogeneous 
catalysis. 
 
18. Activity of catalyst: The ability of a catalyst to increase the rate of a 
chemical reaction is called activity of a catalyst. 
 
19. Selectivity of catalyst: It is the ability of catalyst to direct a reaction to 
yield a particular product (excluding others). For example: 
CO and H
2
 react to form different products in presence of different 
catalysts as follows: 
 a) 
Ni
2 4 2
CO(g) 3H (g) CH (g) H O(g) + ??? ? + 
 b) 
Cu/ZnO Cr O
2 3
2 3
CO(g) 2H (g) CH OH(g)
-
+ ???????? ? 
 c) 
Cu
2
CO(g) H (g) HCHO(g) + ???? 
 
20. Shape–selective catalysis: The catalysis which depends upon the pore 
structure of the catalyst and molecular size of reactant and product 
molecules is called shape- selective catalysis. E.g. Zeolites are shape–
selective catalysts due to their honey- comb structure.  
 
21. Enzymes: Enzymes are complex nitrogenous organic compounds which 
are produced by living plants and animals. They are actually protein 
molecules of high molecular mass. They are biochemical catalysts. 
 
 
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22. Steps of enzyme catalysis:  
(i) Binding of enzyme to substrate to form an activated complex 
E + S 
 
 
 ES
?
  
ii) Decomposition of the activated complex to form product 
E S
?
 ? E + P 
 
23. Characteristics of enzyme catalysis: 
i) They are highly efficient. One molecule of an enzyme can transform 
10
6
 molecules of reactants per minute. 
ii) They are highly specific in nature, e.g., urease catalysis hydrolysis 
of urea only. 
iii) They are active at optimum temperature (298 – 310 K). The rate of 
enzyme catalysed reaction becomes maximum at a definite 
temperature called the optimum temperature.   
iv) They are highly active at a specific pH called optimum pH.  
v) Enzymatic activity can be increased in presence of coenzymes 
which can be called as promoters. 
vi) Activators are generally metal ions Na
+
, Co
2+
 and Cu
2+
 etc. They 
weakly bind to enzyme and increase its activity.  
vii) Influence of inhibitors (poison): Enzymes can also be inhibited or 
poisoned by the presence of certain substances. 
 
24. Distinction between true solution, colloids and Suspension. 
True solution Colloids Suspension 
It is homogeneous 
It appears to be 
homogeneous but is 
actually heterogeneous  
It is heterogeneous 
The diameter of the 
particles is less than 1 
nm 
The diameter of the 
particles is 1 nm to 
1000 nm 
The diameter of the 
particles are larger that 
1000 nm 
It passes through filter 
paper 
It passes through 
ordinary filter paper but 
not through ultra-filters 
It does not pass through 
filter paper 
Its particles cannot be 
seen under a 
microscope 
Its particles can be seen 
by a powerful 
microscope due to 
scattering of light 
Its particles can be seen 
even with naked eye 
 
25. Colloids: A colloid is a heterogeneous system in which one substance is 
dispersed as very fine particles in another substance called dispersed 
medium. 
 
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