NCERT Textbook: Equilibrium | Chemistry Class 11 - NEET PDF Download

 Page 1


Unit 6
Eq Uilibri Um
Chemical equilibria are important in numerous biological 
and environmental processes. For example, equilibria 
involving O
2
 molecules and the protein hemoglobin play 
a crucial role in the transport and delivery of O
2
 from 
our lungs to our muscles. Similar equilibria involving CO 
molecules and hemoglobin account for the toxicity of CO.
When a liquid evaporates in a closed container, 
molecules with relatively higher kinetic energy escape 
the liquid surface into the vapour phase and number of 
liquid molecules from the vapour phase strike the liquid 
surface and are retained in the liquid phase. It gives rise 
to a constant vapour pressure because of an equilibrium in 
which the number of molecules leaving the liquid equals the 
number returning to liquid from the vapour. We say that 
the system has reached equilibrium state at this stage. 
However, this is not static equilibrium and there is a lot of 
activity at the boundary between the liquid and the vapour. 
Thus, at equilibrium, the rate of evaporation is equal to the 
rate of condensation. It may be represented by
H
2
O (l)   H
2
O (vap)
The double half arrows indicate that the processes 
in both the directions are going on simultaneously. The 
mixture of reactants and products in the equilibrium state 
is called an equilibrium mixture.
Equilibrium can be established for both physical 
processes and chemical reactions. The reaction may be 
fast or slow depending on the experimental conditions and 
the nature of the reactants. When the reactants in a closed 
vessel at a particular temperature react to give products, 
the concentrations of the reactants keep on decreasing, 
while those of products keep on increasing for some time 
after which there is no change in the concentrations 
of either of the reactants or products. This stage of the 
system is the dynamic equilibrium and the rates of the 
forward and reverse reactions become equal. It is due to 
After studying this unit you will be 
able to 
• identify dynamic nature of  
equilibrium involved in physical 
and chemical processes;
• state the law of equilibrium; 
• explain characteristics of 
equilibria involved in physical 
and chemical processes;
• write expressions for equilibrium 
constants; 
• establish a relationship between 
K
p
 and K
c
;
• explain various factors that 
affect the equilibrium state of a 
reaction;
• classify substances as acids or 
bases according to Arrhenius, 
Bronsted-Lowry and Lewis 
concepts;
• classify acids and bases as 
weak or strong in terms of their 
ionization constants; 
• explain the dependence of degree 
of ionization on concentration 
of the electrolyte and that of the 
common ion;
• describe pH scale for representing 
hydrogen ion concentration;
• explain ionisation of water and 
its duel role as acid and base;
• describe ionic product (K
w 
) and 
pK
w 
 for water;
• appreciate use of buffer 
solutions;
• calculate solubility product 
constant.
Unit 6.indd   168 9/12/2022   11:58:20 AM
2024-25
Page 2


Unit 6
Eq Uilibri Um
Chemical equilibria are important in numerous biological 
and environmental processes. For example, equilibria 
involving O
2
 molecules and the protein hemoglobin play 
a crucial role in the transport and delivery of O
2
 from 
our lungs to our muscles. Similar equilibria involving CO 
molecules and hemoglobin account for the toxicity of CO.
When a liquid evaporates in a closed container, 
molecules with relatively higher kinetic energy escape 
the liquid surface into the vapour phase and number of 
liquid molecules from the vapour phase strike the liquid 
surface and are retained in the liquid phase. It gives rise 
to a constant vapour pressure because of an equilibrium in 
which the number of molecules leaving the liquid equals the 
number returning to liquid from the vapour. We say that 
the system has reached equilibrium state at this stage. 
However, this is not static equilibrium and there is a lot of 
activity at the boundary between the liquid and the vapour. 
Thus, at equilibrium, the rate of evaporation is equal to the 
rate of condensation. It may be represented by
H
2
O (l)   H
2
O (vap)
The double half arrows indicate that the processes 
in both the directions are going on simultaneously. The 
mixture of reactants and products in the equilibrium state 
is called an equilibrium mixture.
Equilibrium can be established for both physical 
processes and chemical reactions. The reaction may be 
fast or slow depending on the experimental conditions and 
the nature of the reactants. When the reactants in a closed 
vessel at a particular temperature react to give products, 
the concentrations of the reactants keep on decreasing, 
while those of products keep on increasing for some time 
after which there is no change in the concentrations 
of either of the reactants or products. This stage of the 
system is the dynamic equilibrium and the rates of the 
forward and reverse reactions become equal. It is due to 
After studying this unit you will be 
able to 
• identify dynamic nature of  
equilibrium involved in physical 
and chemical processes;
• state the law of equilibrium; 
• explain characteristics of 
equilibria involved in physical 
and chemical processes;
• write expressions for equilibrium 
constants; 
• establish a relationship between 
K
p
 and K
c
;
• explain various factors that 
affect the equilibrium state of a 
reaction;
• classify substances as acids or 
bases according to Arrhenius, 
Bronsted-Lowry and Lewis 
concepts;
• classify acids and bases as 
weak or strong in terms of their 
ionization constants; 
• explain the dependence of degree 
of ionization on concentration 
of the electrolyte and that of the 
common ion;
• describe pH scale for representing 
hydrogen ion concentration;
• explain ionisation of water and 
its duel role as acid and base;
• describe ionic product (K
w 
) and 
pK
w 
 for water;
• appreciate use of buffer 
solutions;
• calculate solubility product 
constant.
Unit 6.indd   168 9/12/2022   11:58:20 AM
2024-25
169 EQUILIBRIUM
this dynamic equilibrium stage that there is 
no change in the concentrations of various 
species in the reaction mixture. Based on the 
extent to which the reactions proceed to reach 
the state of chemical equilibrium, these may 
be classified in three groups .
(i) The reactions that proceed nearly 
to completion and only negligible 
concentrations of the reactants are 
left. In some cases, it may not be even 
possible to detect these experimentally.
(ii) The reactions in which only small 
amounts of products are formed and 
most of the reactants remain unchanged 
at equilibrium stage.
(iii) The reactions in which the concentrations 
of the reactants and products are 
comparable, when the system is in 
equilibrium.
The extent of a reaction in equilibrium 
varies with the experimental conditions such 
as concentrations of reactants, temperature, 
etc. Optimisation of the operational conditions 
is very important in industry and laboratory 
so that equilibrium is favorable in the 
direction of the desired product. Some 
important aspects of equilibrium involving 
physical and chemical processes are dealt in 
this unit along with the equilibrium involving 
ions in aqueous solutions which is called as 
ionic equilibrium.
6.1 EqUil ibri Um in PHYSiCAl 
Pr OCESSES
The characteristics of system at equilibrium 
are better understood if we examine some 
physical processes. The most familiar examples 
are phase transformation processes, e.g.,
 solid        liquid 
 liquid        gas
 solid         gas
6.1.1 Solid-l iquid Equilibrium
Ice and water kept in a perfectly insulated 
thermos flask (no exchange of heat between its 
contents and the surroundings) at 273K and 
the atmospheric pressure are in equilibrium 
state and the system shows interesting 
characteristic features. We observe that the 
mass of ice and water do not change with 
time and the temperature remains constant. 
However, the equilibrium is not static. 
The intense activity can be noticed at the 
boundary between ice and water. Molecules 
from the liquid water collide against ice and 
adhere to it and some molecules of ice escape 
into liquid phase. There is no change of mass 
of ice and water, as the rates of transfer of 
molecules from ice into water and of reverse 
transfer from water into ice are equal at 
atmospheric pressure and 273 K. 
It is obvious that ice and water are in 
equilibrium only at particular temperature 
and pressure. For any pure substance at 
atmospheric pressure, the temperature at 
which the solid and liquid phases are at 
equilibrium is called the normal melting point 
or normal freezing point of the substance.  The 
system here is in dynamic equilibrium and we 
can infer the following:
(i) Both the opposing processes occur 
simultaneously.
(ii) Both the processes occur at the same 
rate so that the amount of ice and water 
remains constant.
6.1.2 l iquid-Vapour Equilibrium
This equilibrium can be better understood if 
we consider the example of a transparent box 
carrying a U-tube with mercury (manometer). 
Drying agent like anhydrous calcium chloride 
(or phosphorus penta-oxide) is placed for 
a few hours in the box. After removing the 
drying agent by tilting the box on one side, a 
watch glass (or petri dish) containing water 
is quickly placed inside the box. It will be 
observed that the mercury level in the right 
limb of the manometer slowly increases and 
finally attains a constant value, that is, the 
pressure inside the box increases and reaches 
a constant value. Also the volume of water in 
the watch glass decreases (Fig. 6.1). Initially 
there was no water vapour (or very less) inside 
the box. As water evaporated the pressure in 
the box increased due to addition of water 
molecules into the gaseous phase inside 
the box. The rate of evaporation is constant. 
Unit 6.indd   169 9/12/2022   11:58:20 AM
2024-25
Page 3


Unit 6
Eq Uilibri Um
Chemical equilibria are important in numerous biological 
and environmental processes. For example, equilibria 
involving O
2
 molecules and the protein hemoglobin play 
a crucial role in the transport and delivery of O
2
 from 
our lungs to our muscles. Similar equilibria involving CO 
molecules and hemoglobin account for the toxicity of CO.
When a liquid evaporates in a closed container, 
molecules with relatively higher kinetic energy escape 
the liquid surface into the vapour phase and number of 
liquid molecules from the vapour phase strike the liquid 
surface and are retained in the liquid phase. It gives rise 
to a constant vapour pressure because of an equilibrium in 
which the number of molecules leaving the liquid equals the 
number returning to liquid from the vapour. We say that 
the system has reached equilibrium state at this stage. 
However, this is not static equilibrium and there is a lot of 
activity at the boundary between the liquid and the vapour. 
Thus, at equilibrium, the rate of evaporation is equal to the 
rate of condensation. It may be represented by
H
2
O (l)   H
2
O (vap)
The double half arrows indicate that the processes 
in both the directions are going on simultaneously. The 
mixture of reactants and products in the equilibrium state 
is called an equilibrium mixture.
Equilibrium can be established for both physical 
processes and chemical reactions. The reaction may be 
fast or slow depending on the experimental conditions and 
the nature of the reactants. When the reactants in a closed 
vessel at a particular temperature react to give products, 
the concentrations of the reactants keep on decreasing, 
while those of products keep on increasing for some time 
after which there is no change in the concentrations 
of either of the reactants or products. This stage of the 
system is the dynamic equilibrium and the rates of the 
forward and reverse reactions become equal. It is due to 
After studying this unit you will be 
able to 
• identify dynamic nature of  
equilibrium involved in physical 
and chemical processes;
• state the law of equilibrium; 
• explain characteristics of 
equilibria involved in physical 
and chemical processes;
• write expressions for equilibrium 
constants; 
• establish a relationship between 
K
p
 and K
c
;
• explain various factors that 
affect the equilibrium state of a 
reaction;
• classify substances as acids or 
bases according to Arrhenius, 
Bronsted-Lowry and Lewis 
concepts;
• classify acids and bases as 
weak or strong in terms of their 
ionization constants; 
• explain the dependence of degree 
of ionization on concentration 
of the electrolyte and that of the 
common ion;
• describe pH scale for representing 
hydrogen ion concentration;
• explain ionisation of water and 
its duel role as acid and base;
• describe ionic product (K
w 
) and 
pK
w 
 for water;
• appreciate use of buffer 
solutions;
• calculate solubility product 
constant.
Unit 6.indd   168 9/12/2022   11:58:20 AM
2024-25
169 EQUILIBRIUM
this dynamic equilibrium stage that there is 
no change in the concentrations of various 
species in the reaction mixture. Based on the 
extent to which the reactions proceed to reach 
the state of chemical equilibrium, these may 
be classified in three groups .
(i) The reactions that proceed nearly 
to completion and only negligible 
concentrations of the reactants are 
left. In some cases, it may not be even 
possible to detect these experimentally.
(ii) The reactions in which only small 
amounts of products are formed and 
most of the reactants remain unchanged 
at equilibrium stage.
(iii) The reactions in which the concentrations 
of the reactants and products are 
comparable, when the system is in 
equilibrium.
The extent of a reaction in equilibrium 
varies with the experimental conditions such 
as concentrations of reactants, temperature, 
etc. Optimisation of the operational conditions 
is very important in industry and laboratory 
so that equilibrium is favorable in the 
direction of the desired product. Some 
important aspects of equilibrium involving 
physical and chemical processes are dealt in 
this unit along with the equilibrium involving 
ions in aqueous solutions which is called as 
ionic equilibrium.
6.1 EqUil ibri Um in PHYSiCAl 
Pr OCESSES
The characteristics of system at equilibrium 
are better understood if we examine some 
physical processes. The most familiar examples 
are phase transformation processes, e.g.,
 solid        liquid 
 liquid        gas
 solid         gas
6.1.1 Solid-l iquid Equilibrium
Ice and water kept in a perfectly insulated 
thermos flask (no exchange of heat between its 
contents and the surroundings) at 273K and 
the atmospheric pressure are in equilibrium 
state and the system shows interesting 
characteristic features. We observe that the 
mass of ice and water do not change with 
time and the temperature remains constant. 
However, the equilibrium is not static. 
The intense activity can be noticed at the 
boundary between ice and water. Molecules 
from the liquid water collide against ice and 
adhere to it and some molecules of ice escape 
into liquid phase. There is no change of mass 
of ice and water, as the rates of transfer of 
molecules from ice into water and of reverse 
transfer from water into ice are equal at 
atmospheric pressure and 273 K. 
It is obvious that ice and water are in 
equilibrium only at particular temperature 
and pressure. For any pure substance at 
atmospheric pressure, the temperature at 
which the solid and liquid phases are at 
equilibrium is called the normal melting point 
or normal freezing point of the substance.  The 
system here is in dynamic equilibrium and we 
can infer the following:
(i) Both the opposing processes occur 
simultaneously.
(ii) Both the processes occur at the same 
rate so that the amount of ice and water 
remains constant.
6.1.2 l iquid-Vapour Equilibrium
This equilibrium can be better understood if 
we consider the example of a transparent box 
carrying a U-tube with mercury (manometer). 
Drying agent like anhydrous calcium chloride 
(or phosphorus penta-oxide) is placed for 
a few hours in the box. After removing the 
drying agent by tilting the box on one side, a 
watch glass (or petri dish) containing water 
is quickly placed inside the box. It will be 
observed that the mercury level in the right 
limb of the manometer slowly increases and 
finally attains a constant value, that is, the 
pressure inside the box increases and reaches 
a constant value. Also the volume of water in 
the watch glass decreases (Fig. 6.1). Initially 
there was no water vapour (or very less) inside 
the box. As water evaporated the pressure in 
the box increased due to addition of water 
molecules into the gaseous phase inside 
the box. The rate of evaporation is constant. 
Unit 6.indd   169 9/12/2022   11:58:20 AM
2024-25
170 chemistry However, the rate of increase in pressure 
decreases with time due to condensation 
of vapour into water. Finally it leads to an 
equilibrium condition when there is no net 
evaporation. This implies that the number 
of water molecules from the gaseous state 
into the liquid state also increases till the 
equilibrium is attained i.e.,
rate of evaporation= rate of condensation
H
2
O(l)          H
2
O (vap)
At equilibrium the pressure exerted by 
the water molecules at a given temperature 
remains constant and is called the equilibrium 
vapour pressure of water (or just vapour 
pressure of water); vapour pressure of water 
increases with temperature. If the above 
experiment is repeated with methyl alcohol, 
acetone and ether, it is observed that different 
liquids have different equilibrium vapour 
pressures at the same temperature, and the 
liquid which has a higher vapour pressure is 
more volatile and has a lower boiling point. 
If we expose three watch glasses containing 
separately 1mL each of acetone, ethyl alcohol, 
and water to atmosphere and repeat the 
experiment with different volumes of the 
liquids in a warmer room, it is observed 
that in all such cases the liquid eventually 
disappears and the time taken for complete 
evaporation depends on (i) the nature of the 
liquid, (ii) the amount of the liquid and (iii) the 
temperature. When the watch glass is open 
to the atmosphere, the rate of evaporation 
remains constant but the molecules are 
dispersed into large volume of the room. As 
a consequence the rate of condensation from 
vapour to liquid state is much less than the 
rate of evaporation. These are open systems 
and it is not possible to reach equilibrium in 
an open system.
Water and water vapour are in equilibrium 
position at atmospheric pressure (1.013 bar) 
and at 100°C in a closed vessel. The boiling 
point of water is 100°C at 1.013 bar pressure. 
For any pure liquid at one atmospheric 
pressure (1.013 bar), the temperature 
at which the liquid and vapours are at 
equilibrium is called normal boiling point of 
the liquid. Boiling point of the liquid depends 
on the atmospheric pressure. It depends on 
the altitude of the place; at high altitude the 
boiling point decreases. 
6.1.3 Solid – Vapour Equilibrium
Let us now consider the systems where solids 
sublime to vapour phase. If we place solid 
iodine in a closed vessel, after sometime 
the vessel gets filled up with violet vapour 
and the intensity of colour increases with 
time. After certain time the intensity of 
colour becomes constant and at this stage 
equilibrium is attained. Hence solid iodine 
sublimes to give iodine vapour and the iodine 
vapour condenses to give solid iodine. The 
equilibrium can be represented as,
I
2
(solid)   I
2
 (vapour)
Other examples showing this kind of 
equilibrium are,
Camphor (solid)  Camphor (vapour)
NH
4
Cl (solid)   NH
4
Cl (vapour)
Fig. 6.1  Measuring equilibrium vapour pressure of water at a constant temperature
Unit 6.indd   170 9/12/2022   11:58:21 AM
2024-25
Page 4


Unit 6
Eq Uilibri Um
Chemical equilibria are important in numerous biological 
and environmental processes. For example, equilibria 
involving O
2
 molecules and the protein hemoglobin play 
a crucial role in the transport and delivery of O
2
 from 
our lungs to our muscles. Similar equilibria involving CO 
molecules and hemoglobin account for the toxicity of CO.
When a liquid evaporates in a closed container, 
molecules with relatively higher kinetic energy escape 
the liquid surface into the vapour phase and number of 
liquid molecules from the vapour phase strike the liquid 
surface and are retained in the liquid phase. It gives rise 
to a constant vapour pressure because of an equilibrium in 
which the number of molecules leaving the liquid equals the 
number returning to liquid from the vapour. We say that 
the system has reached equilibrium state at this stage. 
However, this is not static equilibrium and there is a lot of 
activity at the boundary between the liquid and the vapour. 
Thus, at equilibrium, the rate of evaporation is equal to the 
rate of condensation. It may be represented by
H
2
O (l)   H
2
O (vap)
The double half arrows indicate that the processes 
in both the directions are going on simultaneously. The 
mixture of reactants and products in the equilibrium state 
is called an equilibrium mixture.
Equilibrium can be established for both physical 
processes and chemical reactions. The reaction may be 
fast or slow depending on the experimental conditions and 
the nature of the reactants. When the reactants in a closed 
vessel at a particular temperature react to give products, 
the concentrations of the reactants keep on decreasing, 
while those of products keep on increasing for some time 
after which there is no change in the concentrations 
of either of the reactants or products. This stage of the 
system is the dynamic equilibrium and the rates of the 
forward and reverse reactions become equal. It is due to 
After studying this unit you will be 
able to 
• identify dynamic nature of  
equilibrium involved in physical 
and chemical processes;
• state the law of equilibrium; 
• explain characteristics of 
equilibria involved in physical 
and chemical processes;
• write expressions for equilibrium 
constants; 
• establish a relationship between 
K
p
 and K
c
;
• explain various factors that 
affect the equilibrium state of a 
reaction;
• classify substances as acids or 
bases according to Arrhenius, 
Bronsted-Lowry and Lewis 
concepts;
• classify acids and bases as 
weak or strong in terms of their 
ionization constants; 
• explain the dependence of degree 
of ionization on concentration 
of the electrolyte and that of the 
common ion;
• describe pH scale for representing 
hydrogen ion concentration;
• explain ionisation of water and 
its duel role as acid and base;
• describe ionic product (K
w 
) and 
pK
w 
 for water;
• appreciate use of buffer 
solutions;
• calculate solubility product 
constant.
Unit 6.indd   168 9/12/2022   11:58:20 AM
2024-25
169 EQUILIBRIUM
this dynamic equilibrium stage that there is 
no change in the concentrations of various 
species in the reaction mixture. Based on the 
extent to which the reactions proceed to reach 
the state of chemical equilibrium, these may 
be classified in three groups .
(i) The reactions that proceed nearly 
to completion and only negligible 
concentrations of the reactants are 
left. In some cases, it may not be even 
possible to detect these experimentally.
(ii) The reactions in which only small 
amounts of products are formed and 
most of the reactants remain unchanged 
at equilibrium stage.
(iii) The reactions in which the concentrations 
of the reactants and products are 
comparable, when the system is in 
equilibrium.
The extent of a reaction in equilibrium 
varies with the experimental conditions such 
as concentrations of reactants, temperature, 
etc. Optimisation of the operational conditions 
is very important in industry and laboratory 
so that equilibrium is favorable in the 
direction of the desired product. Some 
important aspects of equilibrium involving 
physical and chemical processes are dealt in 
this unit along with the equilibrium involving 
ions in aqueous solutions which is called as 
ionic equilibrium.
6.1 EqUil ibri Um in PHYSiCAl 
Pr OCESSES
The characteristics of system at equilibrium 
are better understood if we examine some 
physical processes. The most familiar examples 
are phase transformation processes, e.g.,
 solid        liquid 
 liquid        gas
 solid         gas
6.1.1 Solid-l iquid Equilibrium
Ice and water kept in a perfectly insulated 
thermos flask (no exchange of heat between its 
contents and the surroundings) at 273K and 
the atmospheric pressure are in equilibrium 
state and the system shows interesting 
characteristic features. We observe that the 
mass of ice and water do not change with 
time and the temperature remains constant. 
However, the equilibrium is not static. 
The intense activity can be noticed at the 
boundary between ice and water. Molecules 
from the liquid water collide against ice and 
adhere to it and some molecules of ice escape 
into liquid phase. There is no change of mass 
of ice and water, as the rates of transfer of 
molecules from ice into water and of reverse 
transfer from water into ice are equal at 
atmospheric pressure and 273 K. 
It is obvious that ice and water are in 
equilibrium only at particular temperature 
and pressure. For any pure substance at 
atmospheric pressure, the temperature at 
which the solid and liquid phases are at 
equilibrium is called the normal melting point 
or normal freezing point of the substance.  The 
system here is in dynamic equilibrium and we 
can infer the following:
(i) Both the opposing processes occur 
simultaneously.
(ii) Both the processes occur at the same 
rate so that the amount of ice and water 
remains constant.
6.1.2 l iquid-Vapour Equilibrium
This equilibrium can be better understood if 
we consider the example of a transparent box 
carrying a U-tube with mercury (manometer). 
Drying agent like anhydrous calcium chloride 
(or phosphorus penta-oxide) is placed for 
a few hours in the box. After removing the 
drying agent by tilting the box on one side, a 
watch glass (or petri dish) containing water 
is quickly placed inside the box. It will be 
observed that the mercury level in the right 
limb of the manometer slowly increases and 
finally attains a constant value, that is, the 
pressure inside the box increases and reaches 
a constant value. Also the volume of water in 
the watch glass decreases (Fig. 6.1). Initially 
there was no water vapour (or very less) inside 
the box. As water evaporated the pressure in 
the box increased due to addition of water 
molecules into the gaseous phase inside 
the box. The rate of evaporation is constant. 
Unit 6.indd   169 9/12/2022   11:58:20 AM
2024-25
170 chemistry However, the rate of increase in pressure 
decreases with time due to condensation 
of vapour into water. Finally it leads to an 
equilibrium condition when there is no net 
evaporation. This implies that the number 
of water molecules from the gaseous state 
into the liquid state also increases till the 
equilibrium is attained i.e.,
rate of evaporation= rate of condensation
H
2
O(l)          H
2
O (vap)
At equilibrium the pressure exerted by 
the water molecules at a given temperature 
remains constant and is called the equilibrium 
vapour pressure of water (or just vapour 
pressure of water); vapour pressure of water 
increases with temperature. If the above 
experiment is repeated with methyl alcohol, 
acetone and ether, it is observed that different 
liquids have different equilibrium vapour 
pressures at the same temperature, and the 
liquid which has a higher vapour pressure is 
more volatile and has a lower boiling point. 
If we expose three watch glasses containing 
separately 1mL each of acetone, ethyl alcohol, 
and water to atmosphere and repeat the 
experiment with different volumes of the 
liquids in a warmer room, it is observed 
that in all such cases the liquid eventually 
disappears and the time taken for complete 
evaporation depends on (i) the nature of the 
liquid, (ii) the amount of the liquid and (iii) the 
temperature. When the watch glass is open 
to the atmosphere, the rate of evaporation 
remains constant but the molecules are 
dispersed into large volume of the room. As 
a consequence the rate of condensation from 
vapour to liquid state is much less than the 
rate of evaporation. These are open systems 
and it is not possible to reach equilibrium in 
an open system.
Water and water vapour are in equilibrium 
position at atmospheric pressure (1.013 bar) 
and at 100°C in a closed vessel. The boiling 
point of water is 100°C at 1.013 bar pressure. 
For any pure liquid at one atmospheric 
pressure (1.013 bar), the temperature 
at which the liquid and vapours are at 
equilibrium is called normal boiling point of 
the liquid. Boiling point of the liquid depends 
on the atmospheric pressure. It depends on 
the altitude of the place; at high altitude the 
boiling point decreases. 
6.1.3 Solid – Vapour Equilibrium
Let us now consider the systems where solids 
sublime to vapour phase. If we place solid 
iodine in a closed vessel, after sometime 
the vessel gets filled up with violet vapour 
and the intensity of colour increases with 
time. After certain time the intensity of 
colour becomes constant and at this stage 
equilibrium is attained. Hence solid iodine 
sublimes to give iodine vapour and the iodine 
vapour condenses to give solid iodine. The 
equilibrium can be represented as,
I
2
(solid)   I
2
 (vapour)
Other examples showing this kind of 
equilibrium are,
Camphor (solid)  Camphor (vapour)
NH
4
Cl (solid)   NH
4
Cl (vapour)
Fig. 6.1  Measuring equilibrium vapour pressure of water at a constant temperature
Unit 6.indd   170 9/12/2022   11:58:21 AM
2024-25
171 EQUILIBRIUM
6.1.4 Equilibrium Involving Dissolution 
of Solid or Gases in Liquids
Solids in liquids
We know from our experience that we can 
dissolve only a limited amount of salt or 
sugar in a given amount of water at room 
temperature. If we make a thick sugar syrup 
solution by dissolving sugar at a higher 
temperature, sugar crystals separate out if we 
cool the syrup to the room temperature. We call 
it a saturated solution when no more of solute 
can be dissolved in it at a given  temperature. 
The concentration of the solute in a saturated 
solution depends upon the temperature. In 
a saturated solution, a dynamic equilibrium 
exits between the solute molecules in the solid 
state and in the solution:
Sugar (solution)  Sugar (solid), and 
the rate of dissolution of sugar = rate of 
crystallisation of sugar.
Equality of the two rates and dynamic 
nature of equilibrium has been confirmed with 
the help of radioactive sugar. If we drop some 
radioactive sugar into saturated solution of 
non-radioactive sugar, then after some time 
radioactivity is observed both in the solution 
and in the solid sugar. Initially there were no 
radioactive sugar molecules in the solution 
but due to dynamic nature of equilibrium, 
there is exchange between the radioactive 
and non-radioactive sugar molecules between 
the two phases. The ratio of the radioactive 
to non-radioactive molecules in the solution 
increases till it attains a constant value.
Gases in liquids
When a soda water bottle is opened, some of 
the carbon dioxide gas dissolved in it fizzes 
out rapidly. The phenomenon arises due 
to difference in solubility of carbon dioxide 
at different pressures. There is equilibrium 
between the molecules in the gaseous state 
and the molecules dissolved in the liquid 
under pressure i.e.,
CO
2
 (gas)  CO
2
 (in solution)
This equilibrium is governed by Henry’s 
law, which states that the mass of a gas 
dissolved in a given mass of a solvent at 
any temperature is proportional to the 
pressure of the gas above the solvent. 
This amount decreases with increase of 
temperature. The soda water bottle is sealed 
under pressure of gas when its solubility in 
water is high. As soon as the bottle is opened, 
some of the dissolved carbon dioxide gas 
escapes to reach a new equilibrium condition 
required for the lower pressure, namely its 
partial pressure in the atmosphere. This is 
how the soda water in bottle when left open 
to the air for some time, turns ‘flat’. It can be 
generalised that:
(i) For solid  liquid equilibrium, there is 
only one temperature (melting point) at  
1 atm (1.013 bar) at which the two 
phases can coexist. If there is no 
exchange of heat with the surroundings, 
the mass of the two phases remains 
constant.
(ii) For liquid  vapour equilibrium, the 
vapour pressure is constant at a given 
temperature.
(iii) For dissolution of solids in liquids, 
the solubility is constant at a given 
temperature.
(iv) For dissolution of gases in liquids, 
the concentration of a gas in liquid 
is proportional to the pressure 
(concentration) of the gas over the liquid. 
These observations are summarised in 
Table 6.1
Table 6.1 Some Features of Physical Equilibria
Process Conclusion
Liquid ? Vapour
H
2
O (l)  H
2
O (g)
p
H
2
O
constant at given 
temperature
Solid  Liquid
H
2
O (s)  H
2
O (l)
Melting point is fixed at 
constant pressure
Solute(s)  Solute
(solution)
Sugar(s)  Sugar
(solution)
Concentration of solute
in solution is constant
at a given temperature
Gas(g)  Gas (aq)
CO
2
(g)  CO
2
(aq)
[gas(aq)]/[gas(g)] is 
constant at a given 
temperature  
[CO
2
(aq)]/[CO
2
(g)] is 
constant at a given 
temperature 
Unit 6.indd   171 11/2/2022   4:10:42 PM
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Page 5


Unit 6
Eq Uilibri Um
Chemical equilibria are important in numerous biological 
and environmental processes. For example, equilibria 
involving O
2
 molecules and the protein hemoglobin play 
a crucial role in the transport and delivery of O
2
 from 
our lungs to our muscles. Similar equilibria involving CO 
molecules and hemoglobin account for the toxicity of CO.
When a liquid evaporates in a closed container, 
molecules with relatively higher kinetic energy escape 
the liquid surface into the vapour phase and number of 
liquid molecules from the vapour phase strike the liquid 
surface and are retained in the liquid phase. It gives rise 
to a constant vapour pressure because of an equilibrium in 
which the number of molecules leaving the liquid equals the 
number returning to liquid from the vapour. We say that 
the system has reached equilibrium state at this stage. 
However, this is not static equilibrium and there is a lot of 
activity at the boundary between the liquid and the vapour. 
Thus, at equilibrium, the rate of evaporation is equal to the 
rate of condensation. It may be represented by
H
2
O (l)   H
2
O (vap)
The double half arrows indicate that the processes 
in both the directions are going on simultaneously. The 
mixture of reactants and products in the equilibrium state 
is called an equilibrium mixture.
Equilibrium can be established for both physical 
processes and chemical reactions. The reaction may be 
fast or slow depending on the experimental conditions and 
the nature of the reactants. When the reactants in a closed 
vessel at a particular temperature react to give products, 
the concentrations of the reactants keep on decreasing, 
while those of products keep on increasing for some time 
after which there is no change in the concentrations 
of either of the reactants or products. This stage of the 
system is the dynamic equilibrium and the rates of the 
forward and reverse reactions become equal. It is due to 
After studying this unit you will be 
able to 
• identify dynamic nature of  
equilibrium involved in physical 
and chemical processes;
• state the law of equilibrium; 
• explain characteristics of 
equilibria involved in physical 
and chemical processes;
• write expressions for equilibrium 
constants; 
• establish a relationship between 
K
p
 and K
c
;
• explain various factors that 
affect the equilibrium state of a 
reaction;
• classify substances as acids or 
bases according to Arrhenius, 
Bronsted-Lowry and Lewis 
concepts;
• classify acids and bases as 
weak or strong in terms of their 
ionization constants; 
• explain the dependence of degree 
of ionization on concentration 
of the electrolyte and that of the 
common ion;
• describe pH scale for representing 
hydrogen ion concentration;
• explain ionisation of water and 
its duel role as acid and base;
• describe ionic product (K
w 
) and 
pK
w 
 for water;
• appreciate use of buffer 
solutions;
• calculate solubility product 
constant.
Unit 6.indd   168 9/12/2022   11:58:20 AM
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169 EQUILIBRIUM
this dynamic equilibrium stage that there is 
no change in the concentrations of various 
species in the reaction mixture. Based on the 
extent to which the reactions proceed to reach 
the state of chemical equilibrium, these may 
be classified in three groups .
(i) The reactions that proceed nearly 
to completion and only negligible 
concentrations of the reactants are 
left. In some cases, it may not be even 
possible to detect these experimentally.
(ii) The reactions in which only small 
amounts of products are formed and 
most of the reactants remain unchanged 
at equilibrium stage.
(iii) The reactions in which the concentrations 
of the reactants and products are 
comparable, when the system is in 
equilibrium.
The extent of a reaction in equilibrium 
varies with the experimental conditions such 
as concentrations of reactants, temperature, 
etc. Optimisation of the operational conditions 
is very important in industry and laboratory 
so that equilibrium is favorable in the 
direction of the desired product. Some 
important aspects of equilibrium involving 
physical and chemical processes are dealt in 
this unit along with the equilibrium involving 
ions in aqueous solutions which is called as 
ionic equilibrium.
6.1 EqUil ibri Um in PHYSiCAl 
Pr OCESSES
The characteristics of system at equilibrium 
are better understood if we examine some 
physical processes. The most familiar examples 
are phase transformation processes, e.g.,
 solid        liquid 
 liquid        gas
 solid         gas
6.1.1 Solid-l iquid Equilibrium
Ice and water kept in a perfectly insulated 
thermos flask (no exchange of heat between its 
contents and the surroundings) at 273K and 
the atmospheric pressure are in equilibrium 
state and the system shows interesting 
characteristic features. We observe that the 
mass of ice and water do not change with 
time and the temperature remains constant. 
However, the equilibrium is not static. 
The intense activity can be noticed at the 
boundary between ice and water. Molecules 
from the liquid water collide against ice and 
adhere to it and some molecules of ice escape 
into liquid phase. There is no change of mass 
of ice and water, as the rates of transfer of 
molecules from ice into water and of reverse 
transfer from water into ice are equal at 
atmospheric pressure and 273 K. 
It is obvious that ice and water are in 
equilibrium only at particular temperature 
and pressure. For any pure substance at 
atmospheric pressure, the temperature at 
which the solid and liquid phases are at 
equilibrium is called the normal melting point 
or normal freezing point of the substance.  The 
system here is in dynamic equilibrium and we 
can infer the following:
(i) Both the opposing processes occur 
simultaneously.
(ii) Both the processes occur at the same 
rate so that the amount of ice and water 
remains constant.
6.1.2 l iquid-Vapour Equilibrium
This equilibrium can be better understood if 
we consider the example of a transparent box 
carrying a U-tube with mercury (manometer). 
Drying agent like anhydrous calcium chloride 
(or phosphorus penta-oxide) is placed for 
a few hours in the box. After removing the 
drying agent by tilting the box on one side, a 
watch glass (or petri dish) containing water 
is quickly placed inside the box. It will be 
observed that the mercury level in the right 
limb of the manometer slowly increases and 
finally attains a constant value, that is, the 
pressure inside the box increases and reaches 
a constant value. Also the volume of water in 
the watch glass decreases (Fig. 6.1). Initially 
there was no water vapour (or very less) inside 
the box. As water evaporated the pressure in 
the box increased due to addition of water 
molecules into the gaseous phase inside 
the box. The rate of evaporation is constant. 
Unit 6.indd   169 9/12/2022   11:58:20 AM
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170 chemistry However, the rate of increase in pressure 
decreases with time due to condensation 
of vapour into water. Finally it leads to an 
equilibrium condition when there is no net 
evaporation. This implies that the number 
of water molecules from the gaseous state 
into the liquid state also increases till the 
equilibrium is attained i.e.,
rate of evaporation= rate of condensation
H
2
O(l)          H
2
O (vap)
At equilibrium the pressure exerted by 
the water molecules at a given temperature 
remains constant and is called the equilibrium 
vapour pressure of water (or just vapour 
pressure of water); vapour pressure of water 
increases with temperature. If the above 
experiment is repeated with methyl alcohol, 
acetone and ether, it is observed that different 
liquids have different equilibrium vapour 
pressures at the same temperature, and the 
liquid which has a higher vapour pressure is 
more volatile and has a lower boiling point. 
If we expose three watch glasses containing 
separately 1mL each of acetone, ethyl alcohol, 
and water to atmosphere and repeat the 
experiment with different volumes of the 
liquids in a warmer room, it is observed 
that in all such cases the liquid eventually 
disappears and the time taken for complete 
evaporation depends on (i) the nature of the 
liquid, (ii) the amount of the liquid and (iii) the 
temperature. When the watch glass is open 
to the atmosphere, the rate of evaporation 
remains constant but the molecules are 
dispersed into large volume of the room. As 
a consequence the rate of condensation from 
vapour to liquid state is much less than the 
rate of evaporation. These are open systems 
and it is not possible to reach equilibrium in 
an open system.
Water and water vapour are in equilibrium 
position at atmospheric pressure (1.013 bar) 
and at 100°C in a closed vessel. The boiling 
point of water is 100°C at 1.013 bar pressure. 
For any pure liquid at one atmospheric 
pressure (1.013 bar), the temperature 
at which the liquid and vapours are at 
equilibrium is called normal boiling point of 
the liquid. Boiling point of the liquid depends 
on the atmospheric pressure. It depends on 
the altitude of the place; at high altitude the 
boiling point decreases. 
6.1.3 Solid – Vapour Equilibrium
Let us now consider the systems where solids 
sublime to vapour phase. If we place solid 
iodine in a closed vessel, after sometime 
the vessel gets filled up with violet vapour 
and the intensity of colour increases with 
time. After certain time the intensity of 
colour becomes constant and at this stage 
equilibrium is attained. Hence solid iodine 
sublimes to give iodine vapour and the iodine 
vapour condenses to give solid iodine. The 
equilibrium can be represented as,
I
2
(solid)   I
2
 (vapour)
Other examples showing this kind of 
equilibrium are,
Camphor (solid)  Camphor (vapour)
NH
4
Cl (solid)   NH
4
Cl (vapour)
Fig. 6.1  Measuring equilibrium vapour pressure of water at a constant temperature
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171 EQUILIBRIUM
6.1.4 Equilibrium Involving Dissolution 
of Solid or Gases in Liquids
Solids in liquids
We know from our experience that we can 
dissolve only a limited amount of salt or 
sugar in a given amount of water at room 
temperature. If we make a thick sugar syrup 
solution by dissolving sugar at a higher 
temperature, sugar crystals separate out if we 
cool the syrup to the room temperature. We call 
it a saturated solution when no more of solute 
can be dissolved in it at a given  temperature. 
The concentration of the solute in a saturated 
solution depends upon the temperature. In 
a saturated solution, a dynamic equilibrium 
exits between the solute molecules in the solid 
state and in the solution:
Sugar (solution)  Sugar (solid), and 
the rate of dissolution of sugar = rate of 
crystallisation of sugar.
Equality of the two rates and dynamic 
nature of equilibrium has been confirmed with 
the help of radioactive sugar. If we drop some 
radioactive sugar into saturated solution of 
non-radioactive sugar, then after some time 
radioactivity is observed both in the solution 
and in the solid sugar. Initially there were no 
radioactive sugar molecules in the solution 
but due to dynamic nature of equilibrium, 
there is exchange between the radioactive 
and non-radioactive sugar molecules between 
the two phases. The ratio of the radioactive 
to non-radioactive molecules in the solution 
increases till it attains a constant value.
Gases in liquids
When a soda water bottle is opened, some of 
the carbon dioxide gas dissolved in it fizzes 
out rapidly. The phenomenon arises due 
to difference in solubility of carbon dioxide 
at different pressures. There is equilibrium 
between the molecules in the gaseous state 
and the molecules dissolved in the liquid 
under pressure i.e.,
CO
2
 (gas)  CO
2
 (in solution)
This equilibrium is governed by Henry’s 
law, which states that the mass of a gas 
dissolved in a given mass of a solvent at 
any temperature is proportional to the 
pressure of the gas above the solvent. 
This amount decreases with increase of 
temperature. The soda water bottle is sealed 
under pressure of gas when its solubility in 
water is high. As soon as the bottle is opened, 
some of the dissolved carbon dioxide gas 
escapes to reach a new equilibrium condition 
required for the lower pressure, namely its 
partial pressure in the atmosphere. This is 
how the soda water in bottle when left open 
to the air for some time, turns ‘flat’. It can be 
generalised that:
(i) For solid  liquid equilibrium, there is 
only one temperature (melting point) at  
1 atm (1.013 bar) at which the two 
phases can coexist. If there is no 
exchange of heat with the surroundings, 
the mass of the two phases remains 
constant.
(ii) For liquid  vapour equilibrium, the 
vapour pressure is constant at a given 
temperature.
(iii) For dissolution of solids in liquids, 
the solubility is constant at a given 
temperature.
(iv) For dissolution of gases in liquids, 
the concentration of a gas in liquid 
is proportional to the pressure 
(concentration) of the gas over the liquid. 
These observations are summarised in 
Table 6.1
Table 6.1 Some Features of Physical Equilibria
Process Conclusion
Liquid ? Vapour
H
2
O (l)  H
2
O (g)
p
H
2
O
constant at given 
temperature
Solid  Liquid
H
2
O (s)  H
2
O (l)
Melting point is fixed at 
constant pressure
Solute(s)  Solute
(solution)
Sugar(s)  Sugar
(solution)
Concentration of solute
in solution is constant
at a given temperature
Gas(g)  Gas (aq)
CO
2
(g)  CO
2
(aq)
[gas(aq)]/[gas(g)] is 
constant at a given 
temperature  
[CO
2
(aq)]/[CO
2
(g)] is 
constant at a given 
temperature 
Unit 6.indd   171 11/2/2022   4:10:42 PM
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172 chemistry 6.1.5 General Characteristics of 
Equilibria Involving Physical 
Processes
For the physical processes discussed above, 
following characteristics are common to the 
system at equilibrium:
(i) Equilibrium is possible only in a closed 
system at a given temperature.
(ii) Both the opposing processes occur at 
the same rate and there is a dynamic 
but stable condition.
(iii) All measurable properties of the system 
remain constant.
(iv) When equilibrium is attained for a 
physical process, it is characterised by 
constant value of one of its parameters 
at a given temperature. Table 6.1 lists 
such quantities.
(v) The magnitude of such quantities at any 
stage indicates the extent to which the 
physical process has proceeded before 
reaching equilibrium.
6.2 EQUILIBRIUM IN CHEMICAL 
PROCESSES – DYNAMIC 
EQUILIBRIUM
Analogous to the physical systems chemical 
reactions also attain a state of equilibrium. 
These reactions can occur both in forward 
and backward directions. When the rates of 
the forward and reverse reactions become 
equal, the concentrations of the reactants 
and the products remain constant. This 
is the stage of chemical equilibrium. This 
equilibrium is dynamic in nature as it consists 
of a forward reaction in which the reactants 
give product(s) and reverse reaction in which 
product(s) gives the original reactants.
For a better comprehension, let us consider 
a general case of a reversible reaction, 
A + B  C + D
With passage of time, there is 
accumulation of the products C and D and 
depletion of the reactants A and B (Fig. 6.2). 
This leads to a decrease in the rate of 
forward reaction and an increase in the rate 
of the reverse reaction,
Fig. 6.2  Attainment of chemical equilibrium.
Eventually, the two reactions occur at the 
same rate and the system reaches a state of 
equilibrium. 
Similarly, the reaction can reach the state 
of equilibrium even if we start with only C and 
D; that is, no A and B being present initially, 
as the equilibrium can be reached from either 
direction.
The dynamic nature of chemical 
equilibrium can be demonstrated in the 
synthesis of ammonia by Haber’s process.  
In a series of experiments, Haber started 
with known amounts of dinitrogen and 
dihydrogen maintained at high temperature 
and pressure and at regular intervals 
determined the amount of ammonia present. 
He was successful in determining also the 
concentration of unreacted dihydrogen and 
dinitrogen. Fig. 6.4 (page 174) shows that after 
a certain time the composition of the mixture 
remains the same even though some of the 
reactants are still present. This constancy in 
composition indicates that the reaction has 
reached equilibrium. In order to understand 
the dynamic nature of the reaction, synthesis 
of ammonia is carried out with exactly the 
same starting conditions (of partial pressure 
and temperature) but using D
2
 (deuterium) 
in place of H
2
. The reaction mixtures starting 
either with H
2
 or D
2
 reach equilibrium with 
the same composition, except that D
2
 and 
ND
3
 are present instead of H
2
 and NH
3
.  After 
Unit 6.indd   172 11/2/2022   4:10:43 PM
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