Equilibrium PPT Chemistry Class 12

 Page 1


EQUILIBRIUM
EQUILIBRIUM IN CHEMICAL PROCESSES – DYNAMIC 
EQUILIBRIUM
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 .
This leads to a decrease in the rate of forward reaction and an increase in he 
rate of the reverse reaction, Eventually, the two reactions occur at the same 
rate and the system reaches a state of
equilibrium.
Page 2


EQUILIBRIUM
EQUILIBRIUM IN CHEMICAL PROCESSES – DYNAMIC 
EQUILIBRIUM
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 .
This leads to a decrease in the rate of forward reaction and an increase in he 
rate of the reverse reaction, Eventually, the two reactions occur at the same 
rate and the system reaches a state of
equilibrium.
SIMILARLY, THE REACTION CAN REACH THE STA TE OF EQUILIBRIUM
EVEN IF WE START WITH ONL Y C AND D; THA T IS, NO A AND B BEING
PRESENT INITIALLY, AS THE EQUILIBRIUM CAN BE REACHED FROM
EITHER DIRECTION.
Page 3


EQUILIBRIUM
EQUILIBRIUM IN CHEMICAL PROCESSES – DYNAMIC 
EQUILIBRIUM
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 .
This leads to a decrease in the rate of forward reaction and an increase in he 
rate of the reverse reaction, Eventually, the two reactions occur at the same 
rate and the system reaches a state of
equilibrium.
SIMILARLY, THE REACTION CAN REACH THE STA TE OF EQUILIBRIUM
EVEN IF WE START WITH ONL Y C AND D; THA T 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.  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 D2
(deuterium) in place of H2 . The reaction mixtures starting either with 
H2or D2 reach equilibrium with the same composition, except that D2
and ND3 are present instead of H 2and NH 3 . After equilibrium is 
attained, these two mixtures (H2 , N2 , NH3and D2 , N2 , ND3) are mixed 
together and left for a while. Later, when this mixture is analysed, it is 
found that the concentration of ammonia is just the same as before.
Page 4


EQUILIBRIUM
EQUILIBRIUM IN CHEMICAL PROCESSES – DYNAMIC 
EQUILIBRIUM
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 .
This leads to a decrease in the rate of forward reaction and an increase in he 
rate of the reverse reaction, Eventually, the two reactions occur at the same 
rate and the system reaches a state of
equilibrium.
SIMILARLY, THE REACTION CAN REACH THE STA TE OF EQUILIBRIUM
EVEN IF WE START WITH ONL Y C AND D; THA T 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.  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 D2
(deuterium) in place of H2 . The reaction mixtures starting either with 
H2or D2 reach equilibrium with the same composition, except that D2
and ND3 are present instead of H 2and NH 3 . After equilibrium is 
attained, these two mixtures (H2 , N2 , NH3and D2 , N2 , ND3) are mixed 
together and left for a while. Later, when this mixture is analysed, it is 
found that the concentration of ammonia is just the same as before.
However, when this mixture is analysed by a mass spectrometer, it is 
found that ammonia and all deuterium containing forms of ammonia 
(NH3, NH2D, NHD2and ND3) and dihydrogen and its deutrated
forms (H2, HD and D2) are present. Thus one can conclude that 
scrambling of H and D atoms in the molecules must result from a 
continuation of the forward and reverse reactions in the mixture. If 
the reaction had simply stopped when they reached equilibrium, then 
there would have been no mixing of isotopes in this way.
Page 5


EQUILIBRIUM
EQUILIBRIUM IN CHEMICAL PROCESSES – DYNAMIC 
EQUILIBRIUM
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 .
This leads to a decrease in the rate of forward reaction and an increase in he 
rate of the reverse reaction, Eventually, the two reactions occur at the same 
rate and the system reaches a state of
equilibrium.
SIMILARLY, THE REACTION CAN REACH THE STA TE OF EQUILIBRIUM
EVEN IF WE START WITH ONL Y C AND D; THA T 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.  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 D2
(deuterium) in place of H2 . The reaction mixtures starting either with 
H2or D2 reach equilibrium with the same composition, except that D2
and ND3 are present instead of H 2and NH 3 . After equilibrium is 
attained, these two mixtures (H2 , N2 , NH3and D2 , N2 , ND3) are mixed 
together and left for a while. Later, when this mixture is analysed, it is 
found that the concentration of ammonia is just the same as before.
However, when this mixture is analysed by a mass spectrometer, it is 
found that ammonia and all deuterium containing forms of ammonia 
(NH3, NH2D, NHD2and ND3) and dihydrogen and its deutrated
forms (H2, HD and D2) are present. Thus one can conclude that 
scrambling of H and D atoms in the molecules must result from a 
continuation of the forward and reverse reactions in the mixture. If 
the reaction had simply stopped when they reached equilibrium, then 
there would have been no mixing of isotopes in this way.
Equilibrium can be attained from both sides, whether we start 
reaction by taking, H2(g) and N2(g) and get NH3(g) or by taking
NH3(g) and decomposing it into N2(g) and H2(g).                                         
N2(g) + 3H2(g)      2NH3(g)
2NH3(g)      N2(g) + 3H2(g)
Similarly let us consider the reaction,
H2(g) + I2(g)  2HI(g). 
If we start with equal initial concentration of H2and I2, the reaction 
proceeds in the forward direction and the concentration of H2 and I2
decreases while that of HI increases, until all of these become 
constant at equilibrium . We can also start with HI alone and make 
the reaction to proceed in the reverse direction; the concentration of 
HI will decrease and concentration of H2 and I2 will increase until 
they all become constant when equilibrium is reached . If total 
number of H and I atoms are same in a given volume, the same 
equilibrium mixture is obtained whether we start it from pure 
reactants or pure product.