Equilibrium
Equilibrium State
At equilibrium state, both forward and reverse reaction occurs and eventually the reaction
seems to stop.
Equilibrium Law
It states that in a reversible reaction, the ratio of rate of the forward reaction to the rate of
reverse reaction is constant for that particular reaction.
A+B ⇌ C+D
HI(g) ⇌ H2(g) + I2 (g)
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In the above reaction, there will be at first an increase in the purple color owing to
the production of iodine gas
After Some time, the increase in the color will stop, indicating that the reaction has
stopped.
The rate of dissociation of HI is fastest in the beginning when the concentration of
HI is greatest and falls as the reaction proceeds.
The reverse reaction which initially has a zero rate starts slowly and increases in
rate as the concentration of H2 and I2 increases therefore the color in the flask
remains same. And equilibrium reaches
It shows a dynamic equilibrium because both forward and backward reactions are
still occurring.
Features of Equilibrium State
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Equilibrium is dynamic as the reaction do not stop but both forward and backward
reactions occur at the same rate.
Equilibrium is achieved in a closed system as it has no exchange of matter with the
surroundings.
The concentration of reactant and products remain constant at equilibrium as they
are being produced and destroyed at an equal rate
There is no change in color or density at equilibrium as these properties depends on
the concentration of components of the mixture.
Equilibrium can be reached from either direction as the same equilibrium mixture
will result under the same conditions, no matter whether the reaction is started
with all reactants, all products or a mixture of both.
Equilibrium Constant (KC)
aA+bB ⇌ cC + dD
[ ] [ ]
] [ ]
KC = [
Value of Kc stays constant in any reaction at a specified temperature
Reaction Quotient (Q)
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Q can be used to determine the direction of reaction
If Q = Kc, the reaction is at equilibrium and there is no net reaction
Q < Kc, then reaction proceeds to the right in favor of product
If Q > Kc, then reaction proceeds to the left in favor of reactant
Difference between Qc and Kc
Reaction Quotient
 Reaction quotient is the ratio
between the concentration of
products and the concentration of
reactants
 It can be used for any point in the
reaction
 Gives an idea about the direction In
which the reaction is proceeding
 The value is different form time to
time during the progression of the
reaction
Inverse of Kc : Kc’ = 1/Kc
Equilibrium Constant
 Equilibrium constant is the ratio
between the concentration of
products and the concentration of
reactants at equilibrium
 It can be used only for the point at
which reaction id in equilibrium
 Does not give any idea about which
direction the reaction is proceeding
 Value is constant for a particular
equilibrium at a particular
temperature.
Le Chatelier’s Principle
It states that a system at equilibrium when subjected to a change will respond in such a
way so as to minimize the effect of change
Change in Concentration of Reactant or Product
A+B ⇌ C + D
[ ] [ ]
] [ ]
KC = [
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If the concentration of one of the reactants is increased, this will cause the rate of
the forward reaction to increase while the backward reaction will not be affected.
The reaction rate will no longer be equal.
When the equilibrium reestablishes itself, the mixture will have new concentration
of all reactants and products and the equilibrium will be shifted in favor of products
Change in Pressure
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Equilibria involving gases will be affected by a change in pressure if the reaction
involves a change in the number of molecules
There is a direct relationship between the number of gas molecules and the
pressure exerted by a gas in a fixed volume.
If there is an increase in pressure, the system responds to decrease this pressure by
favoring the side with the smaller number of molecules
For Example: In the production of methanol
o CO(g) + 2H2(g) ⇌ CH3OH
o There are 3 molecules of gas on the left hand side and only 1 molecule of gas on the
right hand side, so high pressure will shift the equilibrium to right in favor of smaller
number of gas molecules.
Change in Temperature
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Kc is temperature dependent
Exothermic reaction releases energy (-▲h) whereas endothermic reaction absorbs
energy (+▲h)
Note: The enthalpy change for forward and backward reactions are equal and opposite to
each other.
Consider the reaction: 2NO2 ⇌ N2O4
reaction is exothermic)
▲h = -57 kJ/mol (negative sign indicates the
If this reaction at equilibrium is subjected to a decrease in temperature, the system will
respond by producing heat and it does this by favoring the forward exothermic reaction, i.e.
the equilibrium will shift to the right in favor of the product
N2 + O2 ⇌ 2NO
▲h = 181 kJ/mol (positive ▲h indicates reaction is endothermic)
If we decrease the temperature, backward reaction will be favored, therefore the
equilibrium will shift to the left in favor of reactants and K c will decrease.
At higher temperature, the forward endothermic reaction is favored so the equilibrium
shifts to the right and Kc will increase.
Effect of Catalyst
A catalyst speeds up the rate of a reaction by providing an alternate reaction pathway that
has a transition state with a lower activation energy (Ea)
This increases the number of particles that have sufficient energy to react without raising
the temperature.
The catalyst lowers the activation energy by the same amount for forward and backward
reactions. So the rate of both these reactions will be increased by same factor.
Therefore, the catalyst will have no effect on equilibrium or Kc
The catalyst speeds up the attainment of the equilibrium state and forms the product more
quickly.
Haber’s Process (production of Ammonia)
N2(g) + 3H2(g) ⇌ 2NH3(g)
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All reactants and products are in gaseous phase
There is a change in number of gas molecules as the reaction proceeds
The forward reaction is exothermic while the backward is endothermic
The molar ratio of nitrogen and hydrogen is 1:3
The forward reaction involves a decrease in the number of gas molecules, it will be
favored by a high pressure (for Haber’s Process the pressure is 2* 10 7 pa)
As the forward reaction is exothermic, it is favored by lower temperature (for
Haber’s Process it is 450°C
A catalyst of finely divided iron is used with small amount of aluminum and
magnesium oxides for improving its activity
Contact Process (Production of Sulfuric Acid)
S(g) + O2(g) → SO2(g)
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The combustion of sulfur to form sulfur dioxide
The oxidation of sulfur dioxide to sulfur trioxide
2SO2 + O2 ⇌ 2SO3
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Combination of sulfur trioxide with water to produce sulfuric acid
SO3 + H2O → H2SO4
The rate of the reaction depends on step 2
Pressure = 2* 105 pa
Temperature = 450°C
Catalyst: V2O5
Production of Methanol
CO(g) + 2H2(g) ⇌ CH3OH(g)
▲h = -90 kJ/mol
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Optimum Condition for the above reaction
o Pressure (5 – 10 × 106 pa). Forward reaction involves reduction in the number of
molecules of gasses from 3 molecules of reactant to one molecule of product, therefore
high pressure will favor product.
o Temperature (350°C), Forward reaction is exothermic, so low temperature will increase
the equilibrium yield
o Catalyst (Cu-ZnO-Al2O3) Increases the rate of reactions
Equilibrium Law
It states that in a reversible reaction the ratio of the rate of forward reaction to the rate of reverse
reaction is constant for that reaction
Ques:
Ans: Kc =
[
][
]
[
][
]
=
.
× .
.
× .
=
.
.
= 1.839
Free Energy and Equilibrium
Givves free energy (▲G) is a measure of the work that is available from a system.
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Its sign is used to predict spontaneity of the reaction
▲G = GProduct – Greactant
If there is a decrease in total free energy as work is done by the system, the reaction
progresses forward
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Because the free energy decreases in both the directions, the reaction must go through
a composition which corresponds to the minimum value of free energy, this is the
equilibrium state where net reaction stops
The system has the highest possible value of entropy when free energy is at minimum
When ▲G is negative, the reaction is spontaneous
When ▲G is positive, the reaction is non-spontaneous
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Relationship between ▲G and Equilibrium constant
▲Go = - RT log(Kc)
Where ▲Go = standard free energy change of the reaction
R = Gas Constant (8.314 J/K/mol)
T= Temperature in Kelvin
Kc = Equilibrium Constant
Ques: The esterification reaction that produces ethyl ethanoate as a free energy change ▲G o =
-4.38 kJ/mol
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Calculate the Value of Equilibrium Constant of this reaction at 298 K.
▲Go = - RT log(Kc)
-4.38 kJ/mol = -8.314 × 298 × log(Kc)
log(𝐾 ) =
−4.38 × 1000
4.38 × 1000
=
= 1.77
−8.314 × 298
2477.572
Kinetics and Equilibrium
Rusting of iron is a complex process,
4Fe(s) + 3O2(g) ⇌ 2Fe2O3(g)
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It is a heterogenous equilibrium as the reaction components are in different phases
The value of ▲Go = 1490 × 106 J
Now by using formula, ▲Go = - RT log(Kc) we can calculate Kc which comes out to be
10261 which is very large value and indicates that this is a thermodynamically favorable
reaction and proceeds towards completion