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7 .Equilibrium
Some Important Points and Terms of the Chapter


1. Equilibrium represents the state of a process in which the properties like
temperature, pressure etc do not show any change with the passage of time
2. Chemical 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. Equilibrium
is possible only in a closed system at a given temperature. A mixture of reactants
and products in the equilibrium state is called an equilibrium mixture.
3. In a Homogeneous system, all the reactants and products are in the same phase.
For example, in the gaseous reaction, N2 (g) + 3H2(g) 2NH3(g), reactants and
products are in the homogeneous phase.
4. Equilibrium in a system having more than one phase is called heterogeneous
equilibrium. The equilibrium between water vapor and liquid water in a closed
container is an example of heterogeneous equilibrium. H2O(l) H2O(g)
KENDRIYA VIDYALAYA SANGATHAN
Zonal Institute Of Education And Training; Bhubaneswar Downloaded from :-
Study Material- Chemistry- Class XI

, 5. Henry Law:-It 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
6. Law of Chemical Equilibrium: It may be stated as, at a given temperature the
ratio of product of equilibrium concentration of the products to that of the reactants
with each concentration terms raised to power equal to the respective
stoichiometric coefficient in the balanced chemical reaction has a constant value.
This constant value is known as Equilibrium constant. For a general reaction of the
type aA + bB  cC + dD
Kc = [C]c[D]d /[A]a [B]b This expression is known as Law Of Chemical
Equilibrium
7. Relationship between Kp and Kc: Kp = Kc(RT)  n
8. Units of Equilibrium Constant: The value of equilibrium constant Kc can be
calculated by substituting the concentration terms in mol/L and for Kp partial
pressure is substituted in Pa, kPa, bar or atm. This results in units of equilibrium
constant based on molarity or pressure, unless the exponents of both the numerator
and denominator are same. For the reactions (i)H2(g) + I2(g) 2HI, Kc
and Kp have no unit.(ii)N2O4(g)  2NO2 (g), Kc has unit mol/L and Kp has unit
bar
9. Characteristics Of Equilibrium Constant
 Equilibrium constant is applicable only when concentrations of the reactants
and products have attained their equilibrium state.
 The value of equilibrium constant is independent of initial concentrations of
the reactants and products.
 Equilibrium constant is temperature dependent having one unique value for
aparticular reaction represented by a balanced equation at a given temperature.
 The equilibrium constant for the reverse reaction is equal to the inverse of the
equilibrium constant for the forward reaction.
 The equilibrium constant K for a reaction is related to the equilibrium constant
of the corresponding reaction, whose equation is obtained by multiplying or
dividing the equation for the original reaction by a small integer.
10. Applications of equilibrium constant :
 Predict the extent of a reaction on the basis of its magnitude.
KENDRIYA VIDYALAYA SANGATHAN
Zonal Institute Of Education And Training; Bhubaneswar Downloaded from :-
Study Material- Chemistry- Class XI