The Keq (Kc or Kp) values do not depend on the initial concentrations of reactants but depend only on the equilibrium concentration values.

**Characteristics of
Equilibrium constant**

1.
The K_{eq} (K_{c} or K_{p})
values do not depend on the initial concentrations of reactants but depend only
on the equilibrium concentration values.

2.
When K_{eq} values are greater than
unity, the equilibrium is favourable towards product formations and vice-versa.

3.
Keq values do not change in presence of catalyst
catalyst only speeds up the forward and backward reactions.

4.
Temperature changes on the chemical equilibrium
changes the Keq values. For exothermic equilibrium reactions increase in
temperature, lowers the K_{eq} values and for endothermic equilibrium, increase
in temperature increases the K_{eq} values. Generally, when T is
raised, the equilibrium shifts in a direction in which heat is absorbed.

5.
Pressure changes on gaseous equilibrium alters
the K_{p} values. For dissociation equilibrium, increase in pressure
lowers the K_{p} values, while for association equilibrium (number of
product molecules < number of reactant molecules) increase in pressure
increases the K_{p} values. When the number of product and reactant
molecules are equal, there is no pressure effect.

**Equilibrium constant and equilibrium law**

Let us
consider a general reaction

A + B - - K_{f} --- > < ---K_{r}---
C + D

and let [A], [B], [C] and [D] represent the molar concentrations of
A,B,C and D at the equilibrium point. According to the Law of Mass action,

Rate of
forward reaction a[A][B] = K_{f}[A][B]

Rate of
forward reaction a[C][D] = K_{r}[C][D]

where kf
and kr are rate constants for the forward and reverse reactions.

At equilibrium, rate of forward reaction = rate of reverse reaction.

Therefore,

kf[A]
[B] = kr
[C] [D]

kf /
kr = [C] [D] / [A] [B]

At any specific temperature kf/kr is a constant since both kf and kr are
constants. The ratio kf/kr is called **equilibrium
constant** and is represented by the symbol Kc. The subscript `c' indicates
that the value is in terms of concentration of reactants and products. The
equation (1) may be written as

K_{f }(Equilibrium
constant)_{ = }[C] [D] (Products concentration) / [A] [B] (Reactants concentrations)

This equation is known as the equilibrium constant expression or
equilibrium law. Hence [C], [D] [A] and [B] values are the equilibrium
concentrations and are equal to equilibrium concentrations.

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