```Law of Chemical Equilibrium
According to this law, the ratio of product of concentration of products to the product of concentration of reactants, with each concentration term is raised to the power by its coefficient in overall balanced chemical equation, is a constant quantity at a given temperature and it is called equilibrium constant.
Derivation of law of chemical equilibrium
Let us consider for the following equilibrium
aA + bB ⇔ cC + dD
then, from Law of mass action
rate of forward reaction r1 ∝ [A]a [B]b
or         r1 = Ka [A]a [B]b and rate of reverse reaction r­2 ∝ [C]c [D]d
r2 = K2 [C]c [D]d
at equilibrium, r1 = r2
⇒ K1 [A]a[B]b = K2 [C]c[D]d
⇒ K1/K2 = [C]c[D]d/[A]a[B]b
Kc = K1/K2, an equilibrium constant in terms of active masses of reacting species.
For the reaction
SO2Cl­2        ⇔       SO2      +    Cl2
at t = 0                        a                         0                 0
at equilibrium             a - x                      x                  x
equilibrium conc.       a–x/V                    x/V               x/V (V is volume of container)
So,       [SO2Cl2] = a–x/V; [SO2] = x/V = [Cl2]
So,       Kc = (x/V)×(x/V)/(a–x/V) = x2/(a–x)V ⇒ Kc = x2/(a–x)×V```
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