Law of chemical equilibrium and equilibrium constant

LAW OF MASS ACTION

Equilibrium state of a reversible reaction is characterized by its equilibrium constant, which is related to the equilibrium concentrations of all the reactants and products. The method of writing the expression for equilibrium constant was given for the first time by Guldberg and Wage in 1863 in the form of the law of mass action which states: 

The rate at which a substance reacts is proportional to its active mass (molar concentration) and the overall rate of the reaction is proportional to the product of active masses (molar concentrations) of all the reacting substances

 Equilibrium Constant:
An equilibrium mixture is a mixture of reactants and products in the equilibrium state.
Consider a general reversible reaction,
A+B
C+D

Where A and B are reactants, C and D are products in the above-balanced equation.
Kc=[C][D]/[A][B]
Where Kc is the Equilibrium constant and expressed in concentrations of molL−1.

At a given temperature, the product of concentrations of the reaction products raised to the respective stoichiometric coefficient in the balanced chemical equation divided by the product of concentrations of the reactants raised to their individual stoichiometric coefficients has a constant value. This is known as the Equilibrium Law or Law of Chemical Equilibrium.
The equilibrium constant for the general reaction,
aA+bB
cC+dD


Equilibrium constant expressed as,
Kc=[C]c[D]d/[A]a[B]b


Where A, B, C, and D are the equilibrium concentrations of the reactants and products.


The product of the molar concentrations of the products, each raised to the power equal to its stoichiometric coefficient is divided by the product of the molar concentrations of the reactants, each raised to the power equal to its stoichiometric coefficient is constant at constant temperature is called Equilibrium Constant.

 Characteristics of Equilibrium Constant:

The expression of equilibrium constant depends upon the manner in which the chemical equation representing it is written. For the reaction

Similar relationship is also observed in the pressure equilibrium constant K p .Thus the expression of equilibrium constant depends on how the reaction is expressed in the form of a chemical equation.