what is equlibrium constant

Law of mass action also forms the basis which states that the rate of a chemical reaction is directly proportional to the product of the concentrations of the reactants raised to their respective stoichiometric coefficients. Therefore, given the reaction – 

aA(g) + bB(g) ⇔ cC(g) + dD(g)

By using the law of mass action here, 

• The forward reaction rate would be k+ [A]a[B]b
• The backward reaction rate would be k– [C]c[D]d 

where, [A], [B], [C] and [D] being the active masses and k+ and k− are rate constants of forward and backward reactions, also the a, b, c, d are the stoichiometric coefficients related to A, B, C and D respectively. However, at the equilibrium – the forward and the backward rates are equal, stating – 

Rate of forward reaction = Rate of backward reaction

or, 

or, 

where, 

Kc is the equilibrium constant expressed in terms of the molar concentrations. The equation Kc = [ C ]c·[ D ]d / [ A ]a·[ B ]b 

or, Kc = Kf / Kb is the Law of Chemical Equilibrium. The equilibrium constant is therefore related to the standard Gibbs free energy change for the reaction which is stated by the equation –

§Gº= -RT ln Keq

where T states the temperature, R is the universal gas constant and Keq is the equilibrium constant.