LAW OF MASS ACTION

                     
INTRODUCTION :-
                                  Two chemist C.M GULDBERG and P.WAAGE in 1864 proposed the law of mass action to describe the equilibrium state.

DEFINITION  :-
                            It states that the rate at which a substance react is directly proportional to its active mass, and the rate at which the reaction proceed is directly proportional to its active masses of the reactants.

DERIVATION :-
                                  Consider a hypothetical reaction in which (a) moles of reactant A and (b) moles of reactant B react to give (c) moles of product C and (d) moles of product D at equilibrium.
                                             aA+bB     ←→ cC+dD                                                                                                                                                
MATHEMATICAL PROOF :-                                                                                                           Acording to law of mass action :-
 Rate of forward reaction proportional to [A]^a[B]^b
 Rate of forward reaction =Kf [A]^a[B]^b
 Rate of reverse  reaction proportional to [C]^c [D]^d
 Rate of reverse  reaction =Kr [C]^c [D]^d
 Where Kr and Kf are the rate constant for reverse and forward reactions respectively.
At equilibrium state
                                  Rate of forward reaction = Rate of reverse reaction
                         Kf [A]^a[B]^b = Kr [C]^c[D]d          
               
                        Kf / Kr  =[C]^c[D]^d / [A]^a[B]^b
                             
                       Kc=[C]^c[D]^d /  [A]^a[B]^b    IT IS KNOWN AS EQUILIBRIUM EXPRESSION

EQUILIBRIUM CONSTANT :-
                                                     Where Kc=Kf / Kr and is known as equilibrium constant.

UNIT:-

            The unit is mol/ dm^3.