Law of Mass Action-


 source : slide

Law of Mass Action

Law of mass action was given by  .

“At constt. temp, the rate of reacting substance is proportional to its active mass & rate of a chemical reaction is directly proportional to the product of the active masses of the reacting substances.

Rate of Reaction ∝  [A]

A+B <——-> C+D

Rate of reaction ∝  [A][B]

Applications of law of Mass Action-

Consider a homogeneous reaction which has attained equilibrium at particular temp.

A +B <——->C +D

[A],[B],[C] &[D] are the active masses of A,B,C,D, respectively

A/c to law of mass action-

Rate of forward reaction ∝  [A][B]

Rf= Kf[A][B]

Kf is velocity const. of forward reaction

Rate of back ward reaction ∝ [C][D]

Rb= Kb [C][D]

Kb= velocity constt. of back ward reacn

At equi.,

Rate of forward reaction= Rate of bakward reaction

Rf= Rb

Kf[A][B] = Kb[C][D]

Kf /Kb = [C][D] / [A][B]

Kf /Kb = Kc

here Kc=equilibrium  constant

Kc= [C][D] / [A][B]

Suppose a reversible reaction at constant temp. is

aA+ bB <——> cC+ dD

[A]a, [B]b, [C]c, [D]d are the active masses of A  , B , C & D respectively.

A/c to law of mass action,

Rate of forward reaction ∝  [A] a[B]b

Rf= Kf [A] a[B]b

Kf=velocity constant for forward reaction


Rate of backward reaction ∝  [C]c[D]d

Rb= Kb [C] c[D]d

Kb= velocity constt. for back ward reaction

At equilibrium,


Kf [A] a[B]b   =Kb [C] c[D]d

Kf /Kb =[C] c[D]d / [A] a[B]b

Kf /Kb = Kc

here Kc=equilibrium  constant

Kc= [C] c[D]d / [A] a[B]b