Lakhasly

The Law of Mass Action states that the rate of a chemical reaction is directly proportional to the concentrations of the reactants. For the reversible reaction A + B ⇌ C + D, the forward reaction rate (Vf) is proportional to the concentrations of A and B: Vf ∝ [A][B] or Vf = Kf[A][B], where Kf is the forward reaction rate constant. Similarly, the backward reaction rate (Vb) is proportional to the concentrations of C and D: Vb ∝ [C][D] or Vb = Kb[C][D], where Kb is the backward reaction rate constant. At equilibrium, the forward and backward rates are equal: Vf = Vb. This leads to the equation Kf[A][B] = Kb[C][D], which can be rearranged to give the equilibrium constant: K eq = [C][D] / [A][B] = Kf / Kb.

The Law of Mass Action:
The velocity of a chemical reaction is proportional to the active masses of the
reacting substances.
Consider this reversible reaction at constant temperature:
f
A + B C + D
b
The velocity of the forward reaction depends on the molar concentration of
both A & B:

Vf ɑ [A].[B] or Vf = Kf [A].[B]

Where Kf is the velocity constant of the forward reaction.
Similarly, the velocity of the backward reaction depends on the molar
concentration of both C & D:

Vb ɑ [C].[D] or Vb = Kb [C].[D]

Where Kb is the velocity constant of the backward reaction.
At Equilibrium:
Vf = Vb
Kf [A].[B] = Kb [C].[D]
[C].[D] / [A].[B] = Kf / Kb = K eq "Equilibrium Constant