Norwegian Chemists Waage and Gulberg were the ones who gave us law of mass action. This law of mass action suggest a quantitative relationship between the rates of the reaction and the concentration of the reacting substances.
Law of mass action states that:
The rate of a chemical reaction is directly proportional to the product of the molar concentrations of the reactants at any constant temperature at any given time.
The molar concentration i.e. number of moles per liter also known as active mass. It is expressed by enclosing the symbols of formulae of the substance in square brackets. For example, molar concentration of A is expressed as [A].
Equilibrium constant in terms of law of mass action:
For any reversible reaction, The law of mass action is applied to derive a mathematical expression for equilibrium constant. This is known as law of chemical equilibrium.
Let us suppose a simple reversible reaction:
X + Y ⇌ A + B
In which equilibrium exists between the reactants (X and Y) and the products (A and B). The forward reaction is:
X + Y ————> A + B
According to law of mass action,
Rate of forward reaction = kb [A] [B]
Where kb is the rate constant for the backward reaction and [A] and [B] are molar concentrations of the products A and B respectively.
At equilibrium, the rate of two opposing reactions becomes equal. Therefore, at equilibrium:
Rate of forward reaction = Rate of backward reaction i.e.
kf [X] [Y] = kb [A] [B]
kf / kb = [A] [B] / [X] [Y]
The combined constant, K which is equal to kf / kb is called equilibrium constant and has value for a reaction at a given temperature. The above equation is known as law of chemical equilibrium.
For a general reaction:
xX + yY⇌ aA+ bB
The equilibrium constant will be given as:
K = [A] a [B] b / [X] x [Y] y
Where x, y, a and b have the same values as those in the balanced chemical equation. Hence the equilibrium constant may be given as:
The ratio between the products of molar concentrations of the products to that of the molar concentrations of the reactants with each other concentration term raised to the power equal to its Stoichiometric coefficient in the balanced chemical reaction as constant temperature.
