The Law of Mass Action

The Law of Mass Action

Guldberg and Waage gave this law in 1869. It states that “the rate of chemical reaction is directory proportional to the product of the molar concentrations (active mass) of the reactants at the given temperature”

The number of moles of a substance in one liter is called its molar concentration or active mass. Consider for example, the general reversible reaction.

           Kf

A + B ↔ C +D

          Kr

According to law of mass action

The rate of forward reaction α [A] [B]

Or

The rate of forward reaction = K_f [A] [B]

Where K_f is forward rate constant .

Similarly, rate of backward reaction α [C] [D]

                                                        =K_r [C] [D]

Were K_r is reverse rate constant.

At the Chemical equilibrium stat;

.

, the rates of both the reactions are same at the state of equilibrium.

K_f [A]^a [B]^b =K_r [C]^c [D]^d

k_f            [C]^c [D]^d
----     =    -----------
K_r             [A]^a [B]^b

K_f and K_r are constant at constant temperature and the ratio of K_f / K_r is also constant at constant temperature. It is represented by K and is termed as equilibrium constant. The above reaction is given as

         [C]^c [D]^d
K = -----------
         [A]^a [B]^b

and the expression is termed as law of chemical equilibrium.

The law of chemical equilibrium states the product of molar concentration of the products raised to the power equal to its co-efficient, divided by the product of the molar concentration of the reactants raised to its co-efficient, is constant at constant temperature and is termed as equilibrium constant.

Characteristics of equilibrium constant

1. Its value remains constant at a given temperature irrespective of the direction of approach.

2. The value of the equilibrium constant remains constant at given temperature and pressure irrespective of the concentration of the reactants and products.

3. The value of equilibrium constant depends on the nature and temperature of the reaction but it remains unaffected in the presence or absence of catalyst.

4. It gives information about the reaction proceeding in a particular direction at a given temperature.

Calculation of Molar concentration

From the above discussion, it can be seen that it is essential to determine the molar concentration of the reactants and products in order to determine the equilibrium constant.

Here is an example for the calculation of molar concentration.

Consider the reaction

                          H₂ + I₂ ↔ 2HI

Let a and b be the initial moles of the two reactants.

Let x be the number of moles of the reactants that have reacted at the point of equilibrium.

Now, the above reaction can be given as follows:

Initial moles                                    a         b          0
                                                      H₂ +     I₂ ↔    2HI