Law of Mass Action
For the reaction
2 NO2 = N2O4
in a sealed tube the ratio
is a constant. This phenomenon is known as chemical equilibrium. The ratio is called equilibrium constant (K).
[N2O4] and [NO2] are molar concentrations of N2O4 and NO2.
Such a law of nature is called the law of mass action or mass action law.
Of course, when conditions, such as pressure and temperature, change, a period of time is required for the system to establish an equilibrium.
For systems that are not at equilibrium yet, the ratio calculated from the mass action law is called a reaction quotient Q. The Q values of a closed system have a tendency to reach a limiting value called equilibrium constant K over time. A system has a tendency to reach an equilibrium state.
The law of mass action may be written as:
The rate of a chemical reaction at any particular temperature is proportional to the product of the molar concentrations of reactants with each concentration term raised to the power equal to the number of molecules of the respective reactants taking part in the reaction.
In the chemical kinetics chapter we come to know that chemical reactions can be elementary reactions or complex reactions having number of elementary reactions.
Law of mass action is valid for elementary reactions.