Law of mass action is applicable toa)     Chemical reactions [Reversible]b)     Chemical reactions [Irreversible]c) Homogeneous equilibria and heterogeneous equilibriad)    Physical equilibria

Law of mass action states that the rate of  a reaction is directly propotional to product of active masses of reactants.Active masses in any physical process are taken as unity.Thus law of mass action cannot be applied to a physical process.Law of mass action can be applied to any chemical process which may be reversible (or) irreversible. It can be Homogeneous (or) Heterogeneous.