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Law of mass action is applicable to :
(A) homogeneous chemical equilibrium only
(B) heterogeneous chemical equilibrium
(C) physical equilibrium
(D) both homogeneous and heterogeneous chemical equilibrium

Last updated date: 22nd Feb 2024
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IVSAT 2024
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Hint: The law of mass action says that the rate at which a chemical reaction will occur depends upon the concentration of the reacting species present. The reacting species can be present in any state whether solid, liquid or gases and the state can be different from that of products.

Complete step by step solution:
First, let us see what the law of mass action is.
The law of mass action states that the rate of a chemical reaction is proportional to the concentrations of the reacting substances.
The mass that is actually reacting is called the active mass.
The law explains the behaviour of solutions in the equilibrium. This law provides a relationship between the concentrations of electron holes and free electrons when the semiconductor is in state of thermal equilibrium. Thus, it is widely used in the manufacture of electronics and semiconductors.
Most of the chemical reactions are reversible in nature and many times an equilibrium state is present at which the concentration of reactant is equal to the concentration of the product. At this point, the rate of the forward reaction is equal to the rate of backward reaction. This is the equilibrium point.
Let us take a example-
$N{a_2}C{O_3} + CaC{l_2} \rightleftharpoons CaC{O_3} + 2NaCl$
In this reaction, all the reactants and products are in the same state i.e. aqueous state. So, this is a homogeneous system.
The equilibrium constant of this reaction can be written as -
${K_C} = \dfrac{{[CaC{O_3}][NaCl]}}{{[N{a_2}C{O_3}][CaC{l_2}]}}$
Where ${K_C}$ is the equilibrium constant.
Similarly, we can find the equilibrium constant with any heterogeneous reaction.
Thus, the law is applicable for both the homogeneous and heterogeneous chemical equilibrium.

So, option (D) is the correct answer.

Note: The reaction in which reactants and products are in the same state represents the homogeneous system while the reaction in which the reactants and products are in different states represent the heterogeneous system. It does not matter the state of reacting species until activation energy for the reaction to occur is present.