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# Law of mass action cannot be applied to:A. Decomposition of gaseous $HI$B. Decomposition of gaseous $PC{{l}_{5}}$ .C. Transition of Rhombic Sulphur to monoclinic sulphurD. Decomposition of Calcium carbonate

Last updated date: 13th Jun 2024
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Hint: Law of mass action says that the rate of a chemical reaction is directly proportional to the concentration of the substances involved in the chemical reaction. Law of mass action can be applicable to the reaction where chemical equilibrium exists.

- In the question it is given that the law of mass action cannot be applied to which option among the given options.
- Coming to the given options. Option A, Decomposition of gaseous HI. It is a chemical reaction in which HI is going to dissociate into hydrogen and iodine and the products are in chemical equilibrium with the reactants.
- The dissociation of HI is as follows.
$2HI\rightleftarrows {{H}_{2}}+{{I}_{2}}$
- The option A is an example where chemical equilibrium exists.
- Coming to option B, Decomposition of gaseous $PC{{l}_{5}}$ .
- It is a chemical reaction in which $PC{{l}_{5}}$ is going to dissociate in to $PC{{l}_{3}}$ and $C{{l}_{2}}$ , these products are in chemical equilibrium with the reactants.
- The dissociation of $PC{{l}_{5}}$ is as follows.
$PC{{l}_{5}}\rightleftarrows PC{{l}_{3}}+C{{l}_{2}}$
- The option B is an example where chemical equilibrium exists.
- Coming to option C, Transition of Rhombic Sulphur to monoclinic sulphur. We know that it is a physical equilibrium means change in physical property occurs.
- Therefore on option C Law of mass action cannot be applied.
- Coming to option D, Decomposition of Calcium carbonate.
- The chemical reaction of decomposition of Calcium carbonate is as follows.
$CaC{{O}_{3}}\rightleftarrows CaO+C{{O}_{2}}$
- The option D is an example where chemical equilibrium exists.
- Therefore, Transition of Rhombic Sulphur to monoclinic sulphur is a physical equilibrium not a chemical equilibrium so law mass action cannot be applied.

So, the correct option is C.

Note: Physical equilibrium won’t come under the law of mass action. Law of mass action is applicable where chemical equilibrium exists. For physical changes, the law of mass action cannot be applied in any case.