Law of mass action can not be applied to

Options

A. `2HI Leftrightarrow H_(2)+I_(2)`
B. `PCl_(5) Leftrightarrow PCl_(3)+Cl_(2)`
C. `"Water "Leftrightarrow " Ice"`
D. `CaCO_(3) Leftrightarrow CaO+CO_(2)`

Answer

To answer the question regarding the applicability of the law of mass action, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Law of Mass Action**:
The law of mass action states that the rate of a chemical reaction is directly proportional to the product of the concentrations (or masses) of the reactants, each raised to the power of their respective stoichiometric coefficients in the balanced chemical equation.

2. **Identify the Context of the Law**:
The law of mass action is applicable only in the context of chemical equilibrium. This means it can be used to describe reactions that reach a state where the rate of the forward reaction equals the rate of the reverse reaction.

3. **Differentiate Between Chemical and Physical Equilibrium**:
Chemical equilibrium involves reactions where reactants convert to products and vice versa, while physical equilibrium involves changes in state (e.g., solid to liquid) without any chemical change.

4. **Analyze the Given Options**:
- Option 1: Dissociation of \(2HI \rightleftharpoons H_2 + I_2\) - This is a chemical equilibrium.
- Option 2: Dissociation of \(PCl_5 \rightleftharpoons PCl_3 + Cl_2\) - This is also a chemical equilibrium.
- Option 3: Conversion of water to ice (\(H_2O (l) \rightleftharpoons H_2O (s)\)) - This is a physical equilibrium.
- Option 4: Dissociation of \(CaCO_3 \rightleftharpoons CaO + CO_2\) - This is a chemical equilibrium.

5. **Determine the Correct Answer**:
Since the law of mass action cannot be applied to physical equilibrium, the correct answer is **Option 3**, which describes the conversion of water to ice.

### Final Answer:
The law of mass action cannot be applied to the conversion of water to ice (Option 3).

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