Among the following chemical reactions the irreversible reaction is

Options

A. `PCl_(5)(g) to PCl_(3)(g)+Cl_(2)(g)`
B. `Pb(NO_(3))_(2)(l) +2KI(l) to PbI_(2)(s)+2KNO_(3)(l)`
C. `CaCO_(3)(s) to CaO(s) +CO_(2)(g)`
D. `2SO_(2)(g)+O_(2)(g) to 2SO_(3)(g)`

Answer

To determine which of the given chemical reactions is irreversible, we need to analyze each reaction based on the definitions of reversible and irreversible reactions.

### Step-by-Step Solution:

1. **Understanding Irreversible Reactions**:
- An irreversible reaction proceeds in one direction only, meaning the reactants convert to products, and the products do not revert back to the original reactants. This is typically denoted with a single arrow (→).

2. **Analyzing Each Reaction**:
- **Reaction 1**: \( \text{PCl}_5(g) \rightarrow \text{PCl}_3(g) + \text{Cl}_2(g) \)
- This reaction involves only gaseous substances. The products can react to form the reactants again, indicating that this is a reversible reaction.

- **Reaction 2**: \( \text{Pb(NO}_3)_2(l) + \text{KI}(l) \rightarrow \text{PbI}_2(s) + \text{KNO}_3(l) \)
- Here, lead iodide (\( \text{PbI}_2 \)) forms a solid precipitate. The formation of a precipitate suggests that this reaction is irreversible, as the solid does not revert back to the reactants under normal conditions.

- **Reaction 3**: \( \text{CaCO}_3(s) \rightarrow \text{CaO}(s) + \text{CO}_2(g) \)
- This reaction is a decomposition reaction. Calcium carbonate decomposes to form calcium oxide and carbon dioxide. The reverse reaction can occur, making this a reversible reaction.

- **Reaction 4**: \( 2 \text{SO}_2(g) + \text{O}_2(g) \rightarrow 2 \text{SO}_3(g) \)
- This reaction involves gaseous reactants and products. The reaction can proceed in both directions, indicating that it is also reversible.

3. **Conclusion**:
- Among the given reactions, the only irreversible reaction is **Reaction 2**: \( \text{Pb(NO}_3)_2 + \text{KI} \rightarrow \text{PbI}_2 + \text{KNO}_3 \), as it results in the formation of a precipitate and does not revert back to the original reactants.

### Final Answer:
The irreversible reaction among the given options is **Reaction 2: \( \text{Pb(NO}_3)_2 + \text{KI} \rightarrow \text{PbI}_2 + \text{KNO}_3 \)**.

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