Which of the following behaves as an irreversible reaction when conducted in a closed vessel

Options

A. synthesis of ammonia
B. decomposition of `PCI_5` solid
C. formation of `SO_(2)` from `SO_2 & O_2`
D. precipitation of `Cl^(-)` by `AgNO_3`

Answer

To determine which of the given reactions behaves as an irreversible reaction when conducted in a closed vessel, we need to analyze each reaction provided in the question.

### Step-by-Step Solution:

1. **Identify the Reactions**: The reactions given are:
- Synthesis of ammonia: \( N_2 + 3H_2 \rightleftharpoons 2NH_3 \)
- Decomposition of \( PCl_5 \): \( PCl_5 (s) \rightleftharpoons PCl_3 (g) + Cl_2 (g) \)
- Formation of \( SO_3 \): \( SO_2 (g) + O_2 (g) \rightleftharpoons 2SO_3 (g) \)
- Precipitation of \( CN^- \) by \( AgNO_3 \): \( CN^- (aq) + AgNO_3 (aq) \rightarrow AgCN (s) + NO_3^- (aq) \)

2. **Analyze Each Reaction**:
- **Synthesis of Ammonia**: This reaction is reversible because the products can react to form the reactants again under certain conditions.
- **Decomposition of \( PCl_5 \)**: This reaction is also reversible, as \( PCl_3 \) and \( Cl_2 \) can recombine to form \( PCl_5 \).
- **Formation of \( SO_3 \)**: This is a reversible reaction as well, where \( SO_2 \) and \( O_2 \) can react to form \( SO_3 \) and vice versa.
- **Precipitation of \( CN^- \) by \( AgNO_3 \)**: This reaction is irreversible because once \( AgCN \) precipitates, it does not revert back to \( CN^- \) and \( AgNO_3 \) under normal conditions.

3. **Conclusion**: Among the given reactions, the precipitation reaction of \( CN^- \) by \( AgNO_3 \) behaves as an irreversible reaction when conducted in a closed vessel.

### Final Answer:
The reaction that behaves as an irreversible reaction when conducted in a closed vessel is the precipitation of \( CN^- \) by \( AgNO_3 \).

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