Amongst the following chemical reactions, the unimolecular reaction is:

Options

A. `H_2(g)+Cl_2(g) to 2HCl(g)`
B. `PCl_3(g)+Cl_2(g) to PCl_2(g)`
C. `N_2O_5(g) to N_2O_4(g)+1/2 O_2(g)`
D. `2HI(g) to H_2(g)+I_2(g)`

Answer

To determine which of the given chemical reactions is a unimolecular reaction, we need to understand the concept of molecularity. Here’s a step-by-step solution:

### Step 1: Understand Molecularity
Molecularity refers to the number of reactant molecules involved in an elementary reaction.
- **Unimolecular Reaction**: Involves only one reactant molecule.
- **Bimolecular Reaction**: Involves two reactant molecules.

### Step 2: Analyze the Given Reactions
Now, we will analyze the provided options to identify the number of reactants in each reaction.

1. **Option 1**: H2 + Cl2 → Products
- Reactants: 2 (H2 and Cl2)
- Conclusion: This is a bimolecular reaction.

2. **Option 2**: 2NO → Products
- Reactants: 2 (2 molecules of NO)
- Conclusion: This is also a bimolecular reaction.

3. **Option 3**: N2O5 → Products
- Reactants: 1 (only N2O5)
- Conclusion: This is a unimolecular reaction.

4. **Option 4**: 2H2 + O2 → Products
- Reactants: 3 (2 molecules of H2 and 1 molecule of O2)
- Conclusion: This is a bimolecular reaction.

### Step 3: Identify the Unimolecular Reaction
From the analysis, we see that:
- **Option 3** (N2O5 → Products) involves only one reactant molecule and is therefore a unimolecular reaction.

### Final Answer
The unimolecular reaction among the given options is **Option 3**.

---