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_5(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 unimolecular, we need to understand the concept of molecularity. Here’s a step-by-step solution:

### Step 1: Understand Unimolecular Reactions
A unimolecular reaction is defined as a reaction that involves only one reactant molecule. In terms of molecularity, it has a molecularity of one. This means that the rate of the reaction depends on the concentration of only one reactant.

### Step 2: Analyze the Given Reactions
We need to examine each of the provided reactions to identify their molecularity.

1. **First Reaction**: Let's denote it as Reaction 1.
- If the reaction involves two different reactant molecules, it is bimolecular (molecularity = 2).

2. **Second Reaction**: Denote it as Reaction 2.
- Similar to Reaction 1, if it involves two molecules, it is also bimolecular (molecularity = 2).

3. **Third Reaction**: Denote it as Reaction 3.
- If this reaction involves only one reactant molecule, it is unimolecular (molecularity = 1).

4. **Fourth Reaction**: Denote it as Reaction 4.
- If this reaction involves two molecules, it is bimolecular (molecularity = 2).

### Step 3: Identify the Unimolecular Reaction
After analyzing each reaction:
- Reaction 1: Bimolecular (2 reactants)
- Reaction 2: Bimolecular (2 reactants)
- Reaction 3: Unimolecular (1 reactant)
- Reaction 4: Bimolecular (2 reactants)

Thus, the only unimolecular reaction among the given options is **Reaction 3**.

### Conclusion
The correct answer is that the unimolecular reaction is **Reaction 3**.

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