In any unimolecular reaction

Options

A. the order and molecularity of the slowest step are equal to one
B. molecularity of the reaction can be zero, one or one step
C. more then one reacting species are involved in one step
D. molecularity of the reaction can be determined only experimentally.

Explanation

To solve the question regarding unimolecular reactions, we will break down the concepts step by step.

### Step 1: Understand Unimolecular Reactions
Unimolecular reactions are reactions that involve a single reactant molecule undergoing a transformation. They are characterized as elementary reactions, meaning they occur in one step without any intermediates.

**Hint:** Remember that unimolecular reactions involve only one reactant molecule.

### Step 2: Define Molecularity
Molecularity refers to the number of reactant molecules involved in an elementary reaction. For unimolecular reactions, the molecularity is always 1 because only one molecule is involved in the reaction.

**Hint:** Molecularity is determined by counting the number of reactant molecules in the elementary step.

### Step 3: Define Order of Reaction
The order of a reaction is the sum of the powers of the concentration terms in the rate law expression. For unimolecular reactions, since they involve only one reactant, the order is also 1.

**Hint:** The order can be determined from the rate law, which is based on the concentrations of reactants.

### Step 4: Relationship Between Order and Molecularity
In unimolecular reactions, since there are no intermediates and the reaction occurs in a single step, the order of the reaction and the molecularity are equal. Therefore, both are 1.

**Hint:** In unimolecular reactions, the absence of intermediates means that the order and molecularity will match.

### Step 5: Analyze the Given Statements
Now, let’s analyze the statements provided in the question:

1. **Statement A:** In any unimolecular reaction, the order and the molecularity of the slowest step are equal to one.
- This statement is **correct** because both order and molecularity are 1 for unimolecular reactions.

2. **Statement B:** Molecularity of the reaction can be 0, 1, or more than 1.
- This statement is **incorrect**. For unimolecular reactions, molecularity cannot be 0; it must be at least 1.

3. **Statement C:** In any unimolecular reaction, more than one reacting species are involved in one step.
- This statement is **incorrect**. Unimolecular reactions involve only one reacting species.

4. **Statement D:** Molecularity of the reaction can be determined only experimentally.
- This statement is **incorrect**. While the order of the reaction is determined experimentally, molecularity can be inferred from the reaction mechanism.

### Conclusion
From the analysis, the only correct statement is Statement A.

**Final Answer:** The correct option is A.