Which of the following is always true for a reaction

Options

A. Order is equal to molecularity
B. Order is always different from molecularity
C. Order and molecularity may be same or different
D. Molecularity may be zero or fractional

Answer

To determine which statement is always true for a reaction, we need to analyze the options provided in the context of chemical kinetics.

### Step-by-Step Solution:

1. **Understanding Order and Molecularity**:
- **Order of a Reaction**: This is an experimentally determined quantity that indicates the power to which the concentration of a reactant is raised in the rate law expression. It can be a whole number, zero, or a fraction.
- **Molecularity**: This refers to the number of reactant species involved in an elementary reaction. It is always a whole number (1, 2, 3, etc.) and cannot be fractional.

2. **Evaluating the First Option**:
- The first option states that "order is equal to molecularity." This is only true for elementary reactions. For complex reactions, the order may differ from molecularity. Therefore, this statement is not always true.

3. **Evaluating the Second Option**:
- The second option claims that "order is always different from molecularity." This is incorrect because, in some cases (like elementary reactions), the order can be equal to the molecularity. Thus, this statement is also not always true.

4. **Evaluating the Third Option**:
- The third option states that "molecularity may be 0 or fractional." This is false because molecularity is defined as the number of reactant molecules participating in the reaction and cannot be zero or fractional.

5. **Evaluating the Fourth Option**:
- The fourth option states that "order can be the same as molecularity or different." This is true because for elementary reactions, the order is equal to the molecularity, while for non-elementary reactions, they can differ.

### Conclusion:
The correct statement that is always true regarding reactions is that the order can be the same as molecularity or different, making the fourth option the correct answer.