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 solve the question "Which of the following is always true for a reaction?", we need to analyze the definitions and properties of order and molecularity in chemical reactions. Here’s a step-by-step breakdown:

### Step 1: Understand Molecularity
Molecularity refers to the number of reactant molecules that participate in a single reaction step. It can be classified as:
- **Unimolecular**: Involves one reactant (e.g., A → products).
- **Bimolecular**: Involves two reactants (e.g., A + B → products).
- **Trimolecular**: Involves three reactants (e.g., A + B + C → products).

**Key Point**: Molecularity can never be zero or fractional, and it does not exceed three.

### Step 2: Understand Order of Reaction
The order of a reaction is defined as the sum of the powers of the concentration terms in the rate law expression. It can be determined experimentally and does not necessarily correlate directly with molecularity.

**Key Point**: The order can be zero, fractional, or whole numbers, depending on the reaction mechanism.

### Step 3: Analyze the Options
Now, let's analyze the provided options:
1. **Order is equal to molecularity**: This is not always true. They can be the same in some cases (like unimolecular reactions) but differ in others.
2. **Order is always different from molecularity**: This is also incorrect, as they can be the same in certain reactions.
3. **Order and molecularity may be same or different**: This statement is true. There are cases where they are the same and cases where they are different.
4. **Molecularity may be zero or fractional**: This is false, as molecularity cannot be zero or fractional.

### Conclusion
Based on the analysis, the correct answer is:
**Order and molecularity may be same or different.**