In which of the reaction of the following orders the molecularity and order can never be same ?

Options

A. zero order
B. First order
C. Second order
D. Third order

Answer

To solve the question of which reaction order has molecularity and order that can never be the same, we need to understand the definitions of molecularity and order in chemical kinetics.

### Step-by-Step Solution:

1. **Define Molecularity**:
- Molecularity refers to the number of reactant molecules that are involved in an elementary reaction step. It can take whole number values (1, 2, 3, etc.) and cannot be zero.

2. **Define Order of Reaction**:
- The order of a reaction is the sum of the powers of the concentration terms in the rate law expression. It can take any non-negative value, including fractions and zero.

3. **Identify the Key Difference**:
- The key difference between molecularity and order is that molecularity is always a whole number (1, 2, 3, etc.) while the order can be zero, fractional, or even negative.

4. **Consider Zero Order Reactions**:
- In a zero-order reaction, the rate of reaction is independent of the concentration of the reactants. The rate law for a zero-order reaction is expressed as:
\[ \text{Rate} = k \]
- Here, the order of the reaction is zero, which implies that the concentration of reactants does not affect the rate.

5. **Determine Molecularity in Zero Order Reactions**:
- Since molecularity refers to the number of molecules involved in the reaction, it cannot be zero. Therefore, for a zero-order reaction, the molecularity is not defined as zero.

6. **Conclusion**:
- Thus, in a zero-order reaction, molecularity and order are not the same. Molecularity cannot be zero, while the order can be zero.

### Final Answer:
In a zero-order reaction, the molecularity and order can never be the same.