Which statement of the following is incorrect .

Options

A. Molecularity of a reaction is always a whole number
B. Order and molecularity of a reaction need not be same
C. order of reaction may be zero
D. Order of a reaction depends upon the mechanism of the reaction

Answer

To solve the question of which statement is incorrect regarding molecularity and order of reactions, we will analyze each option step by step.

### Step 1: Analyze Option A
**Statement:** Molecularity of a reaction is always a whole number.

**Explanation:**
- Molecularity refers to the number of reactant species (atoms, ions, or molecules) that collide simultaneously to bring about a reaction.
- Since you cannot have a fraction of a molecule or atom, molecularity is always expressed as a whole number.

**Conclusion:** This statement is **correct**.

### Step 2: Analyze Option B
**Statement:** Order and molecularity of a reaction need not to be the same.

**Explanation:**
- The order of a reaction is determined from the rate law expression, which can be different from the molecularity of the reaction.
- For example, in a complex reaction, the order can differ from the molecularity. In some cases, the order can be determined experimentally and may not reflect the stoichiometry of the reaction.

**Conclusion:** This statement is **correct**.

### Step 3: Analyze Option C
**Statement:** Order of a reaction may be zero.

**Explanation:**
- There are reactions where the rate is independent of the concentration of the reactants, which leads to a zero-order reaction.
- For example, in the decomposition of hydrogen iodide on a gold catalyst, the rate does not depend on the concentration of hydrogen iodide, thus making the order zero.

**Conclusion:** This statement is **correct**.

### Step 4: Analyze Option D
**Statement:** Order of a reaction depends upon the mechanism of the reaction.

**Explanation:**
- The order of a reaction is determined by the rate law expression and is not necessarily related to the mechanism of the reaction.
- The order can be determined experimentally and does not always reflect the molecularity or the steps involved in the mechanism.

**Conclusion:** This statement is **incorrect**.

### Final Answer
The incorrect statement is **Option D**: "Order of a reaction depends upon the mechanism of the reaction."

---