To solve the question, we need to evaluate the statements provided and determine which one is incorrect. Let's analyze each statement step by step.
### Step 1: Analyze the first statement
**Statement 1:** Molecularity of a reaction is always a whole number.
- **Explanation:** Molecularity refers to the number of reactant particles (atoms, ions, or molecules) that collide simultaneously to bring about a reaction. Since you cannot have a fraction of a particle colliding, molecularity must always be a whole number (1, 2, 3, etc.). This statement is **correct**.
### Step 2: Analyze the second statement
**Statement 2:** Order and molecularity of a reaction need not be the same.
- **Explanation:** The order of a reaction is determined from the rate law and can be a whole number, fraction, or even zero. However, molecularity is strictly a whole number. In some cases, the order and molecularity can be the same, but they do not have to be. For example, in the reaction of ethyl acetate with sodium hydroxide, both the order and molecularity can be equal to 2. Therefore, this statement is **incorrect**.
### Step 3: Analyze the third statement
**Statement 3:** Order of reaction may be 0.
- **Explanation:** Yes, the order of a reaction can indeed be zero. This occurs in certain reactions where the rate is independent of the concentration of the reactants. An example is the reaction between hydrogen and chlorine on the surface of water to form hydrogen chloride, which has a zero order. This statement is **correct**.
### Step 4: Analyze the fourth statement
**Statement 4:** Order of a reaction depends upon the mechanism of the reaction.
- **Explanation:** The order of a reaction is derived from the rate law, which is influenced by the mechanism of the reaction. The coefficients in the rate law (which determine the order) are determined experimentally and reflect how the concentration of reactants affects the rate of reaction. Therefore, this statement is **correct**.
### Conclusion
After analyzing all the statements, we find that the **incorrect statement is Statement 2**: "Order and molecularity of a reaction need not be the same."
### Final Answer
**The incorrect statement is:** Statement 2.
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