Which of the following statements is not correct about order of a reaction ?

Options

A. The order of a reaction can be a fractional number.
B. Order of a reaction is experimentally determined quantity
C. Order of a reaction is always equal to the sum of the stoichiometric coefficients of reactants in the balanced chemical equation for a reaction.
D. The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression.

Explanation

To determine which statement is not correct about the order of a reaction, we need to analyze the definitions and properties of reaction order. Here’s a step-by-step solution:

### Step 1: Understand the Definition of 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 reflects how the rate of reaction is affected by the concentration of reactants.

**Hint:** Recall that the order can be determined from the rate law, which is derived from experimental data.

### Step 2: Analyze Each Statement
We need to evaluate the given statements about the order of a reaction:

1. **Statement 1:** The order of reaction can be a fractional number.
- This statement is **true**. The order can indeed be fractional (e.g., 1.5, 2.5) based on experimental observations.

2. **Statement 2:** The order of reaction is an experimentally determined quantity.
- This statement is also **true**. The order is determined through experiments and is not directly related to stoichiometric coefficients.

3. **Statement 3:** The order of reaction is always equal to the sum of the stoichiometric coefficients of the reactants in the balanced chemical equation.
- This statement is **not true**. The order of a reaction does not always equal the stoichiometric coefficients. For example, in the decomposition of N2O5, the rate depends only on the concentration of N2O5, giving it an order of 1, while the stoichiometric coefficients sum to 2.

4. **Statement 4:** The order of reaction is the sum of the powers of the molar concentration of reactants.
- This statement is **true** as it aligns with the definition of order.

### Step 3: Identify the Incorrect Statement
From the analysis, we see that the third statement is incorrect. The order of a reaction is not always equal to the sum of the stoichiometric coefficients.

**Final Answer:** The statement that is not correct about the order of a reaction is: "The order of reaction is always equal to the sum of the stoichiometric coefficients of the reactants in the balanced chemical equation."