Which of the following is incorrect about the order of reaction?

The order of reaction cannot be fractional

The correct answer is option 3. The order of reaction cannot be fractional.

Let us delve deeper into each statement about the order of reaction to understand their correctness and the concepts associated with reaction kinetics.

Statement 1: It is calculated experimentally

The order of a reaction is determined through experimental observation rather than being predicted from theoretical considerations or stoichiometric coefficients alone.  The rate of reaction is measured under different initial concentrations of reactants. By varying one reactant's concentration while keeping others constant and observing how the rate changes, the order with respect to that reactant can be determined. The overall order of the reaction is the sum of the individual orders with respect to each reactant, as determined experimentally.

Example: For a reaction \( \text{A} + \text{B} \rightarrow \text{Products} \), if the rate law is found to be \( \text{Rate} = k[\text{A}]^2[\text{B}]^1 \), then the order with respect to A is 2 and with respect to B is 1.

Statement 2: It is sum of powers of concentration in rate law expression

The order of reaction is defined by the exponents of the concentration terms in the rate law expression. For a general reaction \( \text{A} + \text{B} \rightarrow \text{Products} \), the rate law can be expressed as:

\(\text{Rate} = k[\text{A}]^m[\text{B}]^n \)

where \( m \) and \( n \) are the orders with respect to A and B, respectively. Therefore, the overall order of the reaction is \( m + n \).

Example: If \( \text{Rate} = k[\text{A}]^2[\text{B}]^1 \), then the overall order is \( 2 + 1 = 3 \).

Statement 3: The order of reaction cannot be fractional

This statement is incorrect. The order of reaction can indeed be fractional under certain conditions. Fractional orders typically arise in complex reaction mechanisms where the rate-determining step involves intermediates whose concentrations do not directly correlate with the reactant concentrations in the rate law.

Example: For a hypothetical reaction \( \text{A} \rightarrow \text{Products} \), if the rate law is found to be \( \text{Rate} = k[\text{A}]^{1.5} \), then the reaction order with respect to A is 1.5, which is fractional.

Statement 4: There is not necessarily a connection between order and stoichiometry of a reaction

The order of reaction is determined by experimental kinetics and does not always correspond to the stoichiometric coefficients in the balanced chemical equation. While some reactions have orders that match their stoichiometry (e.g., elementary reactions), many complex reactions involve mechanisms where intermediates affect the observed reaction rate independently of the stoichiometric ratios.

Example: In a decomposition reaction where \( \text{A}_2 \rightarrow 2\text{A} \), the rate law might be \( \text{Rate} = k[\text{A}_2]^{0.5} \), indicating a fractional order with respect to \( \text{A}_2 \) that does not match its stoichiometric coefficient.

Conclusion: The incorrect statement among the options provided is: 3.The order of reaction cannot be fractional.

This statement is false because reaction orders can indeed be fractional, depending on the specific kinetics and mechanism of the reaction as determined experimentally. The other statements correctly describe fundamental aspects of reaction order determination and kinetics.