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

The order of reaction is always equal to the sum of the stoichiometric coefficient of reactants in the balanced chemical equation of a reaction.

The correct answer is option 2. The order of reaction is always equal to the sum of the stoichiometric coefficient of reactants in the balanced chemical equation of a reaction.

The order of a reaction refers to the exponent to which the concentration of each reactant is raised in the rate law equation. It's a crucial concept in chemical kinetics as it indicates how the rate of a reaction depends on the concentration of its reactants.

Let is delve deeper into the explanations for each statement:

1. "The order of a reaction is the sum of the powers of molar concentration of the reactants in the rate law expression." - This statement is correct because the rate law expression is derived experimentally, and the exponents on the concentrations of the reactants in this expression represent the individual orders of the reaction with respect to each reactant. Adding these exponents gives the overall order of the reaction.

2. "The order of reaction is always equal to the sum of the stoichiometric coefficient of reactants in the balanced chemical equation of a reaction." - This statement is incorrect. While sometimes the order of a reaction may coincide with the stoichiometric coefficients, this is not always the case. The stoichiometric coefficients in the balanced chemical equation only indicate the ratios in which reactants are consumed but do not necessarily dictate the rate at which the reaction proceeds.

3. "The order of a reaction can be a fractional number." - This statement is correct. The order of a reaction can indeed be a fraction based on experimental observations. A fractional order suggests a complex reaction mechanism where the rate of reaction does not directly correlate with the concentration of the reactants. Such cases are not uncommon, especially in reactions with multiple steps or intermediate species.

4. "Order of a reaction is an experimentally determined quantity." - This statement is correct. Unlike stoichiometric coefficients, which are determined from the balanced chemical equation, the order of a reaction cannot be predicted solely from the equation. It must be determined experimentally by conducting kinetic experiments, such as measuring the rate of reaction under different initial concentrations of reactants and analyzing the resulting data to derive the rate law and determine the order of the reaction.

In summary, while the stoichiometric coefficients in the balanced chemical equation provide important information about the stoichiometry of the reaction, they do not necessarily dictate the order of the reaction, which is an experimentally determined quantity derived from the rate law expression.