Question

Which of the following statements is not correct about order of a reaction?
A.The order of a reaction can be a fractional number
B.Order of a reaction is experimentally determined quantity
C.The 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.

Answer 1

The order is the sum of the powers to which the concentration of reactants is raised in a rate law expression. The rate of the reaction ${\text{aA + bB}} \to {\text{cC}}$ is ${\text{rate = [A}}{{\text{]}}^{\text{x}}}{\text{ + [B}}{{\text{]}}^{\text{y}}}$ where an order is x + y. On varying the concentration of any of the reactants, the rate of the reaction also changes.

Complete step by step answer:
There are different orders for different reactions. It can be zero order, first order, second order, pseudo-first-order, etc
The exponent on each concentration term is the order of the reaction in that particular reactant
Since it is experimentally determined, it can be integers, fractions.
${\text{xA + yB}} \to {\text{P}}$ , ${\text{rate = k[A}}{{\text{]}}^{\text{p}}}{{\text{[B]}}^{\text{q}}}$ , we could see here that the stoichiometric coefficient and the power terms are different.
${\text{2}}{{\text{N}}_{\text{2}}}{{\text{O}}_{\text{5}}} \to {\text{4N}}{{\text{O}}_{\text{2}}}{\text{ + }}{{\text{O}}_{\text{2}}}$
Rate, ${\text{r = k[}}{{\text{N}}_{\text{2}}}{{\text{O}}_{\text{5}}}{\text{]}}$
It is a first-order reaction that is not equal to the stoichiometry of the reactants in a balanced chemical equation.
Now on checking the options, the order of a reaction can be a fractional number.
The order of a reaction is experimentally determined.
order is the sum of the powers to which the concentration of reactants is raised in a rate law expression.
Therefore, options A, B, and D have correct statements.
And option C is an incorrect statement.

Hence, the correct option is (C).

Note: In very few cases the experimentally determined powers on the concentration term in a rate law are equal to the stoichiometric coefficient from a balanced equation. But this cannot be generalized.
${\text{2NO + }}{{\text{O}}_{\text{2}}} \to {\text{2N}}{{\text{O}}_{\text{2}}}$
${\text{rate = k[NO}}{{\text{]}}^{\text{2}}}{\text{[}}{{\text{O}}_{\text{2}}}{\text{]}}$ which is equal to the stoichiometric coefficient from the balanced equation.
The order is different from molecularity because molecularity is not an experimentally determined value. It is the number of reacting species.