Question

For a reaction rate law expression is:
$Rate = k{\text{ }}{\left[ A \right]^{1/2}}{\left[ B \right]^2}$
Can this reaction be elementary? Explain.

Answer 1

Elementary reactions are those chemical reactions in which one or more reactants chemically react directly to form one or more products in a single step or a single transition state.

Complete step by step answer: For the solution of a reaction let us consider a general reaction
$A + B \to {\text{Product}}$
Rate law is defined as the expression in which rate of reaction is expressed in terms of molar concentration of reactant with each term raised to some power which may or may not be the same as the stoichiometric coefficient of that reactant in a balanced chemical equation.
According to rate law: $rate{\text{ }}of{\text{ }}reaction \propto [A] \times [B]$
Since there is given that rate of reaction = $k{[A]^{1/2}}{[B]^2}$
As we know the order of reaction = sum of the power raised to concentration term. So, $order{\text{ }}of{\text{ }}reaction = \dfrac{1}{2} + 2$
$Order{\text{ }}of{\text{ }}reaction = \dfrac{{1 + 4}}{2} = \dfrac{5}{2} = 2.5$
Since, the order of the reaction is not equal to the sum of the stoichiometric coefficients of each reactant so it is not considered to be an elementary reaction.

Additional Information: The reaction which is not carried out in a single step is considered to be a complex reaction. Every single step of a complex reaction is considered to be an elementary step. And, the slowest step is known as rate determining step.
A series of step reactions or elementary reactions proposed to account for overall reaction is called the mechanism of the reaction.

Note: In case of elementary reaction the order of reaction is equal to the molecularity of the reaction. But in case of change in condition like pressure and temperature the reaction may not remain as elementary reaction hence the molecularity of reaction gets changed.