Rate law for a reaction is given as:
$Rate=K\cdot[A]\cdot[B]^{2}$ What will be the order of this reaction?

Question

Rate law for a reaction is given as: 
$Rate=K\cdot[A]\cdot[B]^{2}$ What will be the order of this reaction?

Accepted Answer

The correct option is (4):

To determine the order of a reaction from its rate law, we sum the exponents of the concentration terms in the rate equation. The rate law given is $Rate=K\cdot[A]^{1}\cdot[B]^{2}$. Here, the exponent for reactant $[A]$ is $1$ (implicitly written as just $[A]$), and the exponent for reactant $[B]$ is $2$. The overall order of the reaction is the sum of these individual orders with respect to each reactant. Therefore, the overall order = $1$ (for $[A]$) + $2$ (for $[B]$) = $3$. This means the reaction is first order with respect to A, second order with respect to B, and third order overall.

The incorrect option is (1):
Option 1 suggests an overall order of 1. This would be the case if the rate law was, for example, $Rate=K\cdot$$A$$^{1}$ or $Rate=K\cdot$$B$$^{1}$. However, our given rate law has a sum of exponents equal to 3.

The incorrect option is (2):
Option 2 suggests an overall order of 2. This would be the case if the rate law was, for example, $Rate=K\cdot$$A$$^{2}$, $Rate=K\cdot$$B$$^{2}$, or $Rate=K\cdot$$A$$^{1}\cdot$$B$$^{1}$. Our rate law's exponents sum to 3, not 2.

The incorrect option is (3):
Option 3 suggests an overall order of 0. A zero-order reaction means the rate is independent of the concentration of the reactants. This would imply the exponents for both $$$A$$$ and $$$B$$$ are zero, which is not the case here.