Question

For a reaction: $ A+{{H}_{2}}O\to B, $ $ Rate\propto [A]. $ What is its molecularity?

Answer 1

Hint :We know that at first think about the chemical reaction. The Molecularity of a reaction is defined as the number of molecules that come together in an elementary reaction and is equal to the sum of stoichiometric coefficients of reactions in elementary reactions.

Complete Step By Step Answer:
Molecularity of a reaction depends on how many molecules come together; a reaction can be unimolecular, bimolecular or termolecular. The kinetic order of any elementary reaction or reaction step is equal to its molecularity, and the rate equation of an elementary reaction can be determined from the molecularity. The kinetic order of a complex reaction cannot be equated to molecularity since it only describes only elementary reactions.. Order of reaction is the sum of the concentration term on which the rate of reaction actually depends and while molecularity is the number of atoms, ions or molecules that must collide to result into a chemical reaction.
Here we see that the Arrhenius equation includes the rate constant, activation energy, gas constant and the temperature, so the question arises what is the application of the Arrhenius equation. So the Arrhenius equation is used to determine the effect of temperature on the rate of reaction because the rate of reaction and rate constant are in direct relation that means if the rate constant gets doubled then the rate of reaction also becomes twice. Thus, the molecularity of this reaction is two as two molecules A and water collide to form product B.

Note :
Remember that the molecularity of a reaction is only applicable to elementary reactions. In complex reactions, the molecularity of a reaction is not applicable to the whole but the slowest step in a complex reaction is used as the molecularity of that reaction.