Chemical reaction involves one or more than one molecule. A single-step reaction which takes place in one single step is called as an elementary reaction. While the chemical reaction that involves series of two or more step is called as a complex or complicated reaction. In the complicated reaction, the sequence of steps through which it takes place is called as the mechanism of chemical reaction. Each sequential step in the mechanism is an elementary step reaction. During the course of chemical reaction, these molecules constantly bump into each other. Based on that, the chemical reaction takes place.
The molecularity of reaction can be defined as the number of molecules reacting with each other in the reaction. In other words, it can be defined as the number of molecules which undergo collision that ultimately resulting in to a chemical reaction. Hence, molecularity of reaction indicates the number of molecules that takes part in the chemical reaction.
“The molecularity of chemical reaction can also be defined as the minimum number of reacting particles (either molecules, atoms, or ions) that come closer and collide in a rate determining step to form one product or more than one product.”
The decomposition of hydrogen peroxide (H2O2) is a complex reaction and it involves more than one step. This reaction takes place in two different steps.
This produces a molecule of water. This reaction can be divided in to following two steps.
From these two steps, first step is slow whereas the second step is fast. Now, as we know, the slowest step of complex reaction is a rate-determining step. In this case, the first step is rate-determining step. This step involves only one molecule of hydrogen peroxide. According to the above-mentioned definition of molecularity, this reaction is an example of unimolecular reaction.
Rate-expression or rate-law of reaction can be defined as the mathematical expression representing the dependence of rate of reaction on the concentration of reactants. The summation of the powers of the concentration terms of every reactant mentioned in the rate-expression is called as order of reaction.
Several reactions have the same molecularity and order of reaction. However, this is not true in every case. They might differ and it depends upon the reaction. Sometimes, these two terms might seem to be a bit confusing. Anyhow, there are several major differences between these two concepts, which are enlisted as follow.