Here are some examples of molecularity:
Example 1: Unimolecular Reaction
Consider the reaction of decomposition of hydrogen iodide (HI) gas:
2 HI(g) → H2(g) + I2(g)
In this reaction, two molecules of HI collide and react to form one molecule of H2 and one molecule of I2. Since only one molecule is directly involved in the reaction, it is classified as a unimolecular reaction.
Example 2: Bimolecular Reaction
An example of a bimolecular reaction is the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O):
2 H2(g) + O2(g) → 2 H2O(g)
In this case, two molecules, one of H2 and one of O2, collide and react to form two molecules of H2O. Therefore, it is a bimolecular reaction.
Example 3: Termolecular Reaction
A termolecular reaction involves the collision and reaction of three molecules simultaneously. An example of a termolecular reaction is the reaction between ozone (O3) and nitric oxide (NO) to form nitrogen dioxide (NO2) and oxygen gas (O2):
O3(g) + NO(g) → NO2(g) + O2(g)
Since three molecules participate in this reaction at the same time, it is classified as a termolecular reaction.
It is important to note that molecularity only provides information about the number of reactant species involved in the elementary step of a reaction, not the overall reaction mechanism. Many reactions may proceed through a series of elementary steps with different molecularities, leading to complex reaction mechanisms.
In summary, molecularity refers to the number of molecules or ions involved in a specific elementary reaction step, and can be classified as unimolecular, bimolecular, termolecular, or even more for reactions involving a larger number of species.
