Understanding Molecularity in Elementary Reactions

Molecularity is a fundamental concept in chemical kinetics that describes the number of reactant species (atoms, ions, or molecules) that are involved in a single step of a chemical reaction. This concept is specifically applicable to elementary reactions.

What is an Elementary Reaction?

An elementary reaction is a reaction that occurs in a single step. The stoichiometry of an elementary reaction directly represents the molecular event that takes place.

Determining Molecularity

The molecularity of an elementary reaction is determined by simply adding up the number of reactant molecules, atoms, or ions that participate in that specific step.

• Unimolecular reactions: Molecularity is one. Only one reactant species is involved.
• Bimolecular reactions: Molecularity is two. Two reactant species are involved.
• Termolecular reactions: Molecularity is three. Three reactant species are involved.

Reactions with molecularity greater than three are very rare because the probability of more than three species colliding simultaneously in the correct orientation is extremely low.

Analyzing the Given Elementary Reaction

The given elementary reaction is:

\( \text{NH}_4\text{NO}_2 \rightarrow \text{N}_2 + 2\text{H}_2\text{O} \)

In this reaction, we need to look at the reactant side to determine the molecularity because molecularity refers to the species involved in the collision leading to the reaction.

• Reactant(s): \( \text{NH}_4\text{NO}_2 \)
• Number of reactant species: There is only one molecule of \( \text{NH}_4\text{NO}_2 \) on the reactant side involved in this elementary step.

Calculating the Molecularity

Since only one molecule of \( \text{NH}_4\text{NO}_2 \) is participating as a reactant in this elementary reaction step, the molecularity of the reaction is the sum of the number of reactant species, which is 1.

Thus, the reaction \( \text{NH}_4\text{NO}_2 \rightarrow \text{N}_2 + 2\text{H}_2\text{O} \) is a unimolecular reaction.

Comparing Molecularity with Reaction Order

It is important not to confuse molecularity with reaction order. Reaction order is an experimentally determined quantity that describes how the rate of reaction depends on the concentration of reactants. For elementary reactions, the molecularity is equal to the reaction order with respect to that elementary step. However, for complex reactions (reactions occurring in multiple steps), the overall reaction order is determined from the rate law of the slowest step (the rate-determining step) and is not necessarily equal to the overall stoichiometry or molecularity of individual steps.

Conclusion on Molecularity

For the elementary reaction \( \text{NH}_4\text{NO}_2 \rightarrow \text{N}_2 + 2\text{H}_2\text{O} \), the molecularity is determined by the number of reactant molecules involved in this single step. There is one reactant molecule (\( \text{NH}_4\text{NO}_2 \)). Therefore, the molecularity is one.

Revision Table: Key Concepts

Additional Information on Elementary Steps and Molecularity

Most chemical reactions are not elementary reactions. They proceed through a series of elementary steps, which collectively form the reaction mechanism. Each elementary step has its own molecularity.

For example, the reaction \( \text{H}_2 + \text{I}_2 \rightarrow 2\text{HI} \) might appear to be a bimolecular elementary reaction with molecularity 2. However, under certain conditions, its mechanism involves multiple steps, and the overall reaction order might differ from its apparent stoichiometry. This highlights why molecularity is tied strictly to the elementary step, while reaction order is an experimental observation for the overall or elementary reaction.

Understanding molecularity helps chemists propose and analyze reaction mechanisms. The molecularity of the rate-determining step (the slowest step in a mechanism) often corresponds to the overall reaction order, especially when the steps preceding it are fast equilibria.