Question

Reactions can be classified as unimolecular, bimolecular, and so on. Why are there no zero-molecular reactions? Explain why termolecular reactions are rare.

Short answer

Zero-molecular reactions are impossible, as reactions involve at least one molecule. Termolecular reactions are rare due to the improbability of three molecules colliding favorably at once.

Step-by-step solution

Define Reaction Molecularity

The molecularity of a reaction is defined as the number of molecules that come together to react in an elementary reaction. It indicates the number of reacting species or molecules involved in a particular step of the reaction.

Analyze Zero-Molecular Reactions

A zero-molecular reaction would imply that no molecules are involved in the reaction process to convert reactants into products. However, reactions require at least one molecule to interact, therefore zero-molecular reactions are impossible physically or chemically.

Characterize Bimolecular Reactions

Bimolecular reactions involve two reacting molecules or species coming together to form products. These are common because two reactants colliding is practical due to molecular motion and concentration.

Understand Termolecular Reactions

Termolecular reactions involve three molecules colliding simultaneously to give products. Such events are rare because the probability of three reactant molecules simultaneously colliding with the proper orientation and sufficient energy is extremely low.

Summarize Reaction Probability

Reactions involving more than bimolecular collisions, such as termolecular reactions, have a low probability of occurrence. This is due to the complexity and specificity needed for three molecules to collide with correct orientation and energy.

Key concepts

Unimolecular Reactions

Unimolecular reactions are the simplest type of chemical reactions. They occur when a single molecule undergoes a transformation to produce one or more products. This type of reaction can happen when a molecule absorbs energy, often from heat or light, initiating a rearrangement or decomposition process.
An example is the decomposition of ozone (O_3) into oxygen (O_2) and an oxygen atom (O). These reactions typically have the reaction rate depending solely on the concentration of the single reactant.
This results in a first-order kinetic reaction where the rate law can be expressed as $extRate=k[extA]$. Here, $k$ is the rate constant, and $[extA]$ is the concentration of the unimolecular reactant.

Bimolecular Reactions

Bimolecular reactions involved two reactant molecules coming together to form products. This is a common type of reaction because it relies on the collisions of two molecules. When these molecules collide with the right alignment and sufficient energy, a reaction can take place.
A good example is the reaction between nitrogen dioxide (NO_2) and carbon monoxide (CO) to produce nitrogen monoxide (NO) and carbon dioxide (CO_2).
The rate of a bimolecular reaction depends on the concentrations of both reactants, often following second-order kinetics. The rate expression may appear as $extRate=k[extA][extB]$, where $k$ is the rate constant, and $[extA]$ and $[extB]$ are the concentrations of the two reactants.

Termolecular Reactions

Termolecular reactions occur when three molecules collide simultaneously with the correct orientation and energy to form products.
Because these conditions are stringent, termolecular reactions happen less frequently compared to unimolecular and bimolecular reactions.
An example would be the reaction of two nitrogen dioxide (NO_2) molecules with an oxygen (O_2) molecule. Due to the low probability of simultaneous trios of collisions, actual termolecular reactions are rare, and many are believed to proceed through a series of bimolecular steps instead.
Termolecular reactions are typically described by third-order kinetics, with a rate law such as $extRate=k[extA][extB][extC]$, where $k$ is the rate constant, and $[extA],[extB],[extC]$ are the concentrations of the three reactants.