Question

What is an elementary step? What is the molecularity of a reaction?

Short answer

An elementary step is a single reaction step in a mechanism. Molecularity is the number of molecules involved in an elementary step.

Step-by-step solution

Understanding an Elementary Step

An elementary step in a chemical reaction is a single stage within a reaction mechanism that describes a specific set of molecular interactions. This step represents an individual collision or transformation that cannot be broken down into simpler interactions.

Features of Elementary Steps

Elementary steps involve a direct conversion between reactants and products and are often depicted as simple reactions, like unimolecular or bimolecular processes. Their rate laws can be written directly from the stoichiometry, as opposed to non-elementary reactions where this is not possible.

Introduction to Molecularity

Molecularity refers to the number of reactant particles involved in an elementary step. It is always an integer value and describes the actual number of atoms, ions, or molecules that must collide to produce the reaction.

Types of Molecularity

There are three primary types of molecularity: unimolecular (involving one reactant particle), bimolecular (involving two reactant particles), and termolecular (involving three reactant particles). Reactions with higher molecularities are rare due to the improbability of simultaneous collisions.

Key concepts

Molecularity

Molecularity is a term used in chemistry that quantifies the number of reactant molecules involved in an elementary step of a reaction. Understanding molecularity is crucial because it tells us how many molecules come together to form products in a single step of a reaction. It is important to note that molecularity is relevant only for elementary reactions, as it directly represents how many molecules collide in a step. This differs from order of reactions in kinetics, which may not necessarily match the stoichiometry due to more complex reaction pathways.

• Unimolecular: Involves a single molecule undergoing change. Common examples include isomerizations.
• Bimolecular: Involves two molecules colliding and reacting. This type is prevalent in many reactions.
• Termolecular: Very rare, involves three molecules colliding simultaneously, a rare event in chemistry.

Each type indicates a different pathway, but higher molecular holds less likelihood due to the decreased probability of simultaneous multi-particle collisions.

Reaction Mechanism

A reaction mechanism provides a detailed roadmap of how a chemical reaction proceeds from reactants to products. It breaks down the overall reaction into simpler steps known as elementary steps. These steps describe molecular changes and interactions at the atomic level, representing each collision or molecular transformation.
Reactions may seem straightforward on paper but involve complex sequences of events that cannot always be captured in a single step. Thus, reaction mechanisms help chemists predict the speed and products of a reaction, offering insight into the reactivity and stability of compounds.
By studying these mechanisms, scientists can determine which steps are slow or fast, which points to those that limit the overall speed of the reaction known as the rate-determining step. This holistic understanding enables the design of better industrial processes and new synthetic routes in organic chemistry.

Unimolecular Reactions

Unimolecular reactions are elementary processes that involve a single reactant molecule undergoing transformation. These reactions are simple yet fundamental to understanding chemical kinetics. In a unimolecular reaction, the molecule may undergo a structural rearrangement or decomposition.
A classic example is the isomerization in which a compound changes structure without altering its molecular formula, such as the conversion of cyclopropane to propene. Other instances include the decomposition of gaseous nitrous oxide (\(N_2O\) \rightarrow \(N_2\) + \(\frac{1}{2}O_2\)).
Unimolecular processes often involve activation energy, which is the energy needed to reach a transition state. Molecular collisions can provide this energy, but the critical aspect here is that only one reactant molecule is involved during its rate-limiting step, making these reactions simpler to analyze energetically compared to multi-molecular processes.

Bimolecular Reactions

Bimolecular reactions involve the collision and reaction between two reactant molecules. These are perhaps the most common type of elementary reactions, and their study provides significant insight into basic chemical interactions.
The simplicity lies in having two reactant molecules colliding in a single, energetically feasible step, transitioning into products. Common reactions in this category include substitution reactions in organic chemistry and the combination of halogens with alkanes.
The rate of bimolecular reactions depends significantly on the concentration of both reacting species, and their kinetic expressions typically follow second-order kinetics. For example, the reaction between hydrogen molecules and iodine molecules is bimolecular: \[H_2(g) + I_2(g) \rightarrow 2HI(g)\]. This simplicity and clarity make bimolecular reactions excellent subjects for kinetic studies and practical applications in synthesis and industrial processes.