Problem 70 - Chemistry The Central Science

Question

(a) What is meant by the term molecularity? (b) Why are termolecular elementary reactions so rare? (c) What is an intermediate in a mechanism?

Step-by-Step Solution

Answer

(a) Molecularity refers to the number of molecules involved in an elementary reaction step or single-event collisions between participating molecules. (b) Termolecular elementary reactions are rare due to the improbability of simultaneous collision between three molecules with the correct alignment, orientation, and sufficient energy to overcome the activation energy barrier. (c) An intermediate in a reaction mechanism is a transient species formed during one of the elementary reaction steps and consumed in a subsequent step, usually unstable and highly reactive, not observed directly in the overall balanced equation.

Step 1: a) Molecularity Definition

The term "molecularity" refers to the number of molecules (atoms, ions, or radicals) involved in an elementary reaction step or single-event collisions between participating molecules. In other words, molecularity is the count of reactant particles in a given elementary reaction, which determines the reaction rate. Molecularity can be unimolecular (involving one molecule), bimolecular (involving two molecules), or termolecular (involving three molecules).

Step 2: b) Termolecular Reactions Rarity

The rarity of termolecular elementary reactions can be explained by the improbability of simultaneous collision between three molecules. For a termolecular reaction to take place, such collision should result in the correct alignment of the molecules, and also have the accurate orientation and sufficient energy to overcome the activation energy barrier. The chances of these factors aligning adequately in a simultaneous three-molecule collision are minimal compared to unimolecular or bimolecular reactions. Consequently, termolecular elementary reactions are considered rare.

Step 3: c) Intermediate in a Mechanism Definition

An intermediate in a reaction mechanism is a transient species formed during one of the elementary reaction steps and then consumed in a subsequent step. Intermediates are usually unstable and highly reactive, which leads to their fast decay and short lifetimes before reacting further. Unlike reactants or products, intermediates are not observed directly in the overall balanced equation of the reaction. Identifying and understanding intermediates involved in reaction mechanisms are crucial for gaining insight into the reaction's true pathway, rate laws, and kinetics.

Key Concepts

Molecularity of Reactions

In chemical kinetics, molecularity is a term used to describe the number of reacting particles—molecules, atoms, ions, or radicals—that participate in an elementary reaction step. It is a crucial concept to understand the fundamental nature of reactions. Elementary reactions can be unimolecular, which involve a single reactant molecule undergoing a change. For example, the decomposition of a molecule into different products. Bimolecular reactions, on the other hand, involve a collision between two reactant molecules. This is the most common type of molecularity and includes many familiar reaction scenarios. Lastly, termolecular reactions are those rare instances where three molecules must simultaneously collide to result in a reaction. It's important to note that molecularity is specific to an elementary step and does not apply to complex reactions involving multiple steps.

Termolecular Reactions

Termolecular reactions are elementary reaction steps involving the simultaneous collision of three reactant molecules. These reactions are considered rare in the field of chemical kinetics. The primary reason for the rarity of termolecular reactions is the improbability of three molecules colliding simultaneously with the correct orientation and sufficient energy needed to react.

• First, achieving a collision between three molecules at the exact same moment requires high concentrations and precise molecular alignment.
• Secondly, the reaction must occur with enough energy to overcome the activation energy barrier.
• This simultaneous meeting of three conditions is uncommon in typical reaction conditions, making such reactions infrequent.

In real-world settings, when a reaction suggests termolecular conditions, it often proceeds via a series of bimolecular steps, where intermediates can play a role.

Reaction Intermediates

In chemical reactions, intermediates are transient species that form in the course of an overall reaction but do not appear in the final product mix. They are formed in one step and consumed in another. Unlike reactants and products that are often stable and commonly observed, intermediates exist only briefly and are highly reactive.

• Identifying intermediates is crucial because they provide insight into the reaction mechanism, elucidating the transition pathways between reactants and products.
• Understanding intermediates can help in determining the rate laws and kinetics of a reaction.
• They are often detected through techniques like spectroscopy or inferred from studying reaction kinetics.

By analyzing intermediates, chemists can better understand and predict how reactions proceed, tailoring conditions for more efficient chemical processes.