Question

(a) What is meant by the term elementary reaction? (b) What is the difference between a unimolecular and a bimolecular elementary reaction? (c) What is a reaction mechanism?

Short answer

(a) An elementary reaction is a single step in a chemical reaction process, where reactant molecules directly collide and transform into product molecules without forming any intermediate species. The rate of an elementary reaction depends on the collision frequency, molecular orientation, and energy of the reactants. (b) In a unimolecular elementary reaction, one reactant molecule decomposes or rearranges to form the product(s), with a rate law represented as \(Rate = k[A]\). In a bimolecular elementary reaction, two reactant molecules collide and react to form the product(s), with a rate law represented as \(Rate = k[A][B]\). The primary difference between these two reaction types is the number of reactant molecules involved and the factors influencing their reaction rates. (c) A reaction mechanism is a detailed, step-by-step description of the pathway through which a chemical reaction proceeds. It involves the collision, transformation, and formation of reactants, intermediate species, and products, providing crucial information about the sequence of elementary reactions, the rate-determining step, and the overall reaction rate.

Step-by-step solution

(a) Definition of Elementary Reaction)

An elementary reaction is a single step in a chemical reaction process. In an elementary reaction, reactant molecules directly collide and transform into product molecules without the formation of any intermediate species. Typically, the rate of an elementary reaction depends on the collision frequency, molecular orientation, and energy of the reactants.

(b.1) Definition of a Unimolecular Elementary Reaction)

A unimolecular elementary reaction involves a single reactant molecule decomposing or rearranging itself to form the product(s). Its rate law is typically represented as \(Rate = k[A]\), where \(k\) is the rate constant and \([A]\) is the concentration of the reactant molecule A. The rate of a unimolecular reaction mainly depends on the energy of the reactant molecule and its internal vibrations or rotations.

(b.2) Definition of a Bimolecular Elementary Reaction)

A bimolecular elementary reaction involves the collision of two reactant molecules, which consequently react to form the product(s). The rate law for a bimolecular reaction is represented as \(Rate = k[A][B]\), where \(k\) is the rate constant, and \([A]\) and \([B]\) are the concentrations of reactant molecules A and B, respectively. The rate of a bimolecular reaction depends on the collision frequency, molecular orientation, and energy of the two reactant molecules.

(b.3) Difference between a Unimolecular and a Bimolecular Elementary Reaction)

The primary difference between a unimolecular and a bimolecular elementary reaction is the number of reactant molecules involved. A unimolecular reaction involves only one reactant molecule, whereas a bimolecular reaction requires the collision of two reactant molecules. Additionally, the rate laws and factors influencing reaction rates differ between these reaction types.

(c) Definition of a Reaction Mechanism)

A reaction mechanism is a detailed, step-by-step description of the pathway through which a chemical reaction proceeds, involving the collision, transformation, and formation of reactants, intermediate species, and products. It provides crucial information about the sequence of elementary reactions, the rate-determining step, and the overall reaction rate. Understanding the reaction mechanism allows for better prediction and control of the reaction's outcome and can lead to improved synthesis of desired products.

Key concepts

Elementary Reaction

In the realm of chemical kinetics, an elementary reaction is defined as a fundamental process by which chemical reactions occur, represented by a single molecular event. These reactions occur in one step, where reactant molecules collide and directly convert to product molecules. It is essential to emphasize that no intermediate species are formed in an elementary reaction.
The rate at which an elementary reaction proceeds is influenced by several factors:

• Collision Frequency: The number of times reactant molecules collide per unit time affects the speed of the reaction.
• Molecular Orientation: For a successful reaction, molecules must approach each other in specific orientations.
• Energy of Reactants: The molecules must possess sufficient energy to overcome the activation energy barrier.

Understanding elementary reactions serves as a building block for analyzing more complex reaction mechanisms.

Reaction Mechanism

A reaction mechanism provides a comprehensive description of all the steps in a chemical reaction. This includes the specific elementary reactions and the order in which they occur, ultimately forming the products from the reactants. Think of it as a molecular roadmap that details the transformation pathway.
Reaction mechanisms contain the following key components:

• Elementary Steps: These are the individual reactions that constitute the overall process.
• Rate-Determining Step: The slowest step which defines the rate of the entire reaction.
• Intermediate Species: Molecules or atoms formed and consumed during the reaction pathway but not seen in the final products.

By understanding the reaction mechanism, chemists can predict reaction outcomes more accurately and develop strategies to optimize desired chemical processes.

Unimolecular versus Bimolecular Reactions

Chemical reactions can often be categorized based on the number of reactants involved. Two important types are unimolecular and bimolecular reactions.
A unimolecular reaction involves a single reactant molecule. This molecule undergoes a transformation, such as decomposition or rearrangement, to form products. The reaction rate is primarily dependent on the energy within this molecule, reflected by the rate law: \[Rate = k[A]\]Here, \(k\) represents the rate constant, and \([A]\) is the concentration of the reactant. Internal vibrations or rotations of the molecule drive these reactions.
In contrast, a bimolecular reaction requires the interaction of two reactant molecules. These molecules must collide with adequate energy and proper orientation to react and form products. The rate law for bimolecular reactions is expressed as:\[Rate = k[A][B]\]This indicates the dependence of the reaction rate on the concentrations of both reactants \([A]\) and \([B]\). Understanding the differences between these two types of reactions helps in predicting how changing conditions or concentrations will impact the reaction kinetics.