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 chemical process that occurs as indicated by the balanced chemical equation, involving the collision and reactions between individual particles. (b) Unimolecular elementary reactions involve only one reacting species, typically in decomposition or isomerization reactions, with a rate law expression of Rate = k[A]. Bimolecular elementary reactions involve the simultaneous collision and reaction of two molecular species, with a rate law expression of Rate = k[A][B]. (c) A reaction mechanism is a series of elementary steps describing the detailed molecular events in a complex chemical reaction, providing information about intermediates, transition states, and the order of bond breaking and formation. The overall rate depends on the slowest step (rate-determining step).

Step-by-step solution

(a) Definition of Elementary Reaction)

An elementary reaction is a single-step chemical process that happens exactly as the balanced chemical equation indicates. It can involve the collision and react-ions between individual particles (atoms, molecules, or their fragments). Elementary reactions can be classified based on the number of molecular species involved, such as unimolecular, bimolecular, or termolecular reactions.

(b) Unimolecular vs. Bimolecular Elementary Reactions)

Unimolecular elementary reactions involve only one reacting species in the rate-determining step. Typically, they indicate decomposition or isomerization reactions. The rate law expression for a unimolecular reaction is: Rate = k[A], where k is the rate constant and [A] is the concentration of the reacting species. Bimolecular elementary reactions involve the simultaneous collision and reaction of two molecular species in the rate-determining step. Examples include reactions between two different molecules or two identical molecules. The rate law expression for a bimolecular reaction is: Rate = k[A][B], where k is the rate constant, and [A] and [B] are the concentrations of the two reacting species.

(c) Definition of Reaction Mechanism)

A reaction mechanism is a series of elementary steps or reactions that describe the detailed molecular events occurring in a complex chemical reaction. It provides information about the intermediates, transition states, and the order of bond breaking and bond formation throughout the reaction. The overall rate of the reaction depends on the slowest step in the reaction mechanism, which is known as the rate-determining step.

Key concepts

Reaction Mechanism

A reaction mechanism is a detailed blueprint of a chemical reaction at the microscopic level. It breaks down complex reactions into elementary steps, providing a step-by-step narrative of how the reaction proceeds.
Understanding the reaction mechanism helps chemists predict the behavior of chemical reactions under different conditions, and it offers insights into the types of intermediates formed during the reaction process.
These intermediates are usually unstable, transient species that exist for a short time, serving as a bridge between reactants and products.

• **Transition States**: High energy states that occur during a reaction, indicating partial bond formations and breakdowns.
• **Rate-Determining Step**: The slowest elementary step in a reaction mechanism that limits the overall reaction rate.

By studying reaction mechanisms, scientists understand the fundamental processes such as bond making and breaking, which are pivotal in designing new chemical processes or improving existing ones.

Unimolecular Reactions

Unimolecular reactions are a type of elementary reaction where only one molecular species is involved in the rate-determining step. This implies that these reactions occur without requiring collisions with other molecules.
Typical examples include cases such as isomerization and decomposition reactions, where a single molecule undergoes structural change or breaks down into simpler substances.
The general rate law for a unimolecular reaction is represented as:
\[ \text{Rate} = k[A] \]
Here, \( k \) is the rate constant, and \([A]\) represents the concentration of the reactant.

• **Isomerization**: This involves the transformation of a molecule into another molecule with the same molecular formula but a different structure.
• **Decomposition**: A reaction where a single compound breaks down into two or more simpler entities.

The concept of unimolecular reactions is pivotal for understanding natural and industrial processes, such as the decomposition of ozone in the atmosphere.

Bimolecular Reactions

Bimolecular reactions involve the simultaneous collision and interaction between two molecular species in the rate-determining step. This interaction can occur between two identical molecules or two different molecules.
These reactions are more common compared to termolecular reactions due to the lower probability of three molecules successfully colliding simultaneously.
The rate law for bimolecular reactions is depicted as:
\[ \text{Rate} = k[A][B] \]
Here, \( k \) is the rate constant, while \([A]\) and \([B]\) denote the concentrations of the two reacting species.

• **Collisions**: Effective collisions between molecules are vital for successful reactions. These collisions must have the proper orientation and adequate energy to overcome activation barriers.
• **Biodegradation**: A practical example where bimolecular reactions occur extensively, as complex organic substances break down in the presence of enzymes.

Bimolecular reactions are integral in fields like pharmacology, where they help explain how different drugs interact and react within the body.