Question

\(\begin{array}{l}{\text { (a) What is meant by the term elementary reaction? }} \\ {\text { (b) What is the difference between a unimolecular }} \\\ {\text { and a bimolecular elementary reaction? (c) What is a }}\end{array}\) \(\begin{array}{l}{\text {reaction mechanism?}(\mathbf{d}) \text { What is meant by the term rate- }} \\ {\text { determining step? }}\end{array}\)

Short answer

(a) Elementary reaction is a single molecular event in which reactants are transformed into products without the formation of any reaction intermediates. (b) Unimolecular and bimolecular elementary reactions are types of elementary reactions based on the number of molecules involved. Unimolecular reactions involve one molecule, while bimolecular reactions involve the collision of two reactant molecules. (c) A reaction mechanism is a series of elementary reactions that together describe the step-by-step process by which reactants are transformed into products, providing a detailed explanation of the molecular events and intermediates involved. (d) The rate-determining step is the slowest step in a reaction mechanism, which governs the overall rate of the reaction. It limits the speed at which reactants are transformed into products and is important in understanding factors controlling the reaction rate.

Step-by-step solution

Answer (a)

Elementary reaction refers to a single molecular event in which reactants are transformed into products. In an elementary reaction, the reactants directly convert into products through a single step, without the formation of any reaction intermediates.

Answer (b)

Unimolecular and bimolecular elementary reactions are types of elementary reactions based on the number of molecules involved in the reaction. - Unimolecular elementary reaction: A reaction in which only one molecule is involved, leading to a rearrangement of atoms or bonds within that molecule. An example is the decomposition of a molecule into two or more smaller molecules. - Bimolecular elementary reaction: A reaction involving the collision of two reactant molecules, resulting in a new product. An example is the reaction between two molecules, A and B, that combine to form a single product C (A + B -> C).

Answer (c)

A reaction mechanism is a series of elementary reactions that together describe the step-by-step process by which reactants are transformed into products. It provides a detailed explanation of the sequence of molecular events and the intermediates formed during the overall reaction. This includes information about the molecular species, their transformations, and the energy changes involved at each step of the reaction.

Answer (d)

The term "rate-determining step" refers to the slowest step in a reaction mechanism, which governs the overall rate of the reaction. Since the reaction can only proceed as fast as the slowest step, the rate-determining step limits the speed at which the reactants are transformed into products. By identifying the rate-determining step, we can better understand the factors controlling the reaction rate and suggest ways to optimize the reaction conditions.

Key concepts

Unimolecular Reaction

Understanding a unimolecular reaction starts with the recognition that it involves only one molecule undergoing a change to form products. Imagine a single molecule, all by itself, deciding to rearrange its atoms without any influence from another molecule. This process might seem lonely, but it's an important concept in chemical reactions.

For example, consider a molecule of ozone (O_3) breaking down into a molecule of oxygen (O_2) and a single oxygen atom. This single-step reaction doesn't need another molecule to have a 'conversation' with; it proceeds when the molecule has enough energy to rearrange itself. This idea allows us to predict and understand how changes in conditions such as temperature can influence the rate at which the reaction proceeds, as more molecules gain the necessary energy to undergo this transformation.

Bimolecular Reaction

In contrast to the solitary nature of unimolecular reactions, bimolecular reactions require a dance between two molecules. They must 'meet' and collide with sufficient energy and in the correct orientation for a reaction to occur.

Think of it like a perfectly choreographed dance routine where two dance partners need to be in sync. By considering the factors that affect these molecular collisions, such as concentration and temperature, chemists can gain insights into how quickly the reaction will take place. For instance, the iconic reaction between hydrogen and iodine molecules to form hydrogen iodide (2HI) exemplifies a bimolecular reaction where two molecules interact directly to form a product.

Reaction Mechanism

A reaction mechanism is akin to a detailed recipe for a chemical reaction, outlining each step that reactant molecules take on their journey to becoming product molecules. Just as a cook needs to follow a series of steps to prepare a dish, molecules follow a series of elementary reactions in a specific sequence.

This concept enables chemists to visualize the transformation from reactants to products, discovering intermediate species and transition states that might be targets for controlling or altering the reaction's pathway. Overall, the reaction mechanism tells the full story of a chemical reaction, beyond the simple reactants-to-products summary.

Rate-Determining Step

The rate-determining step is often described as the bottleneck of a reaction mechanism, which dictates the reaction's pace much like the narrowest part of a funnel restricts the flow of liquid. This concept is critical in both understanding and optimizing reactions because, by identifying and addressing this slowest step, chemists can strategize ways to accelerate the overall reaction.

It's similar to addressing the slowest runner in a relay race to improve the team's total time. By focusing on the rate-determining step, we can apply various chemical principles to improve reaction efficiency, such as increasing the temperature or using a catalyst to lower the activation energy required for this crucial step.