Question

Which of the following is/are examples of unimolecular reactions? a. \(2 \mathrm{NO}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{NOCl}\) b. \(\mathrm{O}_{3} \rightarrow \mathrm{O}_{2}+\mathrm{O}\) c. C=CCCCC d. \(\mathrm{NO}+\mathrm{O}_{3} \rightarrow \mathrm{NO}_{2}+\mathrm{O}_{2}\)

Short answer

Option b is a unimolecular reaction.

Step-by-step solution

Understand Unimolecular Reactions

A unimolecular reaction is a type of chemical reaction in which the transformation of the reactants involves a single molecule. In these reactions, a single molecular entity undergoes a change in structure or identity to form products.

Analyze Each Option

Analyze each given option to determine if it is a unimolecular reaction:a. In the reaction \(2 \mathrm{NO} + \mathrm{Cl}_{2} \rightarrow 2 \mathrm{NOCl}\), there are multiple molecules involved in the reactants, thus it is not a unimolecular reaction.b. In the reaction \(\mathrm{O}_{3} \rightarrow \mathrm{O}_{2} + \mathrm{O}\), a single \(\mathrm{O}_{3}\) molecule decomposes into \(\mathrm{O}_{2}\) and \(\mathrm{O}\), fitting the definition of a unimolecular reaction.c. The reaction represented by C=CCCCC does not provide sufficient information about the change in the potential reaction, making it impossible to categorize without further details.d. In the reaction \(\mathrm{NO} + \mathrm{O}_{3} \rightarrow \mathrm{NO}_{2} + \mathrm{O}_{2}\), multiple molecules are involved similar to option a, so it is not predefined as unimolecular.

Identify Unimolecular Reactions

From the analysis in Step 2, option b, \(\mathrm{O}_{3} \rightarrow \mathrm{O}_{2} + \mathrm{O}\) is the only option that straightforwardly represents a unimolecular reaction due to the involvement of a single molecule undergoing transformation in the reaction process.

Key concepts

Chemical Reactions

Chemical reactions are processes in which substances, known as reactants, transform into different substances, called products. This transformation occurs when chemical bonds are broken and new ones are formed. Reactions can vary greatly, from simple to complex, but they all involve a change in the arrangement of atoms.
Chemical reactions are classified based on several factors:

• Number of Reactants and Products: Reactions can involve a single reactant breaking down into multiple products, multiple reactants combining into a single product, or any combination thereof.
• Energy Change: Reactions can be exothermic (releasing energy) or endothermic (absorbing energy).
• Rate of Reaction: Some reactions occur almost instantaneously, while others can take years.

Understanding how these factors play out in reactions can help predict the course and outcome of chemical reactions, including special cases like unimolecular reactions.

Reaction Mechanisms

Reaction mechanisms are step-by-step descriptions of how reactions occur at the molecular level. They provide a detailed pathway or sequence of elementary reactions that describe the overall transformation of reactants into products.
Here's what you need to know about reaction mechanisms:

• Elementary Steps: These are the simplest reactions that occur between molecules. They often involve a small number of reactants.
• Intermediates: These are transient species that are formed and then consumed during the reaction process. They appear in the mechanism but not in the overall balanced equation.
• Molecularity: This refers to the number of molecules involved in an elementary step. It can be unimolecular, bimolecular, or even termolecular (though termolecular is less common).
• Rate-Determining Step: Often, one step among several is slower than the rest; this controls the overall reaction rate. Understanding which step this is can provide insights into speeding up or slowing down the reaction.

A clear grasp of reaction mechanisms is essential for learning how reactions are modulated and explains why some reactions, like unimolecular ones, proceed the way they do.

Molecular Decomposition

Molecular decomposition refers to the breaking down of a compound into simpler molecular entities. This process is often a key feature of unimolecular reactions. In a unimolecular reaction, a single molecule undergoes decomposition or rearrangement without the direct interaction of another molecule.
Here's a closer look at molecular decomposition:

• Single Reactant: Only one reactant is involved in the initial step of the reaction. This contrasts with bimolecular reactions, where two molecules collide and interact.
• Activation Energy: Even though only one molecule is involved, these reactions still require sufficient energy to break existing bonds, often provided by environmental factors like heat or light.
• Types of Decomposition: Common types include substitution, where part of the molecule replaces another, and fragmentation, where the molecule splits into smaller fragments.

Understanding molecular decomposition is crucial because it forms the basis for predicting reaction pathways and products in chemical processes. Unimolecular reactions serve as a fundamental model for understanding the stability and breakdown of molecular substances.