Problem 66 - Chemistry The Central Science

Question

(a) Can an intermediate appear as a reactant in the first step of a reaction mechanism? (b) On a reaction energy profile diagram, is an intermediate represented as a peak or a valley? (c) If a molecule like \(C l_{2}\) falls apart in an elementary reaction, what is the molecularity of the reaction?

Step-by-Step Solution

Answer

(a) No, an intermediate cannot appear as a reactant in the first step of a reaction mechanism. (b) In a reaction energy profile diagram, an intermediate is represented as a valley. (c) If a molecule like \(Cl_{2}\) falls apart in an elementary reaction, the molecularity of the reaction is unimolecular.

Step 1: Question (a)

Can an intermediate appear as a reactant in the first step of a reaction mechanism? An intermediate is defined as a species formed during the course of a reaction that is neither a reactant nor a product. In the first step of a reaction mechanism, the species involved are the reactants (starting materials) and will not include any intermediates which are formed in the subsequent steps. So, the answer is no - an intermediate cannot appear as a reactant in the first step of a reaction mechanism.

Step 2: Question (b)

On a reaction energy profile diagram, is an intermediate represented as a peak or a valley? A reaction energy profile diagram depicts the potential energy of a reaction system as it progresses along the reaction pathway. Transition states (a high-energy state where bonds are being broken and formed) are represented as peaks, whereas reactants, products, and intermediates are represented as valleys. Therefore, an intermediate would be represented as a valley on a reaction energy profile diagram.

Step 3: Question (c)

If a molecule like \(Cl_{2}\) falls apart in an elementary reaction, what is the molecularity of the reaction? When a molecule like \(Cl_{2}\) falls apart, it dissociates into two radical species: \[Cl_{2} \rightarrow 2Cl\] This is a single event involving one reactant species, so the molecularity of this reaction is called "unimolecular."

Key Concepts

Reaction Intermediates

In the context of a chemical reaction mechanism, an intermediate is a transient species formed during the transition from reactants to final products. They are not part of the starting materials or the final products. This is why an intermediate cannot be a reactant in the first step of a reaction mechanism, as they are formed from the reactants as the reaction progresses.

• An intermediate typically appears in subsequent steps, after the initial step has produced it.
• Intermediates are crucial for understanding the overall transformation taking place in a reaction.
• These species have finite lifetimes, longer than transition states, but usually short enough to not be isolated under normal conditions.

Recognizing intermediates helps chemists understand the mechanism of complex reactions, allowing them to potentially control and optimize reactions.

Reaction Energy Profile

A reaction energy profile diagram is a vital tool used to visualize the energy changes occurring during a chemical reaction. It maps out the potential energy of the reactants, intermediates, and products as the reaction progresses over time.

• The diagram showcases the energy needed to reach the transition states, depicted as the highest points or peaks.
• Intermediates are shown as valleys, indicating lower energy states compared to the transition states.
• These valleys between peaks help distinguish intermediates from high-energy transition states.

Understanding the energy landscape of a chemical process is essential because it helps identify the rate-determining step in a reaction, which is typically the slowest step due to a high energy barrier needing to be overcome.

Molecularity

Molecularity refers to the number of molecules or atoms participating in an elementary reaction step. It provides insight into the basic event occurring at the molecular level.

• A reaction is termed "unimolecular" if it involves a single reactant molecule breaking apart or rearranging.
• Bimolecular reactions involve two molecules colliding and reacting with each other.
• Termolecular reactions, involving the simultaneous encounter of three molecules, are rare due to the low probability of three particles meeting simultaneously.

For example, when a chlorine molecule (\(Cl_{2}\)) dissociates into two chlorine atoms, the process is classified as a unimolecular reaction. Understanding molecularity helps in detailing the reaction mechanism and predicting the behavior and rate of chemical reactions.