Problem 66 - Chemistry The Central Science In Si Units

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 \(\mathrm{Cl}_{2}\) falls apart in an elementary reaction, what is the molecularity of the reaction?

Step-by-Step Solution

Answer

(a) No, intermediates cannot appear as reactants in the first step. (b) Intermediates are valleys on energy diagrams. (c) The molecularity is unimolecular.

Step 1: Understanding Intermediates in Reaction Mechanisms

Intermediates are species that are produced in a reaction mechanism and then consumed in a later step. By definition, an intermediate cannot exist before it is generated, so it cannot appear as a reactant in the first step of a mechanism because it has not been formed yet.

Step 2: Intermediates on a Reaction Energy Profile Diagram

In a reaction energy profile diagram, an intermediate is represented by a valley. The peaks represent the transition states, which are the highest energy points along the reaction pathway. Between two peaks, the valleys show intermediates, as they are at lower energy states compared to the transition states.

Step 3: Determining Molecularity of a Reaction

Molecularity refers to the number of reacting species involved in an elementary reaction. If a molecule like \(\mathrm{Cl}_{2}\) falls apart in an elementary reaction, it involves only that single molecule. Thus, the reaction is unimolecular.

Key Concepts

Understanding Intermediates

Intermediates are crucial in the study of reaction mechanisms. Think of them as temporary products that appear and disappear before the final reaction output is achieved.
Intermediates are different from reactants or final products, which are either consumed or generated by the reaction system.

• They are formed in one or more steps of the reaction mechanism.
• Once formed, intermediates are usually consumed in subsequent steps.

Importantly, intermediates cannot be present as reactants in the initial step of a reaction. This is because they need to be generated first within the ongoing chemical process. Only after their formation can they participate in further steps of the mechanism. Thus, in a multi-step reaction, observe how intermediates facilitate the transition between initial reactants and final products.

The Role of Intermediates in Reaction Energy Profiles

Reaction energy profiles visually showcase the energy changes that occur during the course of a reaction. Understanding these profiles helps grasp when and how intermediates are involved.
On these diagrams, each stage of a reaction mechanism is depicted as a series of peaks and valleys.

• Peaks correspond to transition states, which show the highest energy points that need to be overcome.
• Valleys represent intermediates, indicating a relative decrease in energy following a transition state.

The position of these valleys is pivotal as they define the existence of intermediates between transition states. This allows chemists to understand the stability and role of intermediates during the reaction pathway.

Demystifying Molecularity

Molecularity is an intrinsic property of an elementary reaction that refers to the number of molecules or atoms that come together to react.
Understanding molecularity helps in predicting how reactions occur on a molecular level.

• Unimolecular reactions involve a single molecule, such as when \( \mathrm{Cl}_{2} \) decomposes into 2 chlorine atoms. Here, the reaction depends on the transformation of just one molecule.
• Bimolecular reactions involve two reacting species, either the same or different entities, colliding and reacting with each other.
• Termolecular reactions involve three molecules colliding at the same time. However, these are rare due to the improbability of three particles simultaneously coming together.

Grasping molecularity provides insight into the complexities and the kinetics of chemical reactions, guiding predictions and interpretations of the reaction mechanisms.