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?

Short answer

(a) No, an intermediate cannot appear as a reactant in the first step of a reaction mechanism, as it is formed and consumed during the reaction process. (b) An intermediate is represented as a valley on a reaction energy profile diagram. (c) The molecularity of the elementary reaction where \(\mathrm{Cl}_{2}\) falls apart is 1, making it a unimolecular reaction.

Step-by-step solution

Answer to part (a)

In a reaction mechanism, an intermediate is a species that is formed and consumed during the sequence of steps. It is important to note that intermediates are not present at the beginning or the end of the reaction. As such, it is not possible for an intermediate to appear as a reactant in the first step of a reaction mechanism because intermediate species must be formed from the reactants during the reaction process.

Answer to part (b)

On a reaction energy profile diagram, the progress of the reaction is represented along the horizontal axis, and the reaction energy is represented along the vertical axis. Transition states, which are the highest energy states along the reaction pathway, are represented as peaks, while reactants, products, and intermediates are represented as valleys. Therefore, an intermediate is represented as a valley on a reaction energy profile diagram.

Answer to part (c)

The molecularity of a reaction refers to the number of species involved in an elementary reaction step. In the case of a molecule like \(\mathrm{Cl}_{2}\) falling apart in an elementary reaction, the reaction can be represented as: \[ \mathrm{Cl}_{2} \rightarrow 2\,\mathrm{Cl} \] In this elementary reaction, only one molecule, \(\mathrm{Cl}_{2}\), is involved in the reaction process. As such, the molecularity of this reaction is 1, which means it is a unimolecular reaction.

Key concepts

Reaction Energy Profile

Reaction energy profiles are helpful diagrams that show the energy changes during a chemical reaction.
They map out the progression from reactants, through transition states and intermediates, to the final products.
The vertical axis typically represents the energy of the system, while the horizontal axis represents the reaction's progress over time or coordinate. In the profile, peaks correspond to transition states which are high-energy states, representing the "energy barrier" that must be overcome for the reaction to proceed.
These transition states are not stable and only exist for a very short time.
Valleys, on the other hand, represent more stable points in the reaction. This includes the reactants and products, as well as any intermediates. Intermediates are represented as valleys because they are typically more stable than the transition states.
However, they are generally less stable than the final products, which is why they don't exist indefinitely.
As the reaction moves from one peak to the next, it often pauses at a valley, marking the presence of intermediates before proceeding to the next transition state.

Intermediates in Chemistry

Intermediates are crucial in understanding reaction mechanisms because they help explain how reactants are transformed into products.
These species are formed during the multi-step process of a reaction but do not appear in the overall reaction equation as they are neither reactants nor final products.
During a chemical reaction, intermediates exist temporarily and are consumed in subsequent steps before the reaction reaches completion.
They can include atoms, ions, or complex molecules, and often play a key role in determining the rate at which the overall reaction proceeds. Because intermediates are not present from start to finish in the reaction, they cannot be a reactant in the very first reaction step.
Instead, they originate from initial reactants during the process and are used as subsequent reactants in later steps.
This transient nature is what makes them uniquely identifiable in a reaction mechanism.

Molecularity of Reactions

Molecularity refers to the number of molecules or atoms colliding and reacting in a single elementary step of a reaction.
It is different from the order of reaction, which is determined from experimental rate laws and can be fractional.
Molecularity is always a whole number because it reflects the actual number of particles involved in a single collision or transformation event.
These numbers can be:

• Unimolecular: Involves one molecule, such as the decomposition of \(Cl_2 \rightarrow 2Cl\).
• Bimolecular: Involves two molecules or species, such as in a collision or exchange reaction.
• Termolecular: Involves three molecules or species, though this is rare since simultaneous collisions of three particles are unlikely.

Unimolecular reactions, like the one described with \(Cl_2\), involve a single molecule transitioning to a new state, often through the breaking of a chemical bond.
Understanding molecularity helps in predicting reaction pathways and rates by considering how many particles must come together in a single reactive event.