[VISUAL] Which of the following reaction energy profiles best corresponds to the proposed mechanism? — Kinetics Chemistry Question

STEPS:

1. Count the number of elementary steps in the mechanism: The proposed mechanism consists of two elementary steps:
* Step 1 (Slow): $\text{Cl}(g) + \text{O}_3(g) \rightarrow \text{ClO}(g) + \text{O}_2(g)$
* Step 2 (Fast): $\text{ClO}(g) + \text{O}_3(g) \rightarrow \text{Cl}(g) + 2\ \text{O}_2(g)$
Because there are two distinct elementary steps, the reaction energy profile must have two activation energy barriers (two peaks) with a single valley in between representing the intermediate state (where the intermediate species $\text{ClO}$ is formed). This immediately eliminates Profile A, which depicts a single peak representing a one-step reaction with no intermediates.
2. Determine if the overall reaction is exothermic or endothermic: The overall reaction enthalpy is given as $\Delta H = -285\text{ kJ/mol}_{\text{rxn}}$. Because $\Delta H$ is negative, the overall reaction is exothermic. In an exothermic reaction, the system releases energy, meaning the potential energy of the final products must be lower than the potential energy of the starting reactants. This eliminates Profile D, which depicts an endothermic process where products are higher in energy than reactants.
3. Compare the activation energies ($E_a$) of the two steps: The rate of a reaction step is inversely related to its activation energy (the height of the barrier from the starting point of that step to the transition state peak):
* Step 1 is the slow step, so it must have a large activation energy. This means the first barrier (from reactants to the first peak) must be large.
* Step 2 is the fast step, so it must have a small activation energy. This means the second barrier (from the intermediate valley to the second peak) must be small.
4. Identify the profile that matches all criteria:
* Profile B shows two peaks (two steps).
* The products are lower in energy than the reactants (exothermic).
* The first peak has a significantly larger barrier than the second peak, indicating that the first step has a larger activation energy and is the slow, rate-determining step.
Therefore, Profile B is the correct representation, confirming Option B.

*

WHY_OTHERS_WRONG:

• Option A is incorrect (Profile A): Profile A has only a single peak. This represents a concerted, single-step reaction with no intermediate, which is inconsistent with the proposed two-step mechanism.
• Option C is incorrect (Profile C): Although Profile C correctly represents an exothermic two-step reaction, it shows the second peak as having a much larger activation energy barrier from the intermediate well than the first peak. This would correspond to a mechanism where the first step is fast and the second step is slow, which is the opposite of the proposed mechanism.
• Option D is incorrect (Profile D): Profile D represents an endothermic reaction because the products are at a higher potential energy level than the reactants. Since the overall reaction is exothermic ($\Delta H = -285\text{ kJ/mol}_{\text{rxn}}$), this profile is thermodynamically incorrect.