Step 1: NO(g) + O3(g) → NO2(g) + O2(g) Step 2: NO2(g) + O(g) → NO(g) + O2(g) A reaction mechanism fo — Kinetics Chemistry Question

STEPS:

1. Analyze the Elementary Steps: Look at the two-step mechanism provided in the sources:
* Step 1: $NO(g) + O_3(g) \rightarrow NO_2(g) + O_2(g)$
* Step 2: $NO_2(g) + O(g) \rightarrow NO(g) + O_2(g)$
2. Track the Species NO(g): Observe the "life cycle" of nitric oxide ($NO$) throughout the mechanism. It appears as a reactant in the very first step and is then regenerated as a product in the final step.
3. Determine the Overall Reaction: Sum the two elementary steps to see the net change. When adding them, species that appear on both the reactant and product sides cancel out:
* $(NO + O_3 + NO_2 + O) \rightarrow (NO_2 + O_2 + NO + O_2)$
* After canceling $NO$ and $NO_2$, the overall reaction is: $O_3(g) + O(g) \rightarrow 2 O_2(g)$.
4. Apply the Definition of a Catalyst: In a reaction mechanism, a catalyst is a substance that is consumed in an early step but is regenerated in a subsequent step. Because it is present at the beginning and the end, it does not appear in the balanced overall equation but serves to provide a new pathway (mechanism) for the reaction.
5. Conclusion: Since $NO(g)$ is used in Step 1 and produced back in Step 2, it perfectly fits the definition of a catalyst.

WHY_OTHERS_WRONG:

• A) inhibitor: An inhibitor is a substance that slows down or stops a reaction. The mechanism shown describes the *destruction* of ozone being facilitated by $NO$, meaning it is speeding the process up, not inhibiting it.
• C) reactant & E) product: While $NO$ appears as a reactant in one step and a product in another, it is neither in the overall reaction. A true reactant is consumed permanently, and a true product is newly formed at the end.
• D) intermediate: This is a common point of confusion. An intermediate is a species that is produced in an early step and consumed in a later step (it is "born and then dies"). In this mechanism, $NO_2(g)$ is the intermediate because it is created in Step 1 and used up in Step 2.