Which of the following best describes how a catalyst can be recognized in a reaction mechanism?

• A. It is present in the rate-determining step but not in any other step.
• B. It is consumed in the first step and regenerated in the final step.
• C. It appears only as a product in the overall reaction.
• D. It is consumed and not regenerated during the reaction.

Answer: (B)

Verified step by step guidance

1. Understand the role of a catalyst in a chemical reaction: a catalyst speeds up the reaction without being consumed in the overall process.
2. Recognize that in a reaction mechanism, a catalyst participates in one or more steps but is regenerated by the end of the mechanism, so it does not appear in the overall balanced equation as a reactant or product.
3. Identify that a catalyst is often consumed in an intermediate step but must be regenerated in a subsequent step, ensuring it is present at the start and end of the mechanism.
4. Note that the catalyst does not only appear as a product or only in the rate-determining step; it must be involved in multiple steps to facilitate the reaction.
5. Conclude that the best description is: the catalyst is consumed in the first step and regenerated in the final step, reflecting its temporary involvement and regeneration.