Final answer:
In the given reaction mechanism, ClO⁻ and OH⁻ serve as catalysts since they are regenerated in the process, whereas H₂O functions as a solvent and Br⁻ is a reactant that does not act as a catalyst.
Step-by-step explanation:
When determining which step in a reaction mechanism is the rate-determining step, we look for the slowest step because it controls the overall reaction rate. In the proposed mechanism, Step 2: Br⁻(aq) + HOCl(aq) → HOBr(aq) + Cl⁻(aq) (slow) is indicated as the slow step, and thus it is the rate-determining step for this reaction. As for identifying catalysts, we look for substances that are consumed and then regenerated in the course of the mechanism such that they are present at the beginning and end of the reaction cycle.
A catalyst lowers the activation energy without being consumed in the overall reaction. Considering all steps, ClO⁻ (reactant in step 1 and product in step 2) and OH⁻ (product in step 1 and reactant in step 3) both get regenerated and hence act as catalysts in this reaction mechanism. Neither H₂O nor Br⁻ gets regenerated; H₂O is a solvent here and Br⁻ is a reactant that does not reappear as a product in subsequent steps, thus they are not catalysts.