Final answer:
The intermediate species in the reaction mechanism A B → C D (slow) followed by B D → X (fast) is D, as it is formed in the first step and used up in the second to produce product X.
Step-by-step explanation:
The student has provided a reaction mechanism with a slow step followed by a fast step. The intermediate species in the given reaction mechanism A B → C D (slow) followed by B D → X (fast) is D. The intermediate is formed in the first, slow step of the reaction and is then consumed in the subsequent fast step to form the final product X.
Understanding that the rate-determining step is the slowest step in a reaction mechanism is crucial. The species that appear in the rate-determining step but not in the overall balanced equation for the reaction are intermediates. In the given reaction, D is produced in the slow first step and consumed in the fast second step. Therefore, it is not present in the overall balanced equation of the reaction, confirming it as an intermediate.