Which statement correctly describes a reaction mechanism?

• A. It is always the same as the overall reaction.
• B. It is a sequence of elementary steps that sum to the overall reaction.
• C. It ignores molecularity.
• D. It determines the rate law regardless of rate-determining step.

Answer: (B)

A reaction mechanism is a sequence of elementary steps that add up to the overall reaction. Each step has its own molecularity and rate expression, and when you sum the steps you cancel intermediate species to recover the overall equation. The observed rate law is usually determined by the slowest step—the rate-determining step—so the mechanism does not fix the rate law regardless of which step is slow. This is why the best description is that the mechanism is a sequence of elementary steps that sum to the overall reaction.

The other statements don’t fit: the mechanism is not simply the same as the overall reaction but a detailed path that explains how it proceeds; each elementary step has molecularity and matters for the rate expressions; and the rate law is governed by the rate-determining step, not determined regardless of which step is slow.