In a multi-step reaction, which step determines the overall rate?

• A. The fastest step
• B. The first step
• C. The rate-determining step
• D. The step with the largest activation energy

Answer: (C)

In a multi-step mechanism, the overall rate is set by the slowest step in the sequence. That step acts as a bottleneck: even if all other steps are fast, the entire reaction cannot proceed faster than this one allows because everything downstream depends on its product being formed. Steps before it quickly build up intermediates, and steps after it wait on that slow step to release products. So the rate-determining step is the one that controls the pace of the whole reaction and often, but not always, has the largest activation energy. The key idea is that the slowest step dictates the overall rate.