Which statement is true about rate-determining steps?

Which statement is true about rate-determining steps?

• A. The slowest step controls the overall rate
• B. The fastest step controls the overall rate
• C. The step with the largest activation energy always controls the rate
• D. Any step can control the rate depending on concentrations

Answer: (A)

In a multi-step reaction, the overall rate is governed by the slowest step, the bottleneck in the sequence. Because this step proceeds the slowest, it sets the pace for converting reactants into products, while the faster steps quickly move intermediates along and don’t limit the overall speed. The rate law for the whole reaction effectively comes from the rate expression of that slowest step, given the concentrations of the species involved. While the slowest step often has the largest barrier, it isn’t an absolute rule that the step with the highest activation energy always controls the rate, since the observed rate also depends on how much of the reactants and intermediates are present. The idea that any step could control the rate depending on concentrations isn’t correct within a fixed mechanism—the controlling step is the slowest one under the given conditions.

In a multi-step reaction, the overall rate is governed by the slowest step, the bottleneck in the sequence. Because this step proceeds the slowest, it sets the pace for converting reactants into products, while the faster steps quickly move intermediates along and don’t limit the overall speed. The rate law for the whole reaction effectively comes from the rate expression of that slowest step, given the concentrations of the species involved. While the slowest step often has the largest barrier, it isn’t an absolute rule that the step with the highest activation energy always controls the rate, since the observed rate also depends on how much of the reactants and intermediates are present. The idea that any step could control the rate depending on concentrations isn’t correct within a fixed mechanism—the controlling step is the slowest one under the given conditions.