In a multi-step mechanism, the overall rate is determined by

In a multi-step mechanism, the overall rate is determined by

• A. The fastest step.
• B. The slowest step.
• C. The initial reactants' energy.
• D. The last step to occur.

Answer: (B)

In a reaction that proceeds through several elementary steps, the overall rate is governed by the slowest step, the bottleneck in the sequence. This happens because the faster steps quickly adjust the intermediates so that their fastest pace can’t push the product formation any faster than the slowest step allows. The rate at which products appear is essentially set by how quickly the rate-determining step can proceed, so the observed rate law reflects that step’s reactants and its rate constant. For example, if the slow step is A + B → C, then the overall rate will be proportional to k_slow[A][B], even though subsequent steps happen quickly. The idea that the fastest step controls the rate isn’t correct because those steps can’t accelerate the overall process beyond the speed of the slowest one. The energy barrier of the initial reactants or the last step being slow don’t by themselves determine the overall rate; it’s specifically the step with the smallest rate constant.

In a reaction that proceeds through several elementary steps, the overall rate is governed by the slowest step, the bottleneck in the sequence. This happens because the faster steps quickly adjust the intermediates so that their fastest pace can’t push the product formation any faster than the slowest step allows. The rate at which products appear is essentially set by how quickly the rate-determining step can proceed, so the observed rate law reflects that step’s reactants and its rate constant. For example, if the slow step is A + B → C, then the overall rate will be proportional to k_slow[A][B], even though subsequent steps happen quickly. The idea that the fastest step controls the rate isn’t correct because those steps can’t accelerate the overall process beyond the speed of the slowest one. The energy barrier of the initial reactants or the last step being slow don’t by themselves determine the overall rate; it’s specifically the step with the smallest rate constant.