Question

The rate-determining step in a reaction mechanism is the one with:

• A. The lowest activation energy
• B. The highest activation energy
• C. No energy barrier
• D. The fastest rate

Hint:

Remember: The rate-determining step is the slowest step in a reaction pathway — it has the highest energy barrier to overcome.

Answer 1

The Correct Option is B

In a multi-step chemical reaction, each elementary step may proceed at a different rate depending on its activation energy.
The rate-determining step (also called the slowest step) is the one that controls the overall reaction rate.
This step is typically characterized by the highest activation energy barrier that must be overcome for the reaction to proceed.
Since higher activation energy means a slower rate (as per the Arrhenius equation: $k = A e^{-E_a/RT}$), this step becomes the bottleneck in the reaction pathway.
Let’s analyze the options:
- (1) The lowest activation energy: This would correspond to a fast step, not rate-determining.
- (3) No energy barrier: A hypothetical step with no energy barrier would proceed instantly — not rate-limiting.
- (4) The fastest rate: The rate-determining step is actually the slowest, not the fastest.
Therefore, the step with the highest activation energy governs the rate of the entire reaction mechanism.