Explain the rate-determining step in a mechanism.

In the subtopic Mechanisms and rate-determining steps (A-level only), this AQA A-Level Chemistry 7405 learning objective focuses on explain the rate-determining step in a mechanism. It belongs to Rate equations (A-level only), so revision should stay anchored to this exact subtopic rather than drifting into a generic GCSE-level chemistry summary. Approved keywords to use include mechanism, rate-determining step. Rate-determining step. means the slowest step in a reaction mechanism that determines the overall reaction rate Avoid the mistake of students often confuse the rate-determining step with the overall reaction equation, failing to identify which step limits the rate of the reaction; instead, to clarify, remember that the rate-determining step is the slowest step in the reaction mechanism that controls the overall rate. Identify the step with the highest activation energy or the one that takes the longest time to occur For exam answers, when analysing a proposed mechanism, sketch the energy profile for each elementary step and locate the step with the highest activation energy. This step is the rate‑determining step (RDS) and controls the overall rate. Use the Arrhenius equation to estimate relative rates if numerical data are available