The Bottleneck Concept: In a multi-step mechanism, the overall rate of the reaction is determined by the slowest elementary step, known as the rate-limiting step. This step has the highest activation energy () relative to its starting point.
Predicting the Rate Law: If the first step is the RLS, the overall rate law is simply the rate law of that first step. Reactants involved in steps occurring after the RLS do not appear in the rate law.
Kinetic Control: Because the RLS is the slowest, any increase in the concentration of reactants for that specific step will significantly increase the overall reaction rate, whereas changes to reactants in fast steps have negligible effects.
Scenario: This method is applied when the first step is fast and reversible (reaches equilibrium) and is followed by a slow rate-limiting step. In these cases, the RLS often involves an intermediate as a reactant.
Substitution Method: Since intermediates cannot be in the final rate law, we use the equilibrium of the first step to express the intermediate's concentration in terms of the initial reactants. This is done by setting the forward rate equal to the reverse rate ().
Resulting Rate Law: The final rate law often contains reactant concentrations raised to powers that do not match the overall stoichiometry, providing a unique signature that supports the proposed multi-step mechanism.
| Feature | Intermediate | Transition State (Activated Complex) |
|---|---|---|
| Stability | Relatively stable; exists in a local energy minimum (trough). | Highly unstable; exists at an energy maximum (peak). |
| Isolation | Can sometimes be detected or isolated under specific conditions. | Cannot be isolated; represents a fleeting state of bond breaking/forming. |
| Representation | Appears as a product in one step and reactant in the next. | Not shown in chemical equations; represents the 'top of the hill'. |
Verify the Sum: Always check that the sum of the proposed elementary steps cancels out all intermediates and catalysts to yield the correct overall balanced equation.
Identify the RLS on Graphs: On an energy profile, the step with the largest energy gap between its starting trough (or reactant level) and its peak is the rate-limiting step. Do not just look for the highest absolute peak.
Check the Rate Law: If a question provides an experimental rate law like , and the first step of the mechanism is , the mechanism is likely incorrect or involves a pre-equilibrium step because the orders do not match.
Intermediate vs. Catalyst: Remember that an intermediate is produced then consumed (P C), while a catalyst is consumed then produced (C P).
