Overview
This lecture covers the concept of rate-determining steps in reaction mechanisms, how rate equations relate to mechanisms, and examples illustrating these principles.
Rate-Determining Step in Reaction Mechanisms
- The rate-determining step is the slowest step in a multi-step reaction and controls the overall reaction rate.
- Only the slowest step restricts how fast the reaction proceeds, regardless of other faster steps.
- The rate equation reveals which reactants are involved in the rate-determining step.
- If a reactant appears in the rate equation, it is involved in the rate-determining step.
- Reactants absent from the rate equation are not involved in the rate-determining step.
- The rate-determining step is not always the first step in a mechanism.
- The mechanism cannot be deduced solely from the balanced overall equation.
Order of Reaction and Mechanism Deductions
- The order of reaction with respect to a reactant shows how many molecules of that reactant are in the rate-determining step.
- If the reaction is second-order with respect to X, two X molecules participate in the rate-determining step.
Example: Reaction Between Chlorine Radicals and Ozone
- The slow step: Cl + O₃ → ClO + O₂ controls the rate.
- The rate equation: rate = k[Cl][O₃], first-order with respect to both Cl and O₃.
Example: Substitution of Cl in Chloromethane by OH⁻
- Mechanism 1: Single-step direct substitution by OH⁻.
- Mechanism 2: Slow breaking of C-Cl bond forming a carbocation, then fast reaction with OH⁻.
- Experimental rate equation: rate = k[CH₃Cl] (does not include [OH⁻]).
- OH⁻ not being in the rate equation supports mechanism 2, where it reacts after the rate-determining step.
Intermediates and Rate-Determining Steps
- Sometimes, the rate-determining step involves intermediates not shown in the overall equation.
- Example: 2NO(g) + O₂(g) → 2NO₂(g) proceeds via a two-step mechanism with N₂O₂ as an intermediate.
- Experimental rate equation: rate = k[NO]²[O₂], indicating step 2 (involving both NO-derived intermediate and O₂) is rate-determining.
Key Terms & Definitions
- Rate-determining step — The slowest step in a multi-step reaction mechanism, controlling the overall rate.
- Rate equation — An equation showing how the rate depends on the concentration(s) of reactant(s).
- Order of reaction — The exponent of a reactant’s concentration in the rate equation, indicating how many molecules participate in the rate-determining step.
- Intermediate — A species formed and consumed during a reaction but not present in the overall equation.
Action Items / Next Steps
- Review different reaction mechanisms and identify the rate-determining steps using rate equations.
- Practice deducing reaction order from experimental rate equations.
- Study the provided diagrams for further clarification if available.