1. Reaction Mechanism A reaction mechanism is a step-by-step sequence of elementary reactions that describes how reactants are converted into products. • It shows how the reaction occurs, not just the overall change. • Involves one or more elementary steps. • Includes intermediates: species formed in one step and used up in another. • The slowest step in the mechanism is called the rate-determining step (RDS). Example: For the reaction: 2NO2(g)→2NO(g)+O2(g) Possible mechanism: 1. NO₂ + NO₂ → NO + NO₃ (slow) 2. NO₃ → NO + O₂ (fast) 2. Elementary Reaction An elementary reaction is a single step in a reaction mechanism. It cannot be broken down further. • Occurs in one collision or molecular event. • The rate law can be written directly from the molecularity. • No intermediates involved in a single elementary step. • 3. Molecularity of Elementary Reactions Molecularity is the number of reactant particles involved in the step. Type Description Example Unimolecular Involves 1 particle A → B (e.g., decomposition) Bimolecular Involves 2 particles A + B → C (most common) Termolecular Involves 3 particles A + B + C → D (rare due to low probability) ________________________________________ 4. Intermediates vs. Transition States Feature Intermediate Transition State Stability Relatively more stable Very unstable (high-energy peak) Lifetime Exists briefly Exists only at the moment of transition Detectable Sometimes can be isolated Cannot be isolated ________________________________________ 5. Overall Rate Law from Mechanism • Only elementary steps have rate laws matching the stoichiometry. • For the overall reaction, rate law depends on the rate-determining step. • If intermediate appears in rate law, it must be replaced using earlier steps (pre-equilibrium method). ________________________________________ 6. Reaction Mechanism Criteria To be valid, a mechanism must: 1. Sum to give the overall reaction. 2. Be consistent with the experimental rate law. ________________________________________