Reaction Mechanisms

Reaction Mechanism Reactions occur by a series of steps = Example: Overall reaction: NO2 + CO  NO + CO2 occurs by following steps Step 1: Step 2:

Intermediates Two molecules of NO2 collide Oxygen is transferred, making NO3, the intermediate Intermediates are temporarily formed during a reaction They are neither a reactant nor a product & Get used up in reaction

Rules for Reaction Mechanisms Sum of elementary steps = overall balanced rxn Mechanism must agree with experimental rate law

Elementary Step Steps in reaction from which a rate law for step can be directly written 2 molecules of NO2 need to collide, therefore… Rate = k [NO2]2

Molecularity Rate law written based on molecularity Number of things that have to collide Unimolecular – rxn depends on 1 molecule Bimolecular – rxn depends on 2 molecules Termolecular – rxn depends on 3 molecules Very rare!

Give molecularity and rate law: Unimolecular (first order) rate=k[A] Bimolecular (second order) rate=k[A][B]

Rules for Reaction Mechanisms Sum of elementary steps = overall balanced rxn Mechanism must agree with experimental rate law

Rate Determining Step The slowest step in mechanism determines overall rate Rate cannot be faster than slowest step Demo: Filling bottle with funnel Overall rate law can be written from molecularity of slowest step

How are mechanisms determined? Rate law is determined using experiment (method of initial rates, etc.) Chemist uses intuition to come up w/ various mechanisms Narrows down choices using rules for mechanisms No mechanism is ever absolutely proven