Reaction Mechanisms

1. Reaction Mechanisms

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

3. 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

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

5. 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

6. 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!

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

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

9. 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

10. 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

11. Practice • Pg 601 #46 Discussion questions Pg597 #5,6