1. Reaction Mechanisms

2. Reaction Mechanism Reactions occur by a series of steps =
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