1. Draw a Circle
Draw a Star
Draw a Smiley Face
PRINT your full name (middle name too)
Write 5 X’s
Draw a pac-man
Fold the paper in half and make a pile.

2. Reaction Mechanisms

3. Collision theory
Collision between 2 particles is necessary for a reaction to occur
One particle can decompose
Three particles colliding with sufficient E and correct orientation is… HIGHLY unlikely

4. Reaction Mechanism Reactions occur by a series of steps called a
Example:
Overall reaction: NO2 + CO  NO + CO2
occurs by following steps
Step 1:
Step 2:
N- blue, C- green O - Red

5. Intermediates Two molecules of NO2 collide in step 1
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
Concept check! How does an intermediate differ from a catalyst?

6. Rules for Reaction Mechanisms
Sum of elementary steps must equal overall balanced reaction.
Mechanism must agree with experimental rate law

7. Elementary Step
Steps in reaction from which a rate law can be directly written
In step 1, 2 molecules of NO2 need to collide, therefore…
Rate = k [NO2]2

8. Molecularity Rate law written based on molecularity
Molecularity equals the 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!

9. Molecularity and rate law:
Unimolecular (first order)
rate=k[A]
Bimolecular (second order)
rate=k[A][B]

10. Rate Determining Step
The slowest step in mechanism determines overall rate
Rate cannot be faster than slowest step
Overall rate law can be written from molecularity of slowest step
Like an assembly line

11. 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

12. Practice problem 1
The rxn: 2NO2 (g) + F2 (g)  2NO2F (g) Experimental rate law: rate = k[NO2][F2] Suggested mechanism: NO2 + F2  NO2F + F Slow F + NO2  NO2F Fast Is this an acceptable mechanism? That is does it satisfy the two requirements?

13. Practice problem 2
The rxn: 2NO (g) + 2H2 (g)  N2 (g) + 2H2O (g) Experimental rate law: rate = k[NO]2[H2] Suggested mechanism: H2 +NO  ZZ Fast ZZ + NO  BB + H2O Slow BB + H2  N2 + H2O Fast Is this an acceptable mechanism? That is does it satisfy the two requirements?