1. WARM UP The rate constant for the 2nd order reaction
2NO2(g)  2NO(g) + O2(g)
is 0.54/M·s at 300°C. How long (in s) would it take for the concentration of NO2 to decrease from 0.62 M to 0.28 M?

2. Reaction Mechanisms
Chemistry II

3. Elementary Steps Reactions occur in a series of small steps
Reaction mechanism is the sequence of these elementary steps
Ex:
2 NO(g) + O2(g)  2 NO2(g)
2 NO  N2O2
N2O2 + O2  2 NO2

4. Elementary Steps Ex: Intermediates are formed and used (ex: N2O2)
2 NO(g) + O2(g)  2 NO2(g)
2 NO  N2O2
N2O2 + O2  2 NO2
Intermediates are formed and used (ex: N2O2)
Catalysts enter and re-appear, while intermediates are produced and disappear

5. Elementary Steps
Necessary because it is much more likely for molecules to meet 2-at-a-time in a chain, rather than 8-at-a-time all at once

6. Molecularity # of reacting molecules in an elementary step
Unimolecular: one reactant
Bimolecular: two reactants
Termolecular: more than 2 (rare)

7. Rate Determining Step Elementary step that determines rate
The slowest “car”

8. Rate Determining Step
Concentration of reactants in rate-determining step gives us the rate law
Ex: 2 NO(g) + O2(g)  2 NO2(g)
2 NO  N2O2
N2O2 + O2  2 NO2
If rate = k [NO], step 1 is rate-determining

9. WRAP UP If the rate law for the reaction, 2 NO(g) + O2(g)  2 NO2(g),
is rate = k[NO][O2]3 , which is the rate-determining step? How do you know this?
Step 1) 2 NO  N2O2
Step 2) N2O2 + O2  2 NO2 

10. WARM UP
“It’s not the load that breaks you down – it’s the way you carry it.”
Do you agree with this? Why?
How does this apply to your own life?

11. WRAP UP
Which type of elementary step is more common – bimolecular or termolecular? Why?