1. Reaction Mechanisms Most reactions occur in a series of steps
Some products of a first step are used up as reactants in a second step and are not seen in an overall reaction (intermediate)
Individual reactions are called elementary steps
TWO REQUIREMENTS

2. Molecularity
Deals with the number of species colliding when a reaction happens
UNImolecular: one molecule = first order
BImolecular: two molecules/species = second order
TERmolecular: three species = third order

3. Elementary Steps Requirements
Sum of elementary steps must give the overall balanced equation for the reaction
Mechanism must agree with the experimentally determined rate law

4. Rate-Determining Step
Helps confirm the #2 from the last slide
The intermediate step that is much slower than the other steps
This slower step determines the rate law used in calculations
After determining the rate law, the mechanism can be hypothesized

5. Example 2NO + 2H2 -> 2H2O + N2 Rate = k[NO2]2[H2]
Mechanism: NO + H2 -> N + H2O (slow)
N + NO -> N2O (fast)
N2O + H2 -> N2 + H2O (fast)
Is this mechanism consistent with the observed rate law?
Answer: (part 1) yes, (part 2) no

6. Example #2 2NO + 2H2 -> 2H2O + N2 Rate = k[NO2]2[H2] Mechanism:
NO + H2 <-> N + H2O (fast, with equal rates)
N + NO -> N2O (slow)
N2O + H2 -> N2 + H2O (fast)
Is this mechanism consistent with the observed rate law?
Answer: (part 1) yes, (part 2) no