1. Reaction Mechanisms
-- the processes by which reactions occur --
i.e., what happens in getting from R to P
-- sometimes, mechanisms are dependent on temp.

2. (or elementary reactions):
elementary steps
(or elementary reactions):
R turn to P in a single step
A reaction’s molecularity is
defined by the number of
particles that participate as
reactants in an elementary step.
unimolecular
bimolecular
termolecular
CH3CN CH3NC
NO + O NO2 + O2
very rare

3. For multistep mechanisms, sequences of elementary steps
are needed to go from R to P.
e.g., For the reaction
NO2 + CO NO + CO2
Elem. Step 1:
Elem. Step 2:
these must
add to give
the overall
reaction
NO2 + NO NO + NO3
NO3 + CO NO2 + CO2
NO2 + CO NO + CO2
Above, NO3 is an intermediate.
All multistep mechanisms have them.

4. rate = k [A] rate = k [A]2 rate = k [A] [B] rate = k [A]3
The rate law and relative speed of each elementary step (no
matter how many there are) determine the overall rate law for
the reaction; that is, the mechanism gives us the overall rate
law. Rate laws for the elementary steps are found as follows...
A + B + C P
ter-
A + A + B P
A + A + A P
A + B P
bi-
A + A P
A P
uni-
Rate Law for that
Elementary Step
Molecularity
rate = k [A]
rate = k [A]2
rate = k [A] [B]
rate = k [A]3
rate = k [A]2 [B]
rate = k [A] [B] [C]
These rate laws are for elementary steps,
not necessarily for the R P reaction.

5. Most reactions have multiple elementary steps.
The slowest of these is the...
rate-determining step.
In producing vegetables from a garden,
the rate-determining step is the growing of the food itself.
The correct rate law for a reaction must:
1) be based on…
the r-d step
2) have only…
REACTANTS in it
(NOT intermediates).

6. CHCl3(g) + Cl2(g) CCl4(g) + HCl(g) has the following elementary steps:
E. S. 1: Cl Cl (fast, eq)
E. S. 2: CHCl3 + Cl  CCl3 + HCl (slow)
E. S. 3: CCl3 + Cl  CCl4 (fast)
State the rate law and the units on the rate constant.
(not a REACTANT)
rate =
k2 [CHCl3][Cl]
For fast eq. steps, you can write a rate law in either dir.
e.g., eq rate = k1f[Cl2] = k1r[Cl]2
 [Cl] = (k1f/k1r [Cl2])1/2
sub. this in for
[Cl] above
So rate = k [CHCl3][Cl2]1/2
units on k = M–1/2 s–1