Chem 2 - Chemical Kinetics VI: Microscopic Aspects of Kinetics - Chemical Reaction Mechanisms

1. Chemical Kinetics (Pt. 6)
Microscopic Aspects of
Kinetics- Chemical Reaction
Mechanisms
By Shawn P. Shields, Ph.D.
This work is licensed by Dr. Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0
International License.

2. Chemical Reactions at the
Molecular Level
How do individual species (molecules,
atoms, etc) interact with each other
during the reaction to produce
product(s)?
This question is answered by the
chemical reaction mechanism.

3. Reaction Mechanisms
So what is a reaction mechanism?
A sequence of elementary reactions
(called “steps”) involved in the
conversion of reactants to products.

4. Elementary Steps
The elementary reactions in a
reaction mechanism are called
“elementary steps.”
An elementary step is a reaction that
occurs in a single step and cannot be
subdivided into simpler parts.

5. Writing the Rate Law for an
Elementary Step
The rate law for an elementary step
can be written using the coefficients
for the reactants.
Example: NO2 + F2 NO2F + F
The rate law for the step is
Rate = k1[NO2]1 [F2]1
k1

6. Molecularity
The “molecularity” of an elementary step
is simply the number of reactants
involved.
Example: NO2 + F2 NO2F + F
The molecularity is 2.
Two molecules collide and react
in this step.
k1

7. Elementary Steps
Caution:
Assume that a reaction is NOT an
elementary step unless
You are specifically told it is an
elementary step.
Or, it is a step in a mechanism.

8. Recall: Example SN1 Reaction
Boiling t-butyl bromide in methanol produces
methyl-t-butyl ether.
The reaction proceeds by a “First-Order
Nucleophilic Substitution” (or SN 1) reaction
mechanism.
t-butyl bromide methanol methyl-t-butyl ether

9. Elementary Step
1 (slow):
Elementary
Step 2 (fast):
Elementary
Step 3 (fast):
(Elementary) Steps in the Mechanism for SN1
Reaction

10. Rate-Determining Step
The slowest step in a chemical
reaction determines the kinetics for
the entire reaction.
The reaction cannot proceed faster
than the slowest step…
The “Rate-Determining Step”

11. The Rate-
Limiting
Step
Rate Determining Step from
http://2012books.lardbucket.org/books/principles-of-
general-chemistry-v1.0m/s18-06-reaction-rates-a-
microscopic-v.html

12. Fast Steps in a Kinetic Mechanism
Steps that are faster than the
slow step are “invisible,”
..in that they do not affect the
overall reaction rate.

13. Reaction Intermediates
An “intermediate” is a species
involved in a mechanism step (or more
than one) that is not a reactant or a
product.
Intermediates are transient
species.

14. Analyzing Mechanisms
We are going to learn to analyze two
types of reaction mechanisms:
Reactions where the first step in
the mechanism is the slow step.
Reactions involving one (or more)
fast equilibrium step(s).

15. Analyzing Mechanisms
Reaction mechanisms can never be
“proven” correct.
Experimental evidence for a mechanism
can be collected and used to support the
proposed mechanism.

16. Analyzing Mechanisms
One way to support a proposed reaction
mechanism:
Show that the rate law derived from
the mechanism matches the
experimentally-derived rate law.

17. Analyzing Mechanisms
Use the method of initial rates or the
integrated rate equation and graphing
to determine the experimental rate law.
Compare this result to the rate law
derived from the mechanism.
If they match, the mechanism may be
correct!

18. Example Problems
will be posted separately.
Next up,
Analyzing Kinetic Mechanisms
and Deriving the Rate Law
(Pt 7)