Chem 2 - Chemical Kinetics I: Introduction and Factors Affecting Reaction Rates

1. Chemical Kinetics (Pt. 1)
Introduction and Factors
Affecting Reaction Rates
By Shawn P. Shields, Ph.D.
This work is licensed by Shawn P. Shields-Maxwell under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0
International License.

2. What is Chemical Kinetics?
Chemical kinetics is the study of chemical
reaction rates.
Kinetics can be more simply described as how
the concentration of a given reactant or
product changes in time during a chemical
reaction.

3. What is Chemical Kinetics?
In kinetics experiments, reaction conditions
are changed in a systematic way.
The effect on the rate of the reaction
(i.e., speed of reaction) is measured.
Reaction rates can be measured using a
variety of spectroscopic methods.

4. Kinetics and Mechanism
Kinetics provides a way to study complicated
reactions and determine how they proceed.
Once we know how a reaction proceeds, we
may be able to learn to control it.
Kinetic studies can also provide evidence and
support for a theory about how a reaction
works (its mechanism), but can never “prove”
the mechanism.

5. Macroscopic vs Microscopic Aspects of Kinetics
The macroscopic aspect involves how fast
the reaction runs (overall).
Rate laws are determined by experiment.
Rate laws show the dependence on
concentrations and/or partial pressures of
reactants.

6. Macroscopic vs Microscopic Aspects of Kinetics
The microscopic aspect of reactions
involves how the reaction proceeds at
the molecular level.
A reaction mechanism is determined,
which includes a series of elementary
steps in converting reactants to
products.

7. Kinetics and Mechanism
Example: SN1 Reaction
When t-butyl bromide is boiled in methanol, the reaction
produces methyl-t-butyl ether.
This looks like a very specific reaction, but it can actually be
placed in a large category of reactions called “First-Order
Nucleophilic Substitution” (or SN 1).
t-butyl bromide methanol methyl-t-butyl ether

8. Step 1 (slow):
Step 2 (fast):
Step 3 (fast):
Mechanism for SN1 Reaction

9. Collision Theory and Reaction Rate
Collision theory offers the basic framework
for understanding chemical kinetics.
The basic assumption is that molecules must
come into contact (collide) to react.
Therefore, (forward) reaction rates are
affected by several factors.

10. Factors Affecting Reaction Rates
1) Concentration of the reactants (higher
concentration leads to a faster rate because
there are more collisions)
2) Temperature (higher T leads to more collisions
and with greater energy, which leads to a faster
rate)

11. Factors Affecting Reaction Rates
3) The physical state of the reactants:
Molecules must mix to react (solutions provide
more opportunities for mixing)
The amount of surface area on a solid available
for reaction
Nanoparticles have more surface area than
larger particles.
More surface area; faster rate

12. Factors Affecting Reaction Rates
4) The addition of a catalyst (usually involves a
different mechanism of reaction)
Trypsin Active Site by Fdardel (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via
Wikimedia Commons
Enzymes, such as
Trypsin, act as catalysts
in chemical reactions.

13. Next up…
Reaction rates and how
they change with
concentration (Pt. 2)