1. Kinetics Part One: Rates
Dr. Joshi
Unless otherwise noted, graphics are from Olmsted and Williams, 3rd ed.

2. Kinetics • how fast a reaction occurs • molecular pathway (mechanism)

3. Isomerization of Cis-2-butene to Trans-2-butene
cis -2-butene can rearrange if collisions give the molecule
enough extra energy to break the C—C p bond.

4. Rate of Isomerization
Rate of isomerization decreases as the number of cis-2-butene
molecules decreases.

6. If an N2O4 molecule possesses enough energy, the vibration can
break the N-N bond to produce two NO2 molecules.

7. The rate of reaction decreases as the number of molecules
decreases because fewer NO2 molecules are present to
undergo collisions.

8. Concentration vs. Time for Decomposition of NO2
Balanced chemical equation?
rate = D[ ]
Dt units ?
Calculate rate over the first ten-second interval in terms of NO2, NO and O2.
Note that the rates relate to the reaction stoichiometry.
Rate of reaction = D[O2] = 1/2 D[NO] = – 1/2 D[NO2]
Dt Dt Dt

9. Concentration vs. Time for Decomposition of NO2 (Cont.)
Rate decreases over time.
To compare relative rates, must compare over the same time interval.
The rate (slope) also depends on the length of the chosen time interval.
When you choose an infinitesimally small time interval (t),
the slope is for a line that is tangent to the curve.
This slope (derivative) is the instantaneous rate
(the rate at that moment in time).
When we take an instantaneous rate at the very beginning of the reaction,
it is called the initial rate.