1. Chapter 12 Chemical Kinetics Speed Steps
(The study of how reaction rates change under varying conditions and what molecular events occur during an overall reaction)
Speed
Steps
(how fast or slow a reaction proceeds)
(a reaction mechanism)
←We’ll look at this today

2. Data for a reaction: 2 N2O5(g) → 4 NO2(g) + O2(g) [N2O5] Time (min)
←This stands for the concentration of dinitrogen pentoxide
The following data shows how the concentration changes as the reaction proceeds.

3. = = -1.42 x 10-5 M/s = -1.71x 10-5 M/s Here is the Rate of Change for
Rate = D[N2O5] M – M
0.240
On the graph paper given, create the scales that will be shown for each axis.
0.220 =
0.200
D time s – 2400 s
Rate = x10-5 M/s
0.180
Called Average Rates (over a time interval)
…for 2400 → 4800 s
Rate = M – M
0.160
2400 s – 4800 s
[N2O5]
= x 10-5 M/s
0.100
Find the points shown in black placed on that red line and mark them on yours, then find the slope of this line
0.080
0.060
Now, draw the best tangent line on your graph where the red dot appeared
Instantaneous Rate Rate = M – M
0.040
Now, plot each point from your data!
1400 s – 4200 s
0.020
= -1.71x 10-5 M/s
1000
2000
6000
7000
8000
Time (sec)

4. rate constant = k [N2O5]x [N2O5] Rate = -D[N2O5] Dtime Time (sec)
0.240
As the reaction proceeds the average reaction rate goes down (look at the calculated values!)
The rate at a given moment can be found by finding the slope of the line tangent to the curve at that point
The rate of the reaction then depends on the concentration of the reactant that remains
Therefore:
0.220
0.200
0.180
0.160
[N2O5]
Called the order
rate constant
0.100
Rate = -D[N2O5]
= k [N2O5]x
0.080
Dtime
Negative because the reactant is decreasing and we want a positive value for the rate
Rate Expression
Rate Law
0.060
…which means that the concentration raised to some power times the constant of proportionality, k, also equals the rate
But since the rate is also directly related to the concentration of the reactant remaining, we could also say…
0.040
0.020
1000
2000
6000
7000
8000
Time (sec)