1. Kinetics: Part I: Rates of Reaction
Jespersen Chapter 14 Sec 1 & 2
Dr. C. Yau
Fall 2014

2. Chemical Kinetics
"Chemical Kinetics" refer to how fast a reaction runs.
Gen Chem I – studied stoichiometry
We predicted how much product will be made.
We assumed reaction has finished.
Actually some reactions are very slow and this can be a reason why sometimes % yield is low.

3. Chemical Kinetics In this chapter, we study the rates of reaction.
It's like going on a trip. Stoichiometry is like noting at the end of the trip how many miles we have traveled.
Kinetics is like noting how fast, in miles per hour, we are traveling during the trip. The speed may fluctuate during the trip.

4. Chemical Kinetics We measure how fast a reaction runs by measuring…
the rate of formation of a product or
the rate of disappearance of a reactant
reactant product.

5. Factors Affecting Reaction Rate
Chemical nature of the reactants
Bond strengths
General reactivity (instability of reactants)
e.g. Reactivity of Group IA metals increases as you go down the column (Li, Na, K, Rb, Cs).
Na in water
K in water
Rb & Cs in water
Why is there such a trend?
What is the reaction of these metals with water?
Figure 13.2 The chemical nature of reactants affects reaction rates. (a) Sodium loses electrons easily, so it reacts quickly with water. (b) Potassium loses electrons even more easily than sodium, so its reaction with water is explosively fast.
Particle size and reaction rate. An explosion in this grain elevator in New Orleans, Louisiana, killed 35 people in December 1977.

6. Factors Affecting Reaction Rate (contd.)
2. Ability of the reactants to come in contact with each other
Physical state (consider solid + solid; solid +liquid etc.)
Surface area for liquids, solids, and heterogeneous mixtures
Amount of Mixing
Particle shape/size
Concentration of reactants
Molarity for solutions (higher conc means reactants are closer to each other.)
Pressure effects for gases
Volume effects for gases
Temperature
Catalysts
Figure 13.4 Effect of concentration on rate. Steel wool, after being heated to redness in a flame, burns spectacularly when dropped into pure oxygen.

7. Which of the following would speed a reaction?
stirring it
dissolving the reactants in water, if ionic
adding a catalyst
grinding any solids
boil off some of the solvent
all of these

8. Measuring Rates
Can be measured using any substance in the reaction. We choose one that can be easily detected.
Units: M/s or
Measured in three ways:
instantaneous rate
initial rate (most often used)
average rate
Figure 13.5 Effect of time on concentration. The data for this plot of the change in the concentration of HI with time for the reaction 2HI(g) ® H2(g) + I2(g) at 508 °C are taken from Table The slope is negative because we’re measuring the disappearance of HI. But when its value is used as a rate of reaction, we express the rate as positive, as we do all rates of reaction.
We often select the substance to monitor due to its ease of detection. For example, the disappearance of a vivid color tells us that that particular reactant is consumed and that the reaction is complete.

9. Fig. 14.5 p.643: Instantaneous Rate
2HI(g) H2(g) + I2 (s)
At time 100s, the rate is determined by the slope of the tangent at 100s.
The graph is for disappearance of HI, thus slope is negative.
BUT rate is always given as a positive number:
Rate = (.027M)/110s
= 2.5x10-4 M/s

10. Why is the graph not linear?
What happens to the rate as time progresses?
What happens to the
slope as time
progresses?
Why?
As HI is consumed,
there are less HI to
collide with each other
to form the products….
& reaction slows down.

11. Instantaneous Rates Changes With Time
At 100s, rate =2.5x10-4M/s
At 250s, rate = 1 x10-4M/s
As time progresses,
reaction slows down.
Slope= -0.01M/100s
rate=1x 10-4 M/s

12. Initial Rate of Reaction
"Initial Rate of Reaction" is the instantaneous rate at time zero.
A tangent is drawn at time zero & the slope is determined.
See Example 14.2 p.643
Do Practice Exercises 3 & 4 p.644

13. Instantaneous Reaction Rates
Instantaneous rate: the slope of the tangent to the curve at any specific time
Initial rate determined at the initial time
NO2 appearance
0.005
0.01
0.015
0.02
0.025
0.03
0.035
200
400
600
800
Time (s)
[NO2]
Instantaneous rate: Very fast. Much smaller reproducibilities for all of the same reasons as above. It is imperative that the same time be used, so auto mixers and cell concentration measurements must be made.
Chem FAQ: Estimate the initial rate of a reaction from experimental data.
Why does this graph look so different from the previous graphs?

14. Average Reaction Rates
Average rate of reaction: the slope of the line connecting the starting and ending coordinates for a specified time frame.
NO2 appearance
0.005
0.01
0.015
0.02
0.025
0.03
0.035
200
400
600
800
Time (s)
[NO2]
e.g. Average rate for time period of seconds is measured thus...
(600s, M)
Average Rate: Faster, because we can choose any time interval. Less reproducible because the (error in time/time frame) factor is larger. Requires that we be able to measure concentration. Requires that we use the same time interval reproducibly.
(200 s, M)

15. What is the average rate of B between 10 and 40 s?
M/s
M/s
M/s
M/s
can’t tell from the information
Time Elapsed in Reaction Progress (s)
Concentration of B (M)
0.0
0.1
0.2
0.3
0.4
X (10s, 0.20M) X
(40s, 0.03M)
What happened to the negative sign?
Is B a reactant or product?

16. Rates And Stoichiometry
Example 14.1 p. 640
Butane, C4H10, the fuel in cigarette lighters, burns in oxygen to give carbon dioxide and water.
If, in a certain experiment, the butane concentration is decreasing at a rate of 0.20 mol L-1 s-1, what is the rate at which the oxygen concentration is decreasing, and what are the rates at which the product concentrations are increasing?
We typically describe rates as positive numbers, no matter on which substance we base the measurement.
Chem FAQ: How is the rate at which a reactant disappears related to the rate at which a product appears?
Do Practice Exercise 1 & 2 on p. 641.

17. In the reaction: 2A + 3B →5D we measured the rate of disappearance of substance A to be 3.5×10-5M/s. What is the rate of appearance of D?
In the reaction 3A + 2B →C, we measured the rate of B. How does the rate of C relate?
Rate of appearance of D = RD = 8.8×10-5 M/s
RC=1/2 RB
Rate of formation of C should be slower than rate of consumption of B (by ½).

18. In the reaction 2CO(g) + O2(g) →2CO2(g), the rate of the reaction of CO is measured to be 2.0 M/s. What would be the rate of the production of CO2?
the same
twice as great
half as large
you cannot tell from the given information