Rates of reactions

1. CHE 116: Week 2
Rates of Reactions

2. Tell me something good!

3. Rates of Reaction – How long does it take for a
reaction to occur?
• Measure rate of anything – quantity per time
• Travelling
• Eating cookies
• Typing
• In chemical reactions, quantity is measured as concentration
• The unit of time we use is seconds
The rate of reaction is then:

4. What are the four factors that influence rate
of reaction?
1)
2)
3)
4)

5. State of reactants
• Surface area of contact
• Homogenous mixtures – well-
mixed reactions will have a
higher reaction rate as a result
of more collisions
Temperature of reaction
• As temp increases, rate
increases
• Molecules move faster, collisions
occur more frequently, more
energy behind each collision,
more reactions
Concentration of reactants
• As we increase concentration,
reaction rate increases
• Higher concentration = more
molecules in a given volume
• More molecules = more chances
for collisions to occur
Presence of a catalyst
• Changes the energy profile of a
reaction
• Reaction is made faster because
the energy barrier of the
reaction is lowered

6. Br2 (aq) + HCOOH (aq) → 2 Br- (aq) + 2 H+ (aq) + CO2 (g)
This reaction was studied, and the concentration of Br2 was tracked to
determine the rate of reaction. The following date was collected:
Time (s) [Br2] (M)
0 0.01200
50 0.01010
100 0.00846
150 0.00710
200 0.00596
250 0.00500
300 0.00420
350 0.00353
400 0.00296
Average rate can be determined between any point in
time within the range of dataset:
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 = −
∆[𝐵𝑟2]
∆𝑡𝑖𝑚𝑒
Where ∆[𝐵𝑟2] =
And ∆𝑡𝑖𝑚𝑒 =
So: Average rate =

7. Br2 (aq) + HCOOH (aq) → 2 Br- (aq) + 2 H+ (aq) + CO2 (g)
Time (s) [Br2] (M)
0 0.01200
50 0.01010
100 0.00846
150 0.00710
200 0.00596
250 0.00500
300 0.00420
350 0.00353
400 0.00296
Concentration of Br2
tracked over time of
reaction detailed above:
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 = −
∆ 𝐵𝑟2
∆𝑡𝑖𝑚𝑒
= −
[𝐵𝑟2]𝑓𝑖𝑛𝑎𝑙−[𝐵𝑟2]𝑖𝑛𝑖𝑡𝑎𝑙
𝑡𝑖𝑚𝑒𝑓𝑖𝑛𝑎𝑙 − 𝑡𝑖𝑚𝑒𝑖𝑛𝑖𝑡𝑖𝑎𝑙
Why is there a negative sign at the
front of the equation?
What should the units of the
reaction rate be?

8. Br2 (aq) + HCOOH (aq) → 2 Br- (aq) + 2 H+ (aq) + CO2 (g)
Time (s) [Br2] (M)
0 0.01200
50 0.01010
100 0.00846
150 0.00710
200 0.00596
250 0.00500
300 0.00420
350 0.00353
400 0.00296
Concentration of Br2
tracked over time of
reaction detailed above:
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 = −
∆ 𝐵𝑟2
∆𝑡𝑖𝑚𝑒
= −
[𝐵𝑟2]𝑓𝑖𝑛𝑎𝑙−[𝐵𝑟2]𝑖𝑛𝑖𝑡𝑎𝑙
𝑡𝑖𝑚𝑒𝑓𝑖𝑛𝑎𝑙 − 𝑡𝑖𝑚𝑒𝑖𝑛𝑖𝑡𝑖𝑎𝑙
What is the rate of the reaction with respect to Br2 for the first 100
seconds of the reaction?
What is the rate of the reaction with respect to Br2 between 100
seconds and 200 seconds of the reaction?

9. Br2 (aq) + HCOOH (aq) → 2 Br- (aq) + 2 H+ (aq) + CO2 (g)
Time (s) [Br2] (M)
0 0.01200
50 0.01010
100 0.00846
150 0.00710
200 0.00596
250 0.00500
300 0.00420
350 0.00353
400 0.00296
Concentration of Br2
tracked over time of
reaction detailed above:
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 = −
∆ 𝐵𝑟2
∆𝑡𝑖𝑚𝑒
= −
[𝐵𝑟2]𝑓𝑖𝑛𝑎𝑙−[𝐵𝑟2]𝑖𝑛𝑖𝑡𝑎𝑙
𝑡𝑖𝑚𝑒𝑓𝑖𝑛𝑎𝑙 − 𝑡𝑖𝑚𝑒𝑖𝑛𝑖𝑡𝑖𝑎𝑙
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0 100 200 300 400 500
Contrantion
of
Br2
(M)
Time of reaction (s)
[Br2] (M) over time of reaction (s)
Thinking about your answers when
calculating reaction rate for the first
100 seconds and the next 100
seconds, how do those rates
compare?
Why is this the case?

10. Average rate vs. Instantaneous rate
Average rate:
Instantaneous rate:
Example: If you drove to Boston from Syracuse (312 miles) in 5 hours,
your average rate of travel was 62.4 miles/hour. Your instantaneous
rate would be checking your speedometer at one point and seeing you
were traveling at a rate of 72 miles/hour
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0 100 200 300 400 500
Contrantion
of
Br2
(M)
Time of reaction (s)
[Br2] (M) over time of reaction (s)

11. Reaction Order & Rate Law
Reaction Order
• Tells us about the relationship between the concentration of species in a
reaction and the rate of reaction
• Power of dependence
• We need to determine it experimentally
• It is used to determine the rate law for a specific reaction
Rate Law
• A mathematical relationship between rate of reaction and the reactant
concentrations
• The concentrations of the products do not show up in rate law! (Why?)

12. You run a reaction several times and determine
the rate law is: Rate = k[X]2[Y]1[Z]2
What is the reaction order with respect to:
[X]:
[Y]:
[Z]:
What is the overall order of reaction?

13. Consider the reaction: A + B → C (run at 25°C)
You collect the following data:
Trial [A] [B] Rate (M/s)
1 1.50 1.50 0.032
2 1.50 3.00 0.064
3 3.00 1.50 0.128
What is the rate law of this
reaction?
Analysis: What happens to the rate?
Trials 1 & 2:
Trials 1 & 3:
Reaction Order with respect to:
[A]:
[B]:

14. Consider the reaction: X + Y → Z (run @ 32°C)
You collect the following data:
Trial [X] [Y] Rate (M/s)
1 0.800 0.220 0.215
2 3.200 0.220 0.860
3 0.800 0.440 0.215
Analysis: What happens to the rate?
Trials 1 & 2:
Trials 1 & 3:
Reaction Order with respect to:
[X]:
[Y]:
What is the rate law of this
reaction?

15. Recall:
Reaction Order
• Tells us about the relationship between the concentration of species
in a reaction and the rate of reaction
• Power of dependence
• If the reaction order with respect to [Y] is zero, what does this tell
us?
• What would happen if the concentration of Y was 10 M? 100 M?
Trial [X] [Y] Rate (M/s)
1 0.800 0.220 0.215
2 3.200 0.220 0.860
3 0.800 0.440 0.215

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