CHM 152
Lab 4: Chemical Kinetics
Last updated Jan. 2013
Introduction
In this lab you will use the initial rate method to determine the rate law of the reaction between persulfate
(a.k.a. peroxodisulfate), S2O82-, and iodide.
S2O82-(aq) + 3I-(aq)  2SO42-(aq) + I3-(aq)
(Eq 1)
The rate will be calculated by measuring the change in concentration of I 3- over over time.
All four species are colorless, so a small amount of starch will be added, which (ideally) forms a blue
complex with triiodide (I3-). However, triiodide will begin to form immediately after mixing the two
reagents, so we will also add thiosulfate ion (S2O32-) to the solution, which reacts with triiodide as shown in
the Equation 2.
2S2O32-(aq) + I3-(aq)  S4O62-(aq) + 3I-(aq)
(Eq 2)
Once the thiosulfate completely reacts, triiodide will begin to accumulate in solution and immediately form
the blue complex with starch. By using a known amount of thiosulfate, we can stoichiometrically
determine how much triode it reacted with during this time period (Eq. 2), which is equal to the amount
produced in the reaction studied (Eq. 1).
Furthermore, if we use the same amount of thiosulfate each time, our observed [I3-] will also be constant.
Only the time required to observe this change ( t) will vary because of changes made in the initial S 2O82and I- concentrations.
You will also study the reaction in Equation 1 at various temperatures to determine it’s activation energy
(Ea) and collision frequency factor (A). Using the Arrhenius equation
ln k = ln A – (Ea/R)(1/T)
a plot of ln k vs. 1/T should be linear with a y-intercept equal to ln A and a slope equal to Ea/R. Here, R is
the gas constant, 8.314 J/mol-K.
Concepts to Review
Reaction rates
Initial rate method
Arrhenius equation
Plotting with Excel
Diluting solutions
Stoichiometry
Procedure
Since you’re essentially performing the same reaction 10 times, you should split the trials between you and
your partner (i.e., you perform half, your partner performs the others half).
Part I: Studying the effect of [S2O82-] on reaction rate
1. Prepare the two solutions, A and B, given in trial 1 of the table below. Use volumetric pipettes to
measure volumes of 2mL or less. For all others, you may use a 10mL graduated cylinder.
Do not slow the rest of the class by measuring individual volumes at the reagent area! Measure out
approximate totals for each solution in a beaker and take it back to your work area.
Solution A
Solution B
starch,
0.012 M
0.20 M
0.20 M
0.20 M
0.20 M
mL
Na2S2O3, mL
KI, mL
KNO3, mL
(NH4)2S2O8, mL
(NH4)2SO4, mL
1
2.0
2.0
4.0
4.0
8.0
0.0
2
2.0
2.0
4.0
4.0
4.0
4.0
3
2.0
2.0
4.0
4.0
2.0
6.0
4
2.0
2.0
4.0
4.0
1.0
7.0
[Note: KNO3 and (NH4)2SO4 are added to keep number of ions in solution constant for each determination.
Neither compound interferes with the reaction being studied.]
Trial
2. Combine the two solutions, stirring a few times to mix. Record the time required for the solution to
change color.
3. Repeat the reaction three more times using the volumes given in trials 2-4.
Part II: Studying the effects of [I-] on reaction rate
Repeat Part I using the following volumes.
Trial
1
2
3
4
starch,
mL
2.0
2.0
2.0
Solution A
Solution B
0.012 M
0.2 0M
0.20 M
0.20 M
0.20 M
Na2S2O3, mL
KI, mL
KNO3, mL
(NH4)2S2O8, mL
(NH4)2SO4, mL
2.0
8.0
0.0
4.0
4.0
Use your results from Part I, trial 2
2.0
2.0
6.0
4.0
4.0
2.0
1.0
7.0
4.0
4.0
Part III. Studying the effect of temperature on reaction rate
Repeat Part I using the following volumes. For trial 1 use an ice bath (or two, if you can’t use a large
enough beaker) to cool both solutions to the same approximate temperature. Return the reaction mixture to
the ice bath after combining the solutions. For Trials 3 and 4, use a hot plate to heat the solutions to the
same approximate temperature. Return the reaction mixture to the hotplate after combining the solutions.
Trial
Temp., C
starch,
mL
1
2
3
4
room-10
room
room+10
room+20
2.0
2.0
2.0
Solution A
Solution B
0.012 M
0.20 M
0.20 M
0.20 M
0.20 M
Na2S2O3,
KI, mL
KNO3,
(NH4)2S2O8,
(NH4)2SO4,
mL
mL
mL
mL
2.0
4.0
4.0
2.0
6.0
Use your results from Part I, trial 3
2.0
4.0
4.0
2.0
6.0
2.0
4.0
4.0
2.0
6.0
Waste Disposal
Reaction solutions can be poured down the drain with running water. Volumetric pipettes should be rinsed
twice with water (fill to the volume line).
Name: _____________________________
Section: ________
Data
Part I: Studying the effect of [S2O82-] on reaction rate
Trial
1
2
3
4
1) [I-] after mixing A & B (M)
__________
__________
__________
__________
2) [S2O82-] after mixing A & B (M)
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
3) Time required for color change (s)
Time data from a second group of students (s)
Time data from a third group of students (s)
Average time observed (s)
4) Moles of S2O32-
__________
5) Moles of I3- produced during time observed
__________
6) [I3-] during time observed, (M)
__________
7) Initial Rate, based on average time (M/s)
__________
__________
__________
__________
In the space below, show your work for calculating the initial rates of Trials 1 and 2
8) In Part I, find three sets of trials where the concentration of persulfate is doubled. List each set below.
a) Trials ____ & ____
b) Trials ____ & ____
c) Trials ____ & ____
9) For each set of trials, determine the effect doubling concentration had on the reaction rate, rounding to
the nearest integer.
a) __________
Show your work for 9a
b) __________
c) __________
10) For each set of trials, determine the reaction order with respect to persulfate
a) __________
11) Average order
(rounded to nearest integer)
b) __________
__________
c) __________
Part II: Studying the effects of [I-] on reaction rate
Trial
1
2
3
4
1) [I-] after mixing A & B (M)
__________
__________
__________
__________
2) [S2O82-] after mixing A & B (M)
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
3) Time required for color change (s)
Time data from a second group of students (s)
Time data from a third group of students (s)
Average time observed (s)
4) Moles of S2O32-
__________
5) Moles of I3- produced during time observed
__________
6) [I3-] during time observed, (M)
__________
7) Initial Rate, based on average time (M/s)
__________
__________
__________
__________
8) In Part II, find three sets of trials where the concentration of iodide is doubled. List each set below.
a) Trials ____ & ____
b) Trials ____ & ____
c) Trials ____ & ____
9) For each set of trials, determine the effect doubling concentration had on the reaction rate, rounding to
the nearest integer.
a) __________
Show your work for 9a
b) __________
c) __________
10) For each set of trials, determine the reaction order with respect to iodide
a) __________
11) Average order
(rounded to nearest integer)
b) __________
c) __________
__________
12) Based on your data from Parts I and II, what is the rate law for this reaction?
Part III. Studying the effect of temperature on reaction rate
Trial
1
2
3
4
1) [I-] after mixing A & B (M)
__________
__________
__________
__________
2) [S2O82-] after mixing A & B (M)
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
3) Time required for color change (s)
Time data from a second group of students (s)
Time data from a third group of students (s)
Average time observed (s)
4) Moles of S2O32-
__________
5) Moles of I3- produced during time observed
__________
6) [ I3-] during time observed (M)
__________
7) Initial Rate, based on average time (M/s)
8) Rate constant, k (based on the rate law
derived in Parts I and II)
9) ln k
10) Reaction temperature ( C)
11) Reaction temperature (K)
12) 1/T (K-1)
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
__________
Prepare a plot of ln k vs. 1/T (print and include with your report)
13) slope of plot
14) Activation energy, Ea (J/mol)
15) y-intercept of plot
16) Collision frequency factor, A (1/M s)
__________
__________
__________
__________
Name: _____________________________
Section: ________
Post-Lab Questions
1. Why would it be important to keep the number of ions constant across each trial?
2. How would you reaction rate calculation differed if you had measured the rate with respect to I- instead
of I3-? Would this have affected your predicted rate law? Why or why not?
3. If the reaction had been zero order with respect to persulfate, how would your data have differed from
what you observed?
4. Another form of the Arrhenius equation allows you to determine the activation energy using only two
sets of rate/temperature data
ln(k1/k2) = Ea/R (1/T2 – 1/T1)
Calculate the activation energy for this reaction using the above equation and your results from
a) Trials 1 and 2
b) Trials 2 and 3
Name: _____________________________
Section: ________
Pre-Lab Questions
1. The following data was collected for the reaction between hydrogen and nitrogen monoxide. The time
recorded corresponds to a N2 concentration change of 5.3 x 10-4 M.
2H2(g) + 2NO(g)  2H2O(g) + N2(g)
Trial
1
2
3
[H2], M
0.01
0.02
0.01
[NO], M
0.02
0.02
0.01
Time, s
74
37
296
a. Calculate the reaction rate for each of the three trials.
b. Determine the order of the reaction with respect to H2.
c. Determine the order of the reaction with respect to NO.
d. What is the rate law for this reaction?
e. What is the rate constant?
2. In the reaction being studied this week, what chemical is being used measure reaction time? What are
we using as an indicator?
3. Calculate the initial reactant concentrations for each of the trials in Pt I.
1
2
3
4
[I-] after mixing A & B (M)
__________
__________
__________
__________
[S2O82-] after mixing A & B (M)
__________
__________
__________
__________
Show your work from Trial 1.