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 The Iodine Clock: Measuring the Rate of a Chemical
Reaction to Determine its Specific Rate Law
Learning Goals:
1. To determine the effect that changing the concentration of the reactants has
upon the rate of a specific reaction.
2. To calculate the order of the specific reaction with respect to the
concentration of the reactants.
3. To obtain the specific rate law for a chemical reaction.
Abstract:
Chemical kinetics is the area of chemistry concerned with the study of the rate
or speed of a chemical reaction. Reactions occur at varying speeds ranging from the
slow rusting of iron to the extremely fast decomposition of TNT. Factors such as
concentration of the reactants, temperature, presence of a catalyst, and/or the
nature of the reactants can affect the rate of a reaction. A set of solutions with which
the speed of a chemical reaction can somehow be determined allow the study of the
effects of concentration on reaction rate. The background for this experiment includes
Sections 13.1 and 13.2 in your text (MSJ pages 592-602). These two sections
discuss the terms, theory and calculations needed to understand the general rate
law so that the specific rate law can be determined.
When peroxydisulfate ions (S2O82-) react with iodide ions (I-), one of the resulting
products is Triiodide ion (I3-). In the presence of a soluble starch indicator, this
produces a dark blue solution.
(Reaction 1) S2O82-­‐ + 3 I-­‐ ⇒ I3-­‐ + 2 SO42-­‐ To determine the rate law for the above reaction, a series of changes in the
concentration of peroxydisulfate, [S2O82-], and the corresponding changes in time, t,
need to be measured and plotted. To measure these changes in concentration over
1 time, Reaction 2 is needed to change the dark blue color back to a colorless solution.
This chemical “clock” is created by adding a series of measured volumes (aliquots) of
thiosulfate ion, S2O32-, to the solution containing Reaction 1.
(Reaction 2) I3-­‐ + 2 S2O32-­‐ ⇒ 3 I-­‐ + S4O62-­‐ Thiosulfate ions quickly react with Triiodide ions (I3-) to produce iodide ions (I-).
With no I3- present to react with the starch, the solution returns to its former colorless
state. This lasts until all the thiosulfate ions have reacted and then since more I3continues to be produced the solution will immediately turn blue. The addition of
another aliquot of thiosulfate makes the solution turn colorless and in effect rewinds
the clock for another time interval.
Reaction 1 between the peroxydisulfate ion and the iodide ion will be carried out
several times using different concentrations of the reactants. Data will be collected
and the Rate Law for Reaction 1 will be determined.
Prelab Assignment:
In your lab notebook, prepare the following information:
1. A brief (2-3 sentence) introduction to the lab.
2. A table of safety information including the chemicals used in the lab and any
safety handling precautions. This information can be obtained from the MSDS
safety sheets.
3. Determine a value for x in each of the following equations:
a. 2X = 1
b. 2X = 2
c. 2X = 4
d. 2X = 8
Give the copies of your Pre-lab to your TA at the beginning of the lab. You will not be
allowed to work in the lab without this information
2 Chemicals
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Starch solution (3%)
Sodium thiosulfate solution (0.20 M
Na2S2O3)
Potassium iodide solution (0.20 M KI)
Ammonium peroxydisulfate solution
(0.20 M (NH4)2S2O8 )
Potassium nitrate solution (0.2 M KNO3)
Glassware and Equipment
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Three 125mL Erlenmeyer flasks
Three 25mL Erlenmeyer flasks
Five 50mL plastic beakers
Digital thermometer
Two 1mL plastic syringes
Three 10mL plastic syringes
Timing device (may use your phone)
Label tape
Procedure:
Preparing the Reaction Solutions
Do not put syringes directly into the solution bottles. Use the small plastic beakers to pour out a
small amount at a time and fill the syringe from the beaker. Any excess in the beaker at the end of
the lab period should be disposed of in the waste bottles, not poured back into the bottles.
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Solution # 1 A Label a 125mL Erlenmeyer flask “#1A”. Use a 10mL syringe to add 10.0
mL of 0.20 M KI to the flask. Label the "KI" syringe. Into the same flask, use a different 10mL
syringe to add exactly 20.0 mL of 0.20 M KNO3. Label the "KNO3" syringe. Next, use a 1mL
syringe to add 1.0 mL of starch indicator. Label the "starch" syringe. Finally, use a separate
1mL syringe to add 1.0 mL 0.20 M Na2S2O3. Label the " Na2S2O3" syringe. Swirl the solution
to mix thoroughly.
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Solution # 1 B Label a 25mL Erlenmeyer flask “#1B”. Use a clean 10mL syringe to
measure 10.0 mL of 0.20 M (NH4)2S2O8 into the flask. Label the "(NH4)2S2O8" syringe. Solution #2 A Label another 125mL flask “#2A” and use the KI syringe to add 10.0 mL
of 0.20 M KI. Into the same flask, add exactly 10.0 mL 0.20 M KNO3 from the KNO3 syringe.
Next, use the starch syringe to add 1.0 mL of starch indicator. Finally, use the Na2S2O3
syringe to add 1.0 mL 0.20 M Na2S2O3. Swirl the solution to mix thoroughly.
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Solution #2 B Use the (NH4)2S2O8 syringe to add 20.0 mL of 0.20 M (NH4)2S2O8 into a
25mL Erlenmeyer flask. Label the flask.
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Solution #3 A Use the KI syringe to add 20.0 mL 0.20 M KI into a labeled 125mL
Erlenmeyer flask. Into the same flask, add exactly 10.0 mL 0.20 M KNO3 from the 10mL
syringe. Next, use the starch syringe to add 1.0 mL of starch indicator. Finally, use the
Na2S2O3 syringe to add 1.0 mL of 0.20 M Na2S2O3. Swirl the solution to mix thoroughly.
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Solution #3 B Use the (NH4)2S2O8 syringe to add 10.0 mL of 0.20 M (NH4)2S2O8 into a
labeled 25mL Erlenmeyer flask.
3 Timing the Reactions
Since temperature affects the reaction rate, all of your solutions should be at room
temperature. Record the temperature of solution 1A and solution 1B. For the next
part, accurate time measurements are crucial. Time should be kept from zero and
be cumulative: zero, 2 minutes and 30 seconds, 5 minutes and 10 seconds, etc.
Mix solution 1A and solution 1B and record the time of mixing (or start the timer). This
is time zero. Watch the reaction flask and record the exact time that the solution goes
completely blue/black. Immediately, add 1.0 mL of 0.20 M Na2S2O3 with the 1mL
syringe. Mix the solution and wait for the dark color to appear again.
When the color appears again, record the time and add another 1mL aliquot of
Na2S2O3. Continue until you have recorded time for 6 or 7 of these cycles.
How does the “Clock” work
To turn the solution back to colorless, a measured portion of thiosulfate is added.
This equal portion is called an aliquot. The aliquot is 1.0 mL of 0.20 M Na2S2O3.
Converting to moles,
0.20 moles/L x 0.001 L = 0.00020 moles = 2.0 x 10-4 moles of S2O32So, 2.0 x 10-4 moles are added for each aliquot of S2O32-. According to the equation:
(Reaction 2) I3-­‐ + 2 S2O32-­‐ ⇒ 3 I-­‐ + S4O62-­‐ two moles of thiosulfate, S2O32-, react with one mole of iodine, I3-. And:
(Reaction 1) S2O82-­‐ + 3 I-­‐ ⇒ I3-­‐ + 2 SO42-­‐ one mole of I3- is produced from one mole of peroxydisulfate, S2O82-. Therefore, the
ratio of moles of thiosulfate ion added to moles of peroxydisulfate ion reacted is 2:1.
We started with 2.0 x 10-4 moles of S2O32-, so 1.0 x 10-4 moles of S2O82- have reacted.
In this experiment, the solution will turn blue-black when exactly
1.0 x 10-4 moles of S2O82- has reacted.
The more rapidly the 1.0 x 10-4 moles of S2O82- is consumed, the faster is the
reaction. From the experimental data, you can fill in the table below to show the
number of moles of S2O82- reacted, the concentration of S2O82-, and the elapsed time
4 at the color change. [S2O82-] can be found from the moles of S2O82- and the total
volume of the reaction solution.
Kinetic Data for Mixture of #1A & #1B
mL of
S 2O 3
2-
added
Zero time, at mixing
1
Color change 1
2
Color change 2
3
Color change 3
4
Color change 4
5
Color change 5
6
Color change 6
7
Color change 7
8
Moles of
S 2O 8
2-
reacted
0
Moles of
S 2O 8
2-
2
Total mL
[S2O8 ]
Time elapsed
of solution
(moles/L)
(from time
unreacted
2.0 x 10
-3
zero)
42.0
0.048 M
0
Creating a plot of [S2O82-] vs. time will give the information needed to find the
instantaneous rate for mixture #1.
Continue the Experiment:
Next, you will time the reaction using Solutions #2A & 2B, to record the appearance
of the blue-black color as before.
Then, repeat using Solutions #3A & 3B.
Prepare and react all three sets of concentrations again.
There should now be a total of six runs of the experimental procedure, two trials for
each of the solution pairs #1, 2 and 3.
Dispose of all of your solutions in the appropriately labeled waste container
Post lab Assignment
Include the following information in a lab report uploaded through ICN:
1. Take the values of [S2O82-] and time from the data tables and use a
spreadsheet or plotter program to construct a graph of [S2O82-] vs. time. Then
determine the slope of the curve at time "Zero", which is the initial slope
5 (using the first couple of data points) from the graph. This is the
instantaneous rate for Reaction(1) at "zero time". Since (NH4)2S2O8 is a
reactant, the concentration will be decreasing over time and therefore the
slope and the rate of the reaction will be negative:
[ (NH4)2 S2O8 ]
Rate = − --------------------Δt
2. Repeat the plotting using the data from the second set of solution #1A & B.
3. Construct plots for both trials for mixture #2A & B, and both trials for mixture
#3A & B.
4. After you have constructed all of these graphs, and determined the
instantaneous rates for each, you will have enough equations to solve for the
three unknowns in the specific rate law, m, n, and k.
Rate Law for Reaction between S2O82- and I[ S2O8 2- ]
− ------------ = k [ S2O8 2- ]m [ I- ]n
Δt
5. What effect did doubling the concentration of the peroxydisulfate ion have on
the rate of the reaction?
6. What effect did doubling the concentration of the iodide ion have on the rate
of the reaction?
7. What is the specific Rate Law for the reaction between Ammonium
peroxydisulfate and Iodide, as determined from your kinetics data?
6 Grading Rubric The Iodine Clock: Measuring the Rate of a Chemical Reaction Claim(s): Summary of claims: Statement(s), derived Clearly state the major conclusions or claims that answer the from evidence, using question: scientific reasoning. What is the specific rate law for this reaction from your (15 pts total) experiment? Evidence: Introduction: Scientific data that supports the claim. The data needs to be appropriate and sufficient to support the claim. Provide background information to put the experiment in context. What is chemical kinetics? What are the factors that can affect the rate of reactions? Summarize the purpose of your experiment. (30 pts total) Procedure: Refer the procedure to ICN website and provide the reference of that web address. Be sure to note any changes made to the experiment. Results: Carefully organize and present the data you collected that supports your argument. Include data tables and graphs of [S2O82-­‐] vs. time for each of the mixtures, sample calculations for [S2O82-­‐] and instantaneous rates. Reasoning: Scientific explanations that use evidence and appropriate chemistry concepts to construct claims. (30 pts total) Discussion: Explain how the evidence you presented supports your claim. What underlying chemical explanations can you provide about your evidence? Explain how the specific rate law was determined from your experiments (values for m, n, and k). You can provide the chemical reactions to illustrate your explanation. Use the lab procedure, outside sources, or talk to your instructor to gather information. Be sure to discuss the phenomena at both the molecular & macroscopic (visible to your eye) level. The calculations and answers to the “Post-­‐Lab Assignment” questions should be incorporated into your Discussion. Copyright (c) 2014 the ICN Team
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