Level 1 Chemistry 11-12
Unit 8: Equilibrium
Starch-Iodine Clock Reaction Lab
Introduction
The reaction that you will carry out in this lab is called a starch-iodine clock reaction. Two colorless
solutions (Solutions A and B) will be mixed, and after a short period of time, an observable reaction will
occur. The amount of time it takes for the reaction to be observed depends upon the concentration of the
solutions and the temperature at which the reactions are carried out.
What solutions are we using?
 Solution A is a 0.020 M solution of potassium iodate, KIO3. The iodate ion IO3-1(aq), is the
reacting species in this solution. You will make multiple dilutions of this before reacting it with
solution B.
 Solution B is a solution containing starch and the other reacting species, the hydrogen sulfite ion,
HSO3-1(aq). Solution B is never diluted in this lab. The concentration remains the same
throughout the lab.
What happens in the reaction?
 When solutions A and B are combined, the iodate ion and hydrogen sulfite ions react to form
iodide, sulfate, and hydrogen ions, as shown below.
IO3- 1(aq) + 3HSO3-1(aq) I-1(aq)+ 3SO4-2(aq) + 3H+1(aq)

When all of the hydrogen sulfite ions, HSO3-1(aq), are used up, the iodide ions, I-1(aq), react with
the remaining iodate ions, IO3-1(aq), and produce molecular iodine, I2(s).
5I-1(aq)+ 6H+1(aq) + IO3-1(aq) 3I2(s) + 3H2O(l)

The molecular iodine reacts with the starch to form a blue substance.
What is the general procedure?
The entire class will examine the reaction rate of ten different dilutions of solution A. With your partner,
you will be assigned two of these dilutions. After each dilution is made, you will combine it with 10 mL
of solution B and will measure the time of the reaction, from the moment the two solutions first touch
until the blue color appears. You will repeat for two more trials of that same dilution, and then repeat for
your second assigned dilution.
Materials
2 (18 x 150 mm) test tubes
stop watch
Burette containing Solution A
Burette containing Solution B
Burette containing DI water
Note: All solutions must be at room temperature.
Dilution Number
1
2
3
4
5
6
7
8
9
10
Solution A
Volume
10.0 mL
9.0 mL
8.0 mL
7.0 mL
6.0 mL
5.0 mL
4.0 mL
3.0 mL
2.0 mL
1.0 mL
Located on the
front lab bench
Water Volume
0.0 mL
1.0 mL
2.0 mL
3.0 mL
4.0 mL
5.0 mL
6.0 mL
7.0 mL
8.0 mL
9.0 mL
You have been assigned to collect
data for dilution #s:
____________
and
___________
Safety & Disposal
Dispose of all chemicals in the appropriately labeled containers.
Pre-lab:
1. Lab component: Using a Burette
A burette is an instrument used to measure volume. The data you record tells you the amount of liquid
removed from the burette.
Initial reading of burette:
4 ml
Final reading of burette:
5 ml
a. How much liquid was dispensed from the burette?
b. Using the burettes at your bench, perform the following to master measuring volume of a liquid:
Part One:
 Record the initial reading on the burette. _________
 Dispense approximately 15 mL of liquid.
 Record the final reading on the burette. __________
 What is the exact volume of liquid dispensed from the burette? __________
Part Two:




Record the initial reading on the burette. _________
Dispense approximately 15 drops of liquid.
Record the final reading on the burette. __________
What is the exact volume of liquid dispensed from the burette? __________
c. How does a burette measure volume differently than a graduated cylinder?
2. What is reaction rate?
3. Considering your two assigned dilution samples, how do you think their reaction rates will differ?
Why?
4. What condition must be met for a reaction to occur?
5. Calculate the molarity of KIO3 in each of the 10 diluted solutions the class will make. The initial
concentration of solution A is 0.020 M KIO3. Record these values in the concentration column in the class
data table on page 3.
2
10 ml
11 ml
Procedure
1. Deliver 10.0 mL of solution B from the burette labeled Solution B into one of the test tubes.
2. Prepare your first dilution of solution A in your second test tube according to the chart below. Obtain
your concentrated solution A from the buret labeled Solution A and your water from the burette labeled
DI H2O. Mix your dilution well. Note that the total volume of your diluted solution A will always be
10.0 mL.
3. Pour solution A into solution B and then pour them back and forth quickly three times to obtain
uniform mixing. Measure the time from when the solutions first come in contact until the first sign of a
reaction using a stopwatch. Record in your data table to the nearest second.
4. Repeat the experiment using the same concentration of solution A two more times. Record times in
your data table.
5. Repeat steps 1-4 for your second assigned concentration of solution A. Record all times in your data table.
Data
Table One: Individual Group Data
Dilution #
Vol. A
Vol. H20
Vol. B
Trial #1
Table Two: Class Data
Dilution Volume Volume
#
A
H 2O
Volume
B
1
10
0
10
2
9
1
10
3
8
2
10
4
7
3
10
5
6
4
10
6
5
5
10
7
4
6
10
8
3
7
10
9
2
8
10
10
1
9
10
Times
3
Time
Trial #2
Average
Time
Trial #3
Molarity of KIO3
solution
(A + H2O)
Graph
As a class, plot a graph of the average reaction time vs. concentration of KIO3 (M).
Analysis Discussion
1. What relationship exists between concentration and reaction rate? Support with evidence from the
lab. Explain why this relationship exists using collision theory.
2. What impact would changing the temperature of the test tubes have on the rate of the reaction?
Address the impact of heating and cooling the test tube. Explain using collision theory.
3. So what? Why do we care about changing the rate of a chemical reaction?
a. Using what you have learned in this lab, why do we store food in a refrigerator instead of on the counter
tops?
b. Antacid tablets (like Tums) work by reacting with acid in the stomach to neutralize it. Using what you
have learned in this lab, why is it more effective for you to take two antacid tablets instead of one tablet?
c. When building a campfire people usually start with thinly cut pieces of wood instead of just logs. Why?
d. A piece of unripe fruit can be quickly ripened (often overnight) by placing it in a brown paper bag with
a piece of ripe fruit. The ripe fruit releases a gas, which reacts with compounds on the surface of the
unripe fruit, triggering a series of biochemical reactions that result in the soft, juicy, and sweet
characteristics associated with ripeness. Explain why placing the fruit in the paper bag allows this to
occur so quicker than if you just left them out on the countertop.
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