Iodine Clock Reaction Lab
Background Information
Concentration and temperature have a big effect on reaction rate. This is
the classic iodine clock reaction lab. This lab will involve the chemical
reaction of iodine with starch. In previous years, you probably put iodine
on a piece of bread or potato and saw how it turned purple. Iodine is an
indicator for the presence of starch.
This lab, involving the iodine clock reaction, will demonstrate how
concentration and temperature affects reaction rate. The collision theory
says that chemical reactions need enough energy to collide to react.
They also need enough effective collisions for a reaction to occur. The
collision theory will be obvious through careful plotting of the data.
Solution A is a dilute solution of potassium KIO3 and solution B
contains starch with HSO3- . When the two solutions are mixed, the ions
of the mixed solution undergo the following reactions:
A. IO3- + 3HSO3-  I- + 3SO4-2 + 3H+
B. 5I- + 6H+ + IO3-  3I2 + 3H2O
C. I2 + HSO3- + H2O  2I- + SO4-2 + 3H+
D. I2 + starch  blue solution
Only when the hydrogen sulfite ions have been entirely consumed in
reactions A, B, and C will iodine molecules accumulate in solution and
react with the starch in reaction D.
Preparation Directions:
Solution A-Iodine solution
4.3 g KIO3 in 1 L
Solution B
4 g starch in warm water + 0.2g sodium metabisulfite Na2S2O5 + 2 mL
18 M sulfuric acid right before lab.
DO NOT warm above 50 °C, as the starch molecules can break down.
KNOW: What do you already know about reaction rates in
chemical reactions?
Objectives
-Compare and contrast different temperatures with reaction rate.
-Compare and contrast different concentrations of reactants with
reaction rate.
-Construct graphs with data collected to prove the collision theory.
-Identify reactions of starch with iodine.
Materials
Test tubes/rack, two 400 mL beakers, thermometer, hot plate, 10 mL
graduated cylinder, safety goggles, solution A and solution B, ice,
stopwatch, permanent marker
Procedure to test concentration rate vs. reaction rate
1. GOGGLES ON
2. Calibrate your test tubes to the 10 mL mark with the graduated
cylinders, and mark them with a permanent marker.
3. Place 10 mL of solution A in one test tube and 10 mL of solution B in
a second test tube.
4. Mix the two test tubes, and begin timing. Have your partner mix the
solutions back and forth about three times for good mixing while you are
timing.
5. Record the time it takes for the first sign of reaction.
6. Repeat this procedure using the mixtures of the table below, and note
that you will be adding distilled water to your solution A to bring the
solution up to 10 mL as indicated in the following table:
The first sign of reaction may occur while you are mixing it. The first
sign may be a light tint and may later turn darker. Record the first light
tint, indicating a reaction has occurred.
Table of Mixtures
mL solution A(Iodine) mL solution B
10 (0.01 M)
10
7 (0.007 M)
10
6 (0.006 M)
10
5 (0.005 M)
10
4 (0.004 M)
10
3 (0.003 M)
10
2 (0.002 M)
10
1 (0.001 M)
10
Procedure to test temperature change vs. reaction rate
1. GOGGLES ON
2. Make an ice bath in a 400 mL beaker.
3. Make a hot water bath by warming 250 mL in a 400 mL beaker to
about 50 °C.
NOT ABOVE 50 °C or starch will breakdown.
4. Place 10 mL of solution A into a test tube and 10 mL of solution B
into a second test tube.
5. Mix the two test tubes and begin timing. Have your partner mix the
solutions back and forth about three times for good mixing while you are
timing.
6. Record the time it takes for the first sign of reaction.
7. Repeat this procedure using 10 mL of both solutions at the indicated
temperatures. Do this by placing both test tubes in the ice/warm water
baths until they hit the desired temperature. Mix them, and time the
reaction.
Data Table 1 for Part 1
Solution
A
10
9
8
7
6
5
4
3
2
Time
1
Data Table for Part 2
Temperature
Range
Time
0–5
10–15
20–25
30–35
40–45
50–55
WHAT: What did you learn about reaction rates in chemical
reactions?
Graph Construction
1. Using EXCEL, construct a graph of concentration vs. time by plotting
the concentration of KIO3 on the y-axis and time on the x-axis.
2. Using EXCEL, construct a graph of temperature vs. time with
temperature on the y-axis and time on the x-axis.
Answer one of the analysis questions with complete sentences.
1. In a detailed paragraph, prove the collision theory with your lab data.
You must provide the proof to back up your thoughts. You must also
relate your proof to the molecular level.
2. In a detailed paragraph, how are concentration and temperature
related to reaction rate? You must describe the relationships
mathematically at the molecular level.
Learn: What do you still want to learn about reaction rates in
chemical reactions?