SCH4U
Lab Iodine Clock
Purpose
The purpose of this investigation is to gather and analyze experimental
observations to determine the rate dependence of a reactant in a chemical
system.
Question
What is the order of reaction with respect to the initial concentration of iodate
ions in the iodine clock reaction?
Materials
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Microtray
100 mL beakers
2, 500 mL solution bottles
sodium bisulfite
Ethanol
2.00 g of 1 M concentrated sulfuric acid
2.14 g potassium iodate
2.00 g starch soluble
Distilled water
Stopwatch
Solution A
Add 2.14 grams of potassium iodate to 500.0 mL of distilled water.
Solution B
In 500.0 mL of distilled water add 0.60 grams of sodium bisulfite. Carefully add
2.00 grams of concentrated sulfuric acid and 5.0 mL of ethanol.
Solution C
Boil water and measure out 50.0 mL. Add 2.00 grams of soluble starch and allow
to cool.
Experimental Design
Using the microtray, place 10 drops of the solution A mixture in the first well, 9
drops of the solution A in the second well, etc. until you reach 1 drop of the
solution A/C mixture.
Add 0 drops of water to well 1, add 1 drop of water to well 2, add 2 drops of water
to well, etc., until well 10 gets 9 drops of water.
In a 100 mL beaker, combine 50.0 mL of Solution B with 10.0 mL of Solution C to
the mixture. Add 10 drops of solution B/C to each well. Observe/time each well.
Observe/time one well at a time so you can make proper time observations.
Dispose of all material down the sink with lots of running water.
Observation
Table 1: Effect of Concentration on Reaction Time
Well
Volume of
solution A
(drops)
Volume
of water
(drops)
[IO3-]
(mol/L)
Volume of
solution B/C
(drops)
[IO3-]
(mol/L)
Elapsed
time
(s)
1
2
3
4
5
6
7
8
9
10
Analysis
1. Using the last two columns of your observation table, complete the table
below.
Table 2: Data of Iodine Clock Reaction to Establish Order of Reaction
[IO3-]
(mol/L)
Elapsed time
(s)
ln [IO3-]
1/[IO3-]
2. Plot and label two graphs ON GRAPH PAPER to identify the order of the
reaction with respect to concentration of iodate ions.
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ln [IO3-] versus t (linear for a 1st order reaction)
1 / [IO3-] versus t (linear for a 2nd order reaction)
3. Answer the question of the lab (based on your graphs).
4. Identify independent, dependent, and constant variables.
5. Why is it important to add specific volumes of water to the wells in the
microtray?
How does it work?
This reaction is referred to as the Landolt Clock Reaction. There are three steps
in the process that cause this amazing reaction. When you prepare the Solutions
A, B, and C, the chemicals begin to mix and form new chemical compounds. This
is a very slow reaction, so you don’t see any outward changes. When you begin
to pour the solutions together a much faster reaction occurs, which leads to the
third reaction which is instant. Suddenly, and immeasurably quickly, the clear
liquids turn into a jet black iodine-starch complex. These reactions happen at
different intervals because different chemicals react at different speeds.
Additional Info
Discussion of the Chemistry...
The sudden change from a colorless solution to the blue-black solution is the
result of four sequential reactions. First, the bisulfite ions (HSO3-) reduce some
of the iodate ions (IO3-) to form iodide ions (I-).
Next, the iodide ions (I-) are oxidized by the remaining iodate ions (IO3-) to form
triiodide ions (I3-). The solution now consists of triiodide ions (I3-) and soluble
starch.
In the third reaction, the triiodide ions (I3-) get reduced by the bisulfite ions
(HSO3-) to become iodide ions (I-). That continues until all of the bisulfite has
been consumed.
Finally, the triiodide ions and starch combine to form the dark blue-black starch
complex that looks like ink.
1-
1-
1-
2-
IO3 + 3 HSO3  I + 3 SO4 + 3 H
1+
1-
1-
1+
1-
6 H + IO3 + 8 I  3 I3 + 3 H2O
1-
1-
1-
2-
H2O + I3 + HSO3  3 I + SO4 + 3 H
1-
-1
1+
2 I3 + Starch  Starch—I5 complex (blue) + I
-1