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SKILL: ANALYZING LINE GRAPHS
The Iodine Clock
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Math 7.MR.2.3
Science 8.9.g
Can a chemical reaction be timed? In the iodine clock reaction,
a sudden color change indicates that the reaction has occurred.
The length of time that passes before the color changes depends
on the concentration ratios of the reactants. As shown in the graph
below, the greater the concentration of the reactants, the faster
the reaction.
Example
Suppose you are given an unknown iodine concentration to test
in the iodine clock reaction. What is the concentration ratio of the
iodine if it takes 40 seconds for the color change to occur?
(1) Find 40 seconds on the x-axis of the graph below and follow
the vertical line up to the plotted data.
Before
After
(2) Draw a horizontal line from that point on the curve to the y-axis
to find the iodine concentration ratio in your sample.
Iodine Clock Reaction
The reactants in the iodine clock
reaction produce a sudden color
change several seconds after the
reactants are mixed.
Concentration Ratio
(iodine: other reactants)
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5.0:5.0
4.0:5.0
3.0:5.0
2.0:5.0
1.0:5.0
0
0
20
Time (sec)
40
60
ANSWER The unknown concentration ratio is
approximately 3.0:5.0.
Answer the following questions using the information in the
graph above.
1. Approximately how long will it take for the reaction to occur if
the concentration ratio is 4.0:5.0? 2.0:5.0?
2. Suppose you could extend the curve on the graph. If the reaction
took 70 seconds to occur, what would be the approximate
iodine concentration ratio?
CHALLENGE Using the following concentration ratios and
times for another reactant, draw a reaction rate graph similar
to the one shown above.
Concentration Ratios = 5.0:5.0, 4.0:5.0, 3.0:5.0, 2.0:5.0
Times = 24 sec, 25 sec, 43 sec, 68 sec
Chapter 9: Chemical Reactions 279
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