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Iodine Clock Experiment
CHEM 251
Week of November 29th, 2010
Alexis Patanarut
Purpose
1. To study the effects of temperature and catalysts
on the rate of reaction
2. To calculate reaction order coefficients and the
rate constant k using the method of initial rates
– Experiments 1, 2 and 3 will be used to final the 3 k
values, then the average k value
– Experiment 4 will be used to show you how a
catalyst affects the reaction rate
– Experiment 5 will be used to show you how
temperature affects the reaction rate
Background
• The name 'iodine clock' is derived from the
fact that, by carefully adjusting the reactants
involved in the reaction, the iodine clock
reaction can come to completion with the
accuracy of a clock
– However, this would require glassware of high
precision and meticulous attention to detail
– No worries; we won't be putting you through this
Background, con’t
• For today, you will be using the iodine clock
reaction to find the reaction order coefficients
and rate constant
– Last time, you determined the reaction order and
rate constant of the crystal violet dye reaction by
graphical means
– This week, you will be determining those values
quantitatively; you will also be seeing how
temperature and catalysts affect the reaction rate
Background, con’t
(I) S2O8 + 2I --> 2SO4 + I2
(II) I2 + 2S2O3 --> 2I + S4O6
• The actual iodine clock reaction between the iodine
and the persulfate is VERY fast, so much so that you
will most likely miss the endpoint
• The reaction is therefore slowed down with the
addition of thiosulfate
– The thiosulfate will consume all of the iodine produced in
the persulfate reaction
– Once all of the thiosulfate is used up, the excess iodine
forming will react with the starch in the solution to
produce the blue color indicative of the endpoint
– This is how to endpoint is made measurable by time
The mathematics – finding [S2O8] and
[I]
• Remember that you are making dilutions of
both compounds, since you are combinding
reagents together into a larger volume (10 mL
total)
• Use C1V1 = C2V2 to find the final
concentration (C2) of [S2O8] and [1]
The mathematics – finding the rate of
reaction
• You will use the mathematical rate expression for this
(1) Rate = -D[S2O8]/Dtime
Where D[S2O8] = 1/2D[S2O3] <-- the thiosulfate
reagent
And D[S2O3] = initial [S2O3] concentration
• Therefore, you can find D[S2O8] by taking the initial
concentration of [S2O3] and dividing it in half.
– Take this value and divide it by the amount of time
required to see the blue reaction endpoint, and you can
find the reaction rate
The mathematics – finding x and y
• Use the method of initial rates
• Example
Trial
[A] (mol/L)
[B] (mol/L)
Rate (M/sec)
1
0.1
0.1
1.0 x 10^-4
2
0.1
0.2
1.0 x 10^-4
3
0.3
0.1
3.0 x 10^-4
The mathematics – finding x and y,
con’t
rate 2/rate 1 = {k[0.1]x[0.2]y)/(k[0.1]x[0.1]y)
(1 x 10^-4)/(1 x 10^-4) = k(0.2/0.1)y
1 = k(0.2/0.1)y
because they are the same
y = log(1)/log(2)
value, A or 0.1 cancels out
y=0
from the equation
rate 3/rate 1 = {k[0.3]x[0.1]y)/(k[0.1]x[0.1]y)
(3 x 10^-4)/(1 x 10^-4) = k(0.3/0.1)x
3 = k(0.3/0.1)x
because they are the same
x = log(3)/log(3)
value, B or 0.1 cancels out
x=1
from the equation
The mathematics – finding the rate
constant k
• Once x and y are found, the rate constant k
can be found by merely plugging in the right
numbers and doing a straight calculation
• Once the k for each experiment is found (for
experiments 1, 2 and 3), take the average of
the three k values to find the average k.
• Use this average k value as a means of
comparison for the k value you calculate for
experiments 4 and 5
The experiment
1.
2.
3.
For experiments #1, 2 and 3, combine the amounts of the first five
reagents in a large test tube. Only add the sixth reagent (the
potassium persulfate) when you are ready to begin timing the
reaction to the nearest second. Time how long it takes for you to
see blue appear in the solution.
For experiment #4, add the first five reagents using the same
amounts as experiment #1. However, this time, you will also add
ONE drop from the "mystery reagent" bottle. Then add the
persulfate reagent as before and record the time you see the blue
color.
For experiment #5, add the first five reagents using the same
amounts as experiment #1. In addition, put the required amount
of persulfate solution into a small test tube. Put both test tubes
into a water bath for at least 30 seconds, then take the
temperature of the five-reagent mixture. Add the persulfate to
the five-reagent mixture and see how long it takes for the blue to
appear. Keep an eye on it, or you may miss this.
The experiment - amounts
Experiment 0.20 M
NaI (mL)
0.20 M
NaCl (mL)
0.010 M
Na2S2O3
(mL)
2% starch
0.20 M
K2SO4
(mL)
0.20 M
K2S2O8
(mL)
1
2.0
2.0
2.0
20 drops
2.0
2.0
2
2.0
2.0
2.0
20 drops
0.0
4.0
3
4.0
0.0
2.0
20 drops
2.0
2.0
4
+ 1 drop of
“mystery”
reagent
2.0
2.0
2.0
20 drops
2.0
2.0
5
Heat
mixtures
for 30 sec
2.0
2.0
2.0
20 drops
2.0
2.0
Experimental notes
• Be sure to rinse your dropper with water between
adding individual reagents to avoid contamination of
your reagent
• Have about 20 mL of each reagent in LABELED beakers
at your lab bench. This will prevent you from having to
run back and forth constantly to get chemicals
• After adding the persulfate, stopper the test tube and
agitate the solution for 10 seconds to ensure mixing
• The blue color will appear suddenly. You are to record
the exact length of time it took for the blue to appear
for each trial
In your lab notebook
Calculate the following values:
1. [S2O8] and [I]
2. Reaction rate
3. Rate constant k
Answer the following questions:
1. What do you think the mystery reagent is? How did
adding it affect your reaction rate?
2. What is the effect of temperature on the reaction rate?
3. What was the order of the reaction?
4. Discuss in 2-3 sentences factors that could have
contributed to errors in your experiment.