Notes on Procedure for Experiment No. 29
There are two parts to this experiment. In the first part, you will determine the rate
expression for the iodination of acetone:
You will make four runs at room temperature, corresponding to Runs #1, #2, #4, and #6 in the
lab manual, in which you will separately vary the concentrations of the acetone, H+ (i.e., HCl)
and I2 reactants to determine their reaction orders in the rate expression. (Because of time
limitations, you will not perform Runs #3 and #5 in the lab manual.) In the second part of the
experiment, you will determine the activation energy of the above reaction by making three runs
using the same concentrations of each component as those in Run #1, but at different
temperatures.
Begin by setting up the various water baths. Cooperate to set up four baths on each side
of a bench. Set up three or four sets for the large classes and two for the small classes. There
should be two of the 10-12°C baths in the middle of the bench and one 30-35°C bath and one 4550°C bath at opposite ends of the bench. It is much more important that each of the water baths
maintains a nearly constant temperature than it is to achieve any exact temperature. For the two
warm baths, start at a low setting on the hot plate and work your way up to the desired
temperature range. The cold baths should be brought to the desired temperature range by adding
small amounts of ice, and then maintained by periodically adding one or two pieces of ice. The
temperatures of the various water baths will be measured and continuously monitored using the
Vernier temperature probes. Although it would be much easier to use an excess of ice to
maintain the temperature of the cold bath at 0°C, the reaction would occur too slowly at that
temperature. An approximate rule-of-thumb is that the rate of a reaction changes by a factor of 2
for every 10 degree change in temperature. Thus, at 0°C, the reaction rate would be less than ¼
that at room temperature.
Get three 16x125 cm test tubes, two sets of 2 mL and 4 mL volumetric pipets, a pipet
pump, and a 25 mL Erlenmeyer flask with a cork. Each of the three solutions that are needed for
the runs (4 M acetone, 1 M HCl, and 0.005 M I2) will already be prepared for you, so you will
not have to perform the preparations that are described on p. 589 in the lab manual. Get ~20 mL
each of the iodine and hydrochloric acid solutions, preferably in graduated cylinders or small
beakers. From the left-hand fume hood, get ~20 mL of acetone solution in your 25 mL
Erlenmeyer flask. Keep the flask corked when not actually pipetting so that the concentration of
the acetone solution does not change because of evaporation.
Place 10 mL of distilled water in one of the test tubes. This will be your reference, not
only for determining the time of completion of the reaction, based on disappearance of the
yellow iodine color, but also for total volume (always 10 mL). The preparation of the reacting
solution is the same for all runs. Using a pipet from one set, pipet the correct amount of iodine
solution into one test tube. Than use the other set of pipets to transfer the required volumes of
water, hydrochloric acid, and acetone into the other test tube. Mix the two solutions by pouring
back and forth three or four times and immediately start the timer. Set the test tubes into your
plastic test tube holder. This will give you a white background to better observe the color change
and prevent heating the solution due to handling with your warm hands. Watch for the
disappearance of the yellow iodine color by looking down through the top of the test tube
through the full column of the solution, rather than from the side.
In the second part of the experiment, you will perform three runs using the same amounts
of the three solutions as were used in Run #1, but at different temperatures. For each of the three
runs, after pipetting the solutions into the two test tubes, place them in the appropriate
temperature bath for three to four minutes to allow them to reach the bath temperature before you
mix them. Then take them out, mix them, start the timer and put the test tube back into the bath.
You can save some time by starting the cold one first. The reaction rate in the lowest temperature
bath is sufficiently slow that you can get another test tube and do the runs in the two warm baths
while the cold one is still reacting. Just keep checking it so that you do not miss the completion
time.
Accuracy is more important for the runs at different temperatures. Assuming that all of
the reaction orders are small whole numbers, for the first four runs you need to measure the
completion time accurately enough to determine if the rate was the same, was twice as fast, or
was four times as fast, corresponding to reaction orders of zero, one or two. Remember that you
are determining the reaction rate in each run by observing the disappearance of the iodine, i.e.,
-∆[I2]/∆t. When you finish, you will have four sets of data at different temperatures, viz., the
three in the cold and warm temperature baths, plus Run #1 at room temperature.
When you are finished, put the waste in the jug in the right-hand fume hood. Return the
test tubes, flask and cork, pipets, and pipet pump to the trays under the window.
Summary of Runs and Volumes of Solutions
Run #
(lab
manual)
Iodine
vol
(mL)
Acetone
vol
(mL)
HCl
vol
(mL)
Water
vol
(mL)
Total
vol
(mL)
Temp
#1
#2
#4
#6
2
2
2
4
2
4
2
2
2
2
4
2
4
2
2
2
10
10
10
10
RT
RT
RT
RT
#8
2
2
2
4
10
RT (–10)
#9
2
2
2
4
10
RT (+10)
#10
2
2
2
4
10
RT (+20)
Note: Because of time limitations, you will skip Runs #3, #5 and #7 in the lab manual, which
involve tripling the amounts of the reactants.