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The University of Lethbridge
Department of Chemistry & Biochemistry
Chemistry 2740 Laboratory
Experiment 2
A KINETIC STUDY OF THE BASE CATALYZED CLEAVAGE OF
DIACETONE ALCOHOL USING A DILATOMETER
The decomposition of diacetone alcohol into two molecules of acetone is catalyzed by
hydroxide ions and is an example of an aldol condensation in reverse.
O
OH
CH3-C-CH2-C(CH3)2
OH-
O
2CH3-C-CH3
The rate of decomposition is first-order with respect to the concentrations of both diacetone
alcohol and hydroxide ion:
Rate = k[OH-][diacetone alcohol]
(1)
However, since hydroxide ion is a catalyst its concentration remains constant during the
reaction. The overall reaction appears first-order (i.e. is a “pseudo first order reaction”)
and follows the observable rate law
Rate = k' [diacetone alcohol]
where k' = k [OH-]
(2)
Since the overall reaction is first-order we can study the kinetics of the reaction by
measuring any property of the system that undergoes a change which is proportional to the
extent of reaction. Such a property in this case is the volume of the reaction solution. The
effective volume of one molecule of diacetone alcohol is not the same as the effective volume
of two molecules of acetone and as a result the total volume of the reaction solution
changes as the reaction proceeds. In this case the solution expands although in some
reactions it contracts.
A simple instrument for measuring volume changes is a dilatometer which consists of a
glass bulb to which is attached a tube with a stopcock (for filling the bulb) and also a piece
of long capillary tubing. The bulb is filled with reaction solution to the point where liquid
just enters the capillary tube and then the stopcock on the filling tube is closed. As the
solution expands it does so into the capillary tube causing the meniscus in the tube to rise.
By measuring the distance up the capillary tube that the meniscus travels one has a
measure of the volume change. One can determine the actual volume change if the crosssectional area of the capillary is known but even that is not necessary in this experiment.
Since the position of the meniscus in the capillary column can be measured accurately
using a cathetometer, this is a good experiment to test the Guggenheim method for
determining first-order rate constants (refer to Appendix A on “First-order Reactions”). In
this method readings are generally made at times t0, t1, t2, t3, etc., with each reading
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Chemistry 2740 Laboratory
Experiment 2
taken at a constant, accurately determined time interval after the preceding measurement.
The resulting data list is divided into equal halves. For example, if there are 20 readings
taken at times t 0 – t19 with corresponding measurements P0 – P19, the data would be
divided in two between readings P9 at t 9 and P10 at t10. Next, the differences between the
measurements in the two data sets are taken, i.e., P0-P10, P1-P11, P2-P12, etc. Notice that
the time interval between each pair of readings is constant. Finally a plot of the natural
logarithm of the differences against time, i.e., ln(P0-P10), ln(P1-P11),… vs. t0, t 1,… should
yield a straight line of slope -k, the first-order rate constant.
Apparatus
Cathetometer, 3 dilatometers, timer.
A dilatometer is a device for measuring the expansion (or contraction) of a liquid. Ours is
of relatively simple design and was made locally by Luis Delgado from various pieces of
glassware. It consists of an expansion bulb to which is attached a fine capillary tube with
a narrow and hopefully uniform bore. The expansion tube is connected at the other end to a
filling tube through a stopcock. When the stopcock is closed, a solution in the expansion
tube can only expand up the capillary tube.
Capillary
tube
Filling tube
Expansion
bulb
The volume of liquid in a capillary or cylinder is given
by the cross-sectional area, A, of the cylinder times
its length, l (V = A x l). Thus by measuring the travel,
Δl, of the liquid up the capillary tube one has a
quantity that is proportional to the change in volume
of the reaction mixture (Δ V = A x Δl). As a result one
can follow first order reactions with a dilatometer
and use the first order equation
ln [(lo – l∞) / (lt – l∞)]= kt
(3)
Stopcock
A Dilatometer
and other equations such as the Guggenheim
equation that are derived from it to analyze the
results. This assumes that Δl (and therefore Δ V) is
proportional to the extent of reaction.
One must be careful with thermostating when using a dilatometer. A dilatometer, after
all, is a glorified thermometer and a quite sensitive one at that. Thus the apparatus and
the reaction solution must be pre-equilibrated to the temperature of the reaction. The
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Chemistry 2740 Laboratory
Experiment 2
dilatometer is filled by pouring reaction mixture into the filling tube. Try to pour down the
centre of the tube and not down the walls of the tube. Also do not fill the filling tube above
the level of the water in the water bath because the part of the filling tube above water
level will not be well thermostated.
Next the reaction mixture must be forced into the expansion bulb by use of a rubber bulb
applied to the top opening of the filling tube. Often air bubbles become trapped just below
the stopcock. These can be removed by sucking back with the rubber bulb. Continue to add
more reaction mixture to the filling tube, as necessary. Force reaction mixture into the
expansion bulb until the liquid level reaches the top of the bulb just below the capillary
tube. Stop forcing liquid into the bulb and allow the liquid level to rise into the capillary
tube as a result of the flow of liquid from the filling tube to the expansion bulb. DO NOT
FORCE LIQUID INTO THE CAPILLARY TUBE. Close the stopcock. The dilatometer is
now ready for making measurements of the meniscus height.
The cathetometer is a device for measuring the relative height of the liquid column in the
capillary. It consists of a vertical steel rod with a scale marked along its length and a
telescope that runs up and down the rod. In operation one measures the height of the
liquid column by moving the telescope so that the cross-hair is focussed on the meniscus of
the liquid column. The position of the telescope (and thus the meniscus) is then read off
the scale on the bar with the aid of a vernier.
Ensure that you can read the vernier scale (refer to Appendix B on “Reading a Vernier”)
and can operate the telescope (focus, movement up and down, and leveling) before
proceeding with measurements.
Reagents
Diacetone alcohol, ~ 0.40 M NaOH.
Waste Disposal
A 4-litre bottle for the collection of wastes is supplied with the experimental set up. All
excess stock reagents and reaction solutions should be disposed of in this bottle. The
glassware can then be given a single small rinse into the waste container before being
cleaned further in the sink.
In preparing reaction solutions only remove as much reagent from the stock container as is
necessary to make the reaction mixtures.
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Chemistry 2740 Laboratory
Experiment 2
Procedure
Notes: 1) In order to finish this lab in the time allotted, students must be well organized and
prepared to start this experiment at the beginning of the period.
2) The ~ 0.40 M NaOH solution will need to be standardized by each group. This can
be done before or after the experiment is completed, but must be done before the
calculations for the report are started. Students can arrange a suitable time for
this with their instructor. (Note: A similar task was performed in Chemistry 1000
lab; it may be helpful for you to review that procedure.)
Three kinetic runs should be performed at hydroxide ion concentrations of approximately
0.100, 0.200 and 0.400 M. Prepare 100 mL each of 0.100 M and 0.200 M sodium hydroxide
solutions from the 0.400 M solution provided.
Allow a dilatometer to thermostat in the 25° C water bath. Pipette exactly 50 mL of 0.100
M NaOH solution into a 200 mL Erlenmeyer flask, stopper the flask, and allow it to
thermostat in the bath as well.
When the dilatometer and sodium hydroxide solution have been thermostated for at least
10 minutes, start the reaction by adding with a pipette 2 mL of diacetone alcohol into the
flask containing the 50 mL of 0.100 M NaOH solution. Stopper the flask, shake it
vigorously to ensure mixing and then let it stand in the water bath for a short period to
allow the bubbles to settle. Pour the settled solution into the filling tube of the
dilatometer and proceed to fill the dilatometer as outlined above. When the solution
enters the capillary close the stopcock on the filling tube ensuring that no bubbles remain
in the bulb. Clamp the dilatometer firmly in place in the bath so that the expansion bulb
is covered with water.
Commence reading the height of the meniscus in the capillary column with the
cathetometer and continue to do so at exactly 3-minute intervals for at least 15 readings
(45 minutes). The first reading can be obtained by clamping the telescope so that the
cross-hair is just above the meniscus; start the clock as the meniscus climbs to the crosshair. (Because the telescope inverts its image, the meniscus will appear to be below the
cross-hair when it is actually above and the meniscus will appear to be travelling down
when it is actually travelling up the capillary.) Subsequent readings will require close
cooperation between lab partners. One person should follow the meniscus with the
telescope while the other partner gives out the time so that the first partner can clamp the
telescope in position at exactly 3-minute intervals.
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Chemistry 2740 Laboratory
Experiment 2
When the readings have been completed put the dilatometer aside and proceed to the
second experiment. While the first experiment is being performed, the dilatometer and the
50 mL of sodium hydroxide solution for the second experiment should be clamped in the
bath to thermostat.
Repeat the procedure using 0.200 M NaOH and 0.400 M NaOH in place of 0.100 M NaOH
and with time intervals of 1.5 and 0.75 minutes respectively. In the case of the run using
0.400 M NaOH, allow the reaction to go to completion and then read the height of the
meniscus.
Before leaving the laboratory, please enter names, date, and experimental data
into the computer. DO NOT FORGET TO ENTER YOUR STANDARDIZATION
DATA INTO THE COMPUTER ONCE YOU HAVE OBTAINED IT.
Calculations and Report
Use the Guggenheim method to calculate the apparent first-order rate constants (k’) for
each run. For the last run, also calculate k’ using equation (3). Compare the rate constants
calculated by the two methods and discuss the validity of using the Guggenheim method to
calculate rate constants (i.e. discuss if the value calculated using the Guggenheim method
compares favourably to the value calculated using the standard method).
Calculate the second-order rate constants (k) in each case and discuss this confirmation of
the first-order dependence on hydroxide ion concentration.
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