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Resonance Stabilization Energy in Benzene
*Wagner, E.P.
(revised May 2015)
Objectives
The purpose of this experiment is to evaluate the resonance energy of benzene using experimental
and theoretical methods. The heat of combustion of a compound will be measured with a bomb
calorimeter. That measurement, plus literature values of combustion and vaporization enthalpies, will
lead to an experimental value of the resonance energy.
Background
In this investigation you will measure the heat evolved during the combustion of an organic
compound in the presence of excess oxygen under constant volume conditions. Under these
conditions, the heat evolved, q, equals ΔU rather than ΔH. Upon determination of ΔUcombustion
we can then determine a wide range of properties of the compounds examined. ΔHcombustion is
readily determined from ΔUcombustion and can be used to determine resonance energetics. The
molecule trans,trans,cis-1,5,9-cyclododecatriene ("CDDT") is used as a chemical intermediate
for elastomers, flame retardants, flavors, fragrances, and polymers. It is the source of many
twelve-membered carbocyclic compounds. Perhaps the labels "E,E,Z" are better than the
common "trans, trans, cis" (2). In examining CDDT we can determine the resonance energy of
benzene from the thermodynamics of the following theoretical reaction.
+

(1)
Benzene
Cyclohexane
CDDT
This type of reaction is an example of an isodesmic reaction in which the number and type of
bonds in CDDT is equal to the sum of benzene and cyclohexane. In the gas phase, the heat of
combustion of CDDT should equal the sum of the heats of combustion of benzene and
cyclohexane. (In the liquid phase, this is not quite true due to the different heats of vaporization
of the three compounds.) However, this analysis assumes that benzene has a set structure or does
not have any resonance structures. In reality, this is not a valid assumption and the heat of
combustion of CDDT does not equal the sum of the heats of combustion of benzene and
cyclohexane. The difference is the resonance energy, Eres, of benzene (3-4). In other words, the
enthalpy of the reaction below is Eres.
C6H6(g) (benzene) + C6H12(g) (cyclohexane) → C12H18(g) (CDDT)
(2)
We have a hard time experimentally measuring the heats of formation or heats of combustion of
these compounds in the gas phase, but using the bomb calorimeter, we can measure them in the
liquid phase. As hinted to above, this introduces some minor difficulties – the differences in the
heats of vaporization (i.e, the heat required to change from the gas phase to the liquid phase) of
the compounds of interest. Since all of these processes are state functions, Hess’s law can be
utilized to ultimately obtain the quantity of interest. In this case Eres of benzene (Figure 1).
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ΔHconden = - ΔHvap benzene
a)
C6H6(g)  C6H6(l)
b)
C6H6(l) + (15/2)O2(g)  6CO2 (g) + 3H2O(l)
ΔHcomb benzene
c)
C6H12(g)  C6H12(l)
ΔHconden = -ΔHvap cyclohexane
d)
C6H12(l) + 9O2(g)  6CO2 (g) + 6H2O(l)
ΔHcomb cyclohexane
e)
12CO2(g) + 9 H2O(l)  C12H18(l) + (33/2)O2(g) -ΔHcomb CDDT
f)
C12H18(l)  C12H18(g)
ΔHvap CDDT
Overall: C6H6(g) + C6H12(g)  C12H18(g)
-Eresonance benzene
.
Figure 1: The complete Hess’s law cycle required to obtain the resonance energy of benzene.
Since this cycle is setup with Reactants -> Products, remember that enthalpy of the reaction is
Products - Reactants. Therefore, the result is - Eres. Literature values will be used for steps a, b
and c in this laboratory experiment. Steps d and e are measured in this experiment and value for
step f will be determined with the data provided below.
In this laboratory experiment, you will measure the heat of combustion for cyclohexane and
CDDT. We will not conduct experiments with benzene due to it health hazards. Instead, you
will need to find a literature value for ΔHcomb benzene. In addition, you need to find literature
values for the heat of vaporization for benzene and cyclohexane. The enthalpy of vaporization of
CDDT will be determined from the temperature dependence of vapor pressure. Recall the integrated
form of the Clausius-Clapeyron equation (4,5).
Temp (K)
Pressure (Pa)
298
323
377
392
433
435
473
484
510
35
140
1333
2666
12266
13330
37464
53320
101308
𝐿𝐿�𝑃𝑣𝑣𝑣 � =
−∆𝐻 1
𝑅
�𝑇� + 𝑐𝑐𝑐𝑐𝑐
(3)
Experimental vapor pressures values for CDDT have already been determined and are given
above (6). This will allow you to determine the enthalpy of vaporization of CDDT.
Combining all of these literature and experimental values through Hess’s law will allow you
to determine Eres.
There is one last small issue we need to address. The calorimeter used in this experiment
conducts experiments at constant volume. Therefore, the heat evolved is the total energy, ΔU.
However, conversion between ΔU and ΔH can be completed using the following
thermodynamic equation of state and the ideal gas law equation.
ΔH =ΔU +Δ(PV) = ΔU + (Δngas)RT
(4)
The heat evolved during the combustion of an unknown, qv, is related to the temperature rise by
qv = ΔUcombustion = - Cbomb (T2 – T1)
(5)
where Cbomb is the heat capacity of the calorimeter; pressure vessel (bomb), the water in the heat
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bath, and the walls of the water bath. To determine Cbomb, A compound with a known heat of
combustion and weight is used to then solve equation (5) for Cbomb. Typically benzoic acid is
used for this calibration step. Again, in the interest of time, the heat capacity of the bomb has
been previously determined. In our lab we have two bomb setups. One matching set has paint
dots on each piece. The heat capacity of this bomb is 10802.26 Joules. The heat capacity of the
bomb with no paint dot on the matching pieces is 10187.67 Joules.
Laboratory Procedures
In the interest of saving time you will actually be running two oxygen bombs in one calorimeter
jacket. While one bomb is in the calorimeter and you are collecting temperature data, the second
bomb can be prepared for detonation. It is important to keep the oxygen bombs with their
corresponding oval buckets because the heat capacities of the two bombs are be different. Set up
the oxygen bomb, steps 1 through 6. Once one bomb is ready for detonation, that group should
complete steps 7 through 16. However, once you are finished with step 12 the second bomb can
be put in the calorimeter and detonated. Continue this cycle of alternating between preparation
and detonation of the two bombs until all of your runs are finished. You need at least 2 runs for
the cyclohexane and at least two runs for the CDDT. If time permits, it is best to get 3 runs on
each sample.
Note: cyclohexane is volatile and is difficult to weigh precisely. Thus you will need to use a
gelatin capsule to contain the sample. Follow the procedure given below to prepare and fill a gel
capsule. You will be placing the filled capsule into the basket and then have the ignition wire
pressed against the top of the capsule. Be sure to weigh the capsule empty so you can subtract
the gelatin’s heat of combustion (3.53 kJ/g).
To fill the capsule, pour a reasonable amount into a wide mouth container. It is important not to
touch the capsule with your bare hands so be sure to wear rubber gloves for this portion. Pull the
two parts of the capsule apart. Submerge both ends of the capsule and remove as much air as
possible. Place the two parts of the capsule back together below the liquid level. It is sometimes
easier to do this with two sets of tweezers. Remove the capsule from the beaker and press the
two ends fully together. Finally dry off the outside of the capsule with a kimwipe. Weigh the
basket empty and then again with the capsule recording the weights each time. Remember when
setting up the ignition wire to have it rest on top of the capsule.
During the calculations, remember to subtract the heat of combustion of the capsule from the
total heat released. Additionally, most if not all of the carbon residue in the basket at the end of
the run will be from the gelatin and thus the residue mass should be subtracted from the starting
weight of the capsule, not the sample.
Because cyclohexane is a more complicated sample preparation, it is recommended that you test
the CDDT first to gain some practice and experience.
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The following bomb calorimeter instructions are adapted from the Parr Instruments’ 1431
Bomb Calorimeter Instructions supplied with the equipment.
1. Getting Started: Disconnect the stirrer and motor pulleys. Take off the calorimeter cover
and remove the oxygen bomb and the oval bucket.
Note: Do not lay the cover on the tabletop. Always set it in the supporting ring on the stand to
protect the thermometer and stirring shafts.
2. Disassemble Oxygen Bomb: Make sure the pressure release valve (Figure 4) is open
(counter-clockwise). Once you are sure the bomb is depressurized, unscrew and remove the
outer locking ring. Remove the top bomb cover and place in the holder as shown below.
Note: If bomb is pressurized and outer locking ring is removed rapid decompression of oxygen
bomb can occur resulting in serious injury.
Figure 4: Oxygen Bomb. (Figure taken from the Parr 1341 bomb calorimeter instruction
manual.)
3. Preparing the Sample: Clean and fully dry the sample basket (Figure 5). Weigh the basket
before and after adding the sample to it to obtain an accurate sample weight. Sample size should
be around one gram.
Note: Once the basket is clean, try not to touch the basket with your hands. You can get oils on
the basket that can affect the weight and alter your results.
4. Setting the Fuse: Place the basket back into the holding arm of the top bomb cover (Figure
5). Lift the wire covers on the top bomb cover to expose two holes the ignition wire must be
threaded through. Use 10 cm (Note: the cardboard that the wire is wrapped around shows length
in terms of calories, not cm: 23 cal=10cm) of wire and form it in the same manner as shown in
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Figure 5 making sure the wire dips into the sample without touching the sample cup. Move the
wire covers back down over the ignition wire holes to secure the wire in place.
Figure 5: Sample basket and fuse setup for bomb. (Figure taken from the Parr 1341 bomb
calorimeter instruction manual.)
5. Charging the Bomb: Add 1 mL of deionized water to the bottom of the oxygen bomb outer
case (this is to collect any acid created in the detonation process). Place the top bomb cover back
on the oxygen bomb outer case and screw the outer locking ring back on making sure it is
fully tightened. Close the pressure release valve. Open the adjacent oxygen tank with the
regulator fully off. Place the gold filing valve onto the pressurizing valve of the oxygen bomb.
Slowly open the regulator on the oxygen take allowing the bomb to pressurize. Once the
pressure reaches 15 atm. turn off the regulator and open the pressure release valve slowly. The
venting process should take at least one minute. Vent the bomb in this manner 2 more times to
ensure only oxygen is in bomb. Once venting is complete, pressurize the bomb to 30 atm. and
remove the gold filing valve by first releasing the line pressure with the release switch on the
tank regulator then pulling the gold fitting straight up. Close the oxygen tank. Be sure not to
shake or tip the bomb or your sample will spill, ruining your results.
6. Fill the calorimeter bucket Replace the oval bucket noting the three dimples in the bottom of
the bucket which rest on supporting pins in the bottom of the jacket (Figure 6). The single dimple
must always face forward when setting the bucket in the jacket. Place the bomb into the oval
buck on the resting indentation on the bottom of the bucket. This is facilitated by holding tongs
that fit into holes on the side of the outer locking ring. Connect the two ignition wire pins (6) to
the electrical ignition ports on the oxygen bomb. Using a 2 L volumetric flask fill the oval
bucket with 2.0 L of room temperature deionized water. Make sure there are no bubbles
leaking from the bomb when it is under water. If there are try tightening the fittings or ask
your TA. Place the top back onto the calorimeter jacket and reconnect the stirrer and motor
pulleys. Turn the motor for the stirrer on with the black knob on the top of the motor.
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At this point the bomb experiment is ready to begin. While taking data on the first experimental
run, coordinate efforts and have your lab partner set up the second oxygen bomb so that it is
ready to go when the first run is completed.
Figure 6: Complete bomb calorimeter schematic and setup. (Figure taken from the Parr 1341
bomb calorimeter instruction manual.)
7. Let the stirrer run for approximately 2-3 minutes: This allows the system to reach
equilibrium before starting a measured run. Be sure to record the temperature for 2 minutes at 15
sec intervals before detonating the bomb.
8. Connect the Ignition Unit: Connect the disconnected wire with screw clamp to the 10 cm
screw on the ignition system (white square box next to calorimeter).
9. Start the data acquisition software “Bombcal” on the computer. Select a data acquisition
rate of 2 sec. Click the run button (arrow) in the menu bar. At this point you should see the
charting begin. Collect data for about 1 minute to get a good base line before pushing the bomb
ignition button.
10. Detonating the Bomb: Hold the ignition button on the ignition system for 5 seconds. Be
sure to record this start time, which will be needed for data analysis later. After firing disconnect
the wire with the screw clamp that was attached to the 10 cm screw on the ignition system to
avoid electric shock. Caution: Do not have your head, hands or any parts of your body over
the calorimeter when firing the bomb.
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11. The bucket temperature will start to rise. This rise will be rapid during the first few
minutes; then it will become slower as the temperature approaches a stable maximum. Be sure
to monitor toe computer to make sure it is acquiring data. Continue recording temperature
readings until the temperature has stabilized at a single temperature for 30-60 seconds. Notice
that the computer program provides digital readouts for the last three temperature reading to aid
with this. Usually the temperature will reach a maximum; then drop very slowly. However, this
is not always true since a low starting temperature may result in a slow continuous rise without
reaching a maximum. If the temperature of the water fails to rise within the first minute of
detonation, the sample has failed to detonate and you will need to reset the bomb (see following
steps).
12. After the last temperature reading, click the stop button on the computer program. Right
click on the chart and copy and paste the data into Excel or use the export to Excel option. Stop
the motor, remove the belt and lift the cover from the calorimeter. Wipe the thermometer and
stirrer with a clean cloth and set the cover on the support stand. Remove the ignition leads and
lift the bomb out of the bucket.
13. Open the pressure release valve on the bomb head to release the gas pressure before
attempting to remove the outer locking ring. This release should proceed slowly over a period of
not less than one minute to avoid entrainment losses. After all pressure has been released,
unscrew the outer locking ring; lift the top bomb cover out of the oxygen bomb housing case and
place it on the support stand. Examine the interior of the bomb for soot or other evidence of
incomplete combustion. If such evidence is found, the test will have to be discarded.
14. Remove all unburned pieces of fuse wire from the bomb electrodes; straighten them and
measure their combined length in centimeters. Subtract this length from the initial length to
obtain the net amount of wire burned. Reweigh the basket to determine the amount of unburned
fuel/capsule.
16. Clean Up: Wipe down all parts of the bomb. Wash out the sample cup. Dispose of the oval
bucket water. If you have another run to complete then start back over at step 3. If you are
finished with the bomb, thoroughly clean the bomb (especially the sample cup), and reassemble.
Bomb Calorimetry Calculations
Qtotal = Cbomb*∆T
(general form of equation solve for Qtotal)
Qtotal - (Qw - Qwu) = Qsample
(equation corrected for heat released by fuse wire)
If a capsule is used, be sure to also subtract it’s heat of combustion (Qcapsule) from Qtotal
Variables
Cbomb- Calorimeter heat capacity
Qw - Ignition wire (1 cm = 2.3 cal) Note: the cardboard that the wire is wrapped around
shows length in terms of calories, not cm (although 23 cal=10cm)
Qwu - Unburned wire
Qcapsule - Gelatin capsule = 3.53 kJ/g
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Lab Report
Data to include in your lab report:
1. Excel graph of temp vs. time for calorimetry experiments annotated with the points used to
determine temperature change. See Appendix A for data analysis method and procedure.
2. Literature values for ΔHcombustion benzene, ΔHvaporization benzene, ΔHvaporization cyclohexane. Make sure
to identify and cite your source(s) for these values. Be careful; enthalpy of formation is not
the same as enthalpy of combustion.
3. Experimental ΔUcombustion data for both the CDDT and cyclohexane. Also be sure to show the
conversion of these values to ΔHcombustion.
4. Experimental ΔHvaporization data for CDDT.
5. Sample calculations for each type of calculation you completed in this experiment.
6. The reaction shown in equations 1 and 2 can be performed through computational chemistry.
Use Scigress to perform this analysis and determine Eres. Compare your computational
results with your experimental results. (See below for Scigress instructions)
Include the following in the discussion section of your lab report
1. There are several other methods that can be used to find Eres of benzene. For example,
determining the heat of combustion of cyclooctatetraene and then relating it to Eres of
benzene (6). Show/explain how this would be possible.
2. Can bond enthalpy information be used to determine Eres of benzene? Explain.
3. Discuss one other method that could be used to find Eres of benzene.
4. Out of all of the above methods including the Scigress modeling and your experiment, which
method do you think is the best? Explain.
5. Resonance energy can be found on any structure that has more than two adjacent sp2
hybridized carbons. For example, cyclooctatetraene, as well as the straight conjugated chain
analogs, C6H8 and C8H10. However, none of these three structures have as much resonance
energy “savings” per carbon atom (Eres) as benzene. A relative comparison in order of
increasing Eres per carbon atom is shown below. Explain this ordering of Eres.
≈
<
<<
Increasing resonance energy savings per carbon
Scigress Modeling Instructions
Draw cyclohexane and benzene in Scigress and perform an optimized geometry experiment for
each at the PM6 level. When it is complete, the heat of formation value should be displayed at
the bottom of the output file. Note that this is the gas phase value. For CDDT you will need to
first find the minimum energy structure before running the experiment. Draw CDDT and the
beautify comprehensive. The resulting structure will be a symmetrical ring, which clearly is not
correct. Perform the experiment Conformations | Conformations from dynamics | MM2
molecular dynamics at 1000K. However, before running edit the temperature in the procedure to
298K. After the experiment completes, toggle over the graph to get the lowest energy
conformation. Then copy the structure shown into a new object window. Next perform the
experiment Chemical sample | Current energy | PM6. When complete you will now have the
heat of formation for the conformer of CDDT that best represents it at 298K. Use the three ΔHf
values to calculate Eres benzene.
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References
1. Rawdah, T. N.; El-Faer, Z. M. Conformational analysis of cis,trans,trans-1,5,9cyclododecatriene. Tetrahedron Letters 1995, 36, 3381-3384.
2. Pauling, Linus; The Chemical Bond, Cornell University Press, Ithaca, New York, 1967. The
resonance energy of benzene is stated on page 121 of Chapter 6, "The Resonance of
Molecules"
3. Silbey, R. J.; Alberty, R. A., Bawendi, M. G. Physical Chemistry, 4th edition; John Wiley &
Sons: New York; 2005. See section 6.3 for the Clausius-Clapeyron equation. (Any physical
chemistry textbook will have this information)
4. McQuarrie, D. A.; Simon, J.D. Physical Chemistry; University Science Books: Sausalito,
1997. See equation 23.13 for enthalpy of vaporization.
5. Rauh, H.-J.; Geyer, W.; Schmidt, H. and Geisler, G. Z. Phys. Chemie (Leipzig) 1973, 253, 43
6. Stevenson, G.R. The determination of the resonance energy of benzene. A physical chemistry
laboratory experiment. Journal of Chemical Education 1972 ,49(11), 781
7. http://webbook.nist.gov/chemistry/
*This lab is adapted from
Halpern, A.M. Experimental Physical Chemistry, 2nd ed; Prentice-Hall: Upper Saddle River, NJ,
1997; Experiment 6.
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Appendix A: Data analysis method for determining temperature change of bomb.
Plot the (temperature, time) data for each run as illustrated in Figure 1. Note the important points
in the graph denoted by ’i’, ’a’, ’b’, ’c’, and ’f’. The point ’a’ denotes the time of firing of the
bomb, ’b’ the position where the temperature reaches 60 % of the total change, and ’c’ the time
of maximum temperature (i.e., end of the reaction). Points ’i’ and ’f’ denote the initial and final
points of measurement, respectively. A simple approach for obtaining the temperature rise would
consist of subtracting the initial and final temperatures (e.g. ∆T = Tc −Ta). However, if the
temperature is not stable in the initial (i < t < a) and final (f > t > c) periods, baseline correction
must be applied. If the baseline is assumed to be linear, the rates of change can be obtained by
using a difference approximation:
𝑟1 =
𝑇𝑓 − 𝑇𝑐
𝑇𝑎 − 𝑇𝑖
𝑎𝑎𝑎 𝑟2 =
𝑎−𝑖
𝑓−𝑐
Figure 1: An example of (temperature, time) data plot showing the positions for reading Ti, Ta,
Tb, Tc, and Tf .
The baseline is evaluated for both initial and final temperatures about point b:
∆𝑇 = �𝑇𝑐 − 𝑟2 (𝑐 − 𝑏)� − �𝑇𝑎 − 𝑟1 (𝑏 − 𝑎)� = 𝑇𝑐 − 𝑇𝑎 − 𝑟1 (𝑎 − 𝑏) − 𝑟2 (𝑐 − 𝑏)
This is the expression for the baseline corrected temperature rise.
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Appendix B: Temperature probe interface programming and reset
The instructions below are provided and should only be performed with the Instructor’s help.
Occasionally, the digital data collection interface will stop working properly. This often caused
by the large RF put out when the bomb is detonated. If this occurs, use the procedure below to
reset/reprogram the interface.
-
First try a new battery in the temp probe. It uses a special 3.7 V Li ion battery. The voltage
output needs to be very close to the 3.7V. Test the voltage output and if it is below 3.5V,
replace the battery and determine if this resolves the issue. If not, then continue below.
-
On the computer go to start menu -> PuTTY -> and open “PuTTY”
Select the “serial” radio button
In the textbox above type in “com5”
Hit enter button on keyboard
Turn on the temp probe
At the point you should see a command window open with the info about the temp probe. It
should read “ ParrThermometer Rev. $Rev:100$”
Type 1 then Enter (single channel mode)
Type c then enter a contrast value of 2000. Hit Enter.
Finally, the units for the measurement need to be set. Type S. each time you type “s”, the
program cycles through the three possible units, Celsius, Fahrenheit, and Ohms. Make sure
you set it to Celsius.
Now close the command box. At this point the temp probe should be working properly, If
not check your settings again. If the problem persists, call Dr. Wagner or Chuck in the
Electronics Shop.
-
-