Bomb Calorimetry

Determination of the Resonance Stabilization Energy of Benzene Using Bomb Calorimetry

Kaitlynn, Jesse , Belinda
Truman State University

Abstract

The resonance stabilization energy of benzene is investigated by the combustion of cyclododecatriene. The heat of combustion for cyclododecatriene was determined using bomb calorimetry and used to solve for the stabilization energy of benzene. The bond stabilization energy of benzene is found to be 167.6 ± 388.3 kJ/mol. This evidence that benzene has a resonance stabilization energy is possibly necessary for determining the structure.

Introduction

The structure of benzene was a focus for many years. Although it was agreed that benzene contained three double bonds, a structure to explain its high stability was not suggested until 1866 by German chemist August Kekulé.1 His theory suggested that benzene was a hexagonal molecule containing three, alternating double bonds. This theory gave insight that lead to the field of aromaticity. After examining benzene’s heat of hydrogenation, Kekulé’s model was still not sufficient in explaining benzene’s stability. His theory was eventually replaced by resonance theory, which asserts that benzene does not have three alternating double bonds, but rather it has six pi electrons that are equally delocalized over the ring. A simple depiction is a hybrid of both of Kekulé’s structures (Scheme 1).
In order to confirm the structure of benzene, the enthalpy of Kekulé’s structure must be determined and then used to calculate the resonance stabilization energy (RSE) for benzene. If the RSE is greater than zero, there is additional stabilization that must come from something besides the enthalpy that corresponds to the isolated double bonds. In modern chemistry, this extra stabilization stems from a “resonance hybrid” of Kekulé’s structure. Since Kekulé’s 1,3,5-cyclohexatriene does not actually exist, scientists must use a compound that is structurally equivalent to the combination of Kekulé’s structure and a known compound. CDDT is theoretically formed by the reaction of benzene and cyclohexane (Figure 1). Benzene and cyclohexane do not react with one another, so the reverse reaction must be analyzed instead. The heat of combustion for CDDT can be used to determine the RSE of benzene.2
The ∆combH for CDDT can be determined using bomb calorimetry. Using the Rauh et Al, Equation 1, to determine the vapor pressure for CDDT, this value can be entered into the Clausius-Clapeyron, Equation 2, to determine the ΔvapH for the overall reaction. By constructing a thermodynamic cycle, the enthalpy at each step can be analyzed in order to calculate ΔfH (Figure 2).

log10(Pvap) = -() + 10.8681 (1)

ln (P/P0)= (∆H/R)*((1/T0)-(1/T) ) (2)

Experimental
Benzoic acid was measured and formed into a pellet with a pellet press. It was then placed into a crucible which was place into a Parr model 1108 oxygen combustion bomb. An ignition wire was placed across the electrode leads with the center touching the top of the benzoic acid pellet. The bomb was then sealed and flushed with 30 atm of oxygen gas before being filled with 30 atm of oxygen gas. Next, the bomb was placed into the Parr 1341 plain jacket calorimeter and filled with 2.00 L of distilled water. Then the electrodes were attached to the ignition unit, and the calorimeter was covered with a lid. The stirrer was turned on, and the Vernier temperature probe was secured into position in the lid with parafilm. Temperature data was recorded for five minutes before igniting the bomb and for five minutes after the temperature change leveled off. This procedure was repeated with CDDT except forming a pellet was unnecessary as CDDT was in liquid form. Results

The heat capacity of the calorimeter was found using

Qtot ∆combH (3)

Qmm∆T (4)

Qmm Qtot Q1 (5)

Q1 (6)

with Q1 as the heat from the wire, Qtot as the total heat, and Qmm as the difference between them. The calculated heat capacity [C] and the change in temperature of the combustion of CDDT were used to calculate Qtot for CDDT (4). From Qtot, Qmm was found (5). Qmm was used to find the change in internal energy for the combustion of CDDT.

∆combU(kJ/g)=Qmm/g (sample) (7)

The change in the internal energy of combustion is equal to the change in enthalpy for CDDT because as shown in Scheme 3 the change in moles is zero. (8)

Using the change in enthalpy for CDDT the resonance of benzene was calculated (Table 2) 1 to be 167.6 ± 388.3 kJ/mol.
While using the bomb, many issues arose that caused several runs to be left out of the data analysis. One of the experimental trials was not used because the o-ring burned. It is possible the o-ring contained a flaw as the change in temperature measured with the calorimeter was over ten degrees, and there was a large amount of soot with only a partial o-ring left after the temperature leveled off. Two of the other runs were not considered in the data processing due to the large amount of soot found all over the inside of the bomb after these trials. The ignition wire failing to ignite or igniting but failing to ignite the pellet was another issue on several runs. Of the runs that were kept for data analysis benzoic acid #1, CDDT #3, and CDDT #4 all had a very small amount of soot that was contained to the crucible. This may have caused an error.

Discussion: Assuming most literature enthalpy values are exact and wire corrections are small, the majority of uncertainty for this experiment stems from uncertainty in the CDDT combustion. The smallest part of this error includes a ±0.0001 error associated with the scale and a ±0.003 error associated with the combined enthalpy. The error associated with the heat capacity (C) was found and then used to calculate the error with the combined U. As these initial error levels are so small, it seems likely that the data is reliable. However, the resonance stabilization energy that was found experimentally was, 167.6 ± 388.3 kJ/mol, where as the literature value is 150.7 kJ/mol.3 The results suggest that benzene does have resonance structures as otherwise it would not have a resonance stabilization energy.
Some of the precision error that is present may have come from the precision limitation of using a temperature probe rather than a calorimetry thermometer. However, this error should have been small as the temperature probe was reading to four decimal places. Another possible source of error would be that the purity of the sample was not analyzed so uncertainty is created as impurities would affect the measured combustion heat.
The experiment had several aspects in it that helped to work towards reliable accurate results. For instance, the bomb was flushed with pure oxygen to remove nitrogen so that nitrogen oxides would not be a contributing source of error. Also, the fuse was weighed before and after ignition so that its contribution could be taken into account. Another thing that was done to make the result reliable was a calibration run with benzoic acid was done between each of the runs of CDDT. This helped to account for other various errors that could not be eliminated such as imperfect insulation of the calorimeter. Despite these steps the overall results, while close to the literature value, have a very large error term.
The resonance stabilization energy that was found experimentally was larger than the literature value. This may have occurred if an incomplete combustion took place as this would make the ΔcombH smaller than it should have been which would cause the calculated resonance stabilization energy to be larger than the literature value. Incomplete combustion is likely the largest source of error in the data. One thing that may have indicated an incomplete combustion is that some of the runs had a tiny amount of soot in the crucible. Another thing that may indicate an incomplete combustion is that there were tiny amounts of wire left uncombusted. Conclusion

The goal of the experiment was to find the resonance stabilization energy of benzene. The resonance stabilization energy of benzene was found to be 167.6 ± 388.3 kJ/mol at the 95% confidence level. Although this value is more than the literature value, it still shows that there is resonance stabilization energy. The implication of this is that it indicates benzene is not 1, 3, 5-cyclohexatriene but is a more stable structure. This helps to explain some of the properties of benzene such as the derivatives of benzene not having isomers and how stable it is. It is helpful to know this because it could suggest how benzene will react under various conditions with various other chemicals. Knowing the resonance energy helps to explain a little about the structure and chemical reactivity of benzene. Acknowledgements
Kevin A. Robb, Michael Delcau, Erika ???, Wendy La, ??? ???

References
1. Wade, L. G. Organic Chemistry, 7th ed.; Pearson: Upper Saddle River, NJ, 2010; p 707-708.
2. McCormick, J.M. Determination of the Resonance Stabilization Energy of Benzene by Bomb
Calorimetry. http://chemlab.truman.edu/PChemLabs/CHEM324Labs/BombCalorimetry.asp (accessed Sept, 12 2012).
3. J. Sherman The heats of hydrogenation of unsaturated hydrocarbons. Journal of the American Oil Chemists ' Society; Volume 16, Number 2; February, 1939

Tables

Table 1. Calculations from Experimental Data to Find Enthalpy.

Run Name
Weight of Sample
(grams)
Qmm kJ q1 kJ Qtot kJ Change in Temp

C
ΔcombU
(kJ/g)
ΔcombH (kJ/g)
Benzoic Acid #1
0.7805
20.63
0.04393
20.68
2.07
9.99 ± 0.002

26.434
± .003
CDDT #1
0.6573
28.5
0.05858
28.6
2.86
9.99 ±
0.002
43.4 ±
0.010
43.4 ±
0.010
Benzoic Acid #3
0.8297
21.93
0.07615
22.01
2.16
10.2±
0.002

26.434
± .003
CDDT #3
0.6638
29.8
0.07029
29.9
2.93
10.2±
0.002
44.9 ±
0.01
44.9 ±
0.01
Benzoic Acid #4
0.8031
21.23
0.06150
21.29
2.06
10.3 ±
0.002

26.434
± .003
CDDT #4
0.6969
32.4
0.06034
32.5
3.13
10.4 ±
0.003
46.5 ±
0.02
46.5 ±
0.02
Benzoic Acid #5
0.7674
20.22
0.06033
20.28
1.94
10.6 ±
0.002

26.434
± .003
CDDT #5
0.6926
32.2
0.06150
32.3
3.11
10.4 ±
0.003
46.5 ±
0.02
46.5 ±
0.02
Benzoic #6
0.8567
22.6
0.06268
22.71
2.23
10.2 ±
0.002

26.434
± .003

Table 2. Resonance Stabilization Energy Calculations.
CDDT Run #
ΔcombH (kJ/g)
ΔcombH (kJ/mol)
RSE (kJ/mol)
1
-43.4
-7040
-149.4
3
-44.9
-7290
100.6
4
-46.5
-7548
358.6
5
-46.5
-7550
360.6
Average (95% confidence)
45.3 ± 2.4
-7357 ± 388
167.6 ± 388.3

Table 3. Thermodynamic Cycle.

Equations
Enthalpy (kJ/mol)
C6H6 (g) →C6H6(l)
-33 ± 2
C6H6(l)+(15/2)O2(g)→6CO2(g)+3H2O(l)
-3267 ± 20
C6H12(g)→C6H12(l)
-33.5 ± 0.3
C6H12(l)+9O2(g)→6CO2(g)+6H2O (l)
-3920
12CO2(g)+9H2O(l)→C12H18(l)+(33/2)O2
7357 ± 388
C12H18(l)→C12H18(g)
64.1
Overall:C6H6 (g) +C6H12(g)→C12H18(g)
167.6

Schemes Scheme 1. Kekulé’s structures.

Scheme 2. Formation Reaction of CDDT.

Scheme 3. Combustion of CDDT.

C12H18 (l) + 12O2 (g) ⇆ 12CO2 (g) + 9H2O (l)

Figure Legends

Figure 1. Benzoic Acid Run #1 (representative of the benzoic acid runs)

Figure 2. CDDT Run #1 (representative of CDDT runs)

References: 1. Wade, L. G. Organic Chemistry, 7th ed.; Pearson: Upper Saddle River, NJ, 2010; p 707-708. 3. J. Sherman The heats of hydrogenation of unsaturated hydrocarbons. Journal of the American Oil Chemists ' Society; Volume 16, Number 2; February, 1939