The enthalpy of combustion of naphthalene was experimentally determined to be -5030. 44 ± 78. 98 kJ/mol which was a 2. 5% error from the literature value of -5160 ± 20 kJ/mol. 2 The theoretical enthalpy of combustion of solid naphthalene was calculated to be -6862. 68 kJ/mol using bond energies for the gaseous molecules, the heat of vaporization of liquid water and the literature value of the heat of sublimation of naphthalene, which was 72 ± 4 kJ/mol.
The absolute value of the difference between the theoretical heat of combustion and the literature value heat of combustion of naphthalene was 1702. 68 ± 20 kJ/mol. 2 This difference indicates that the resonance energy of naphthalene is very high and that its most stable configuration has a much lower energy than the unstable configuration that was combusted during this experiment. The results were obtained using bomb calorimetry where a sample was combusted in a bomb immersed in water, and the variations in water temperature were used to determine the heat of the combustion.
Introduction: The resonance energy of a molecule is the difference in energy between the actual configuration of a molecule and that molecule’s most stable structure. It is useful to know the resonance energy of a species as it describes the difference between an experimentally determined heat of combustion and a theoretical one, calculated using bond energies with Hess’s Law. 1 This comparison gives insight into how stable the molecules being combusted in the experiment are, as well as how the environment in which they were combusted differs from standard pressure and temperature.
There were three distinct goals that were to be accomplished during this experiment. An experimental enthalpy of combustion of naphthalene was to be calculated and compared to a literature value, a theoretical enthalpy of combustion of naphthalene was to be determined, and to discuss the resonance energy in naphthalene by comparing the theoretical enthalpy of combustion to a literature value. To accomplish the goals of this experiment, bomb calorimetry was utilized.
Naphthalene was combusted in excess oxygen at about 30 atm in order to ensure that the products of the combustion were only carbon dioxide and water and that the following chemical equation was followed: 1 C10H8s+12O2g>10CO2g+4H2Ol (1) It was important that equation (1) was followed in order to make the calculation of the theoretical heat of combustion of naphthalene valid. The first step of this experiment was to determine the heat capacity of the entire bomb calorimeter.
To do this benzoic acid, which had a known heat of combustion, was combusted in the bomb and the rate of temperature change of the water surrounding the bomb was recorded. This data was then used to calculate the heat capacity of the calorimeter according to equation (2): Ccalorimeter=-?Usamplemsample?Ufuse?mfuse?T (2) In equation (2), ?U was the heat of combustion, msample was the mass of the benzoic acid sample and ?mfuse was the difference in mass between the fuse wire present at the start and what remained of the fuse wire after the combustion.
It was assumed in equation (2) that heat capacity was not dependent on temperature. The sample of solid naphthalene was then combusted in the bomb calorimeter and data was recorded for its combustion. The heat of combustion was calculated by rearranging equation (2), and then the enthalpy of combustion of naphthalene was calculated using equation (3): ?H=?U+?ngRT (3) In equation (3), ?n was the change in moles of gas from the products to the reactants, R was the ideal gas constant, and T equals T60%, which is the temperature of the ignition after it was 60% complete.
T60% was used in order to account for the heat lost through the bomb cylinder and heat created due to stirring. It was calculated using equation (4): T60%=Ti+0. 6*Tf-Ti (4) In equation (4) Ti and Tf were the y-interecepts of the pre-ignition and post-ignition lines, respectively. It was desired to compare a theoretical value of enthalpy of combustion to a literature value. To do this, the theoretical value was calculated using a literature value for the heat of sublimation of naphthalene, the heat of vaporization of water and average bond energies, given in Table 1 of the lab packet. Equations (1) and (5) were used to calculate the theoretical enthalpy of combustion of gaseous naphthalene, where n was the number of moles, m was the number of bonds, and ?
H was the average bond energy: ?Hcombustion,naphs=ni+mi*?Hireactants-ni+mi*?Hiproducts (5) The theoretical value of the combustion of solid naphthalene was calculated by substituting their given values in the literature into equation (5) in place of their corresponding terms. Experimental: A more detailed explanation of procedures can be found in the lab packet. To summarize, samples of benzoic acid and naphthalene were combusted using the Parr 1341 Calorimeter with a Parr oxygen bomb. A massed benzoic acid pellet was placed onto the center of the combustion pan and 10 cm of nickel alloy fuse wire was massed and then threaded through both electrodes on the head of the bomb so that it lay firmly against the acid pellet. 1mL of distilled water was then placed in the bottom of the bomb in order to ensure that all the water produced by the combustion was liquid.
The bomb was then closed and then purged of air with 15 atm of oxygen twice before being charged with 30 atm of oxygen. The bomb was then placed in the center of the calorimeter bucket, the electrodes were connected to the top of the bomb and then 2 L of deionized water was poured into the bucket so that the bomb was completely immersed. The variations in the temperature of this water would be measured and used to calculate the heat capacity of the calorimeter in the benzoic acid run or the heat of combustion in the naphthalene run.
The calorimeter lid was then used to cover the calorimeter, while making sure that the stirrer would be able to rotate freely. The bomb calorimetry software, which would record the temperature of the water in the calorimeter, was opened and a new run was started. Once a slow, linear temperature change was observed, the room was cleared of people, and the calorimeter was ignited for five seconds. After another 30 seconds, the room was reentered and the run was stopped once the change in temperature became very small, which took about 15 minutes.
The bomb was depressurized, then cleaned using acetone and the calorimeter bucket was emptied and the remaining fuse wire was massed. A pellet of naphthalene was formed using the pellet press and then massed along with a new fuse wire. The entire process that was used for the combustion of benzoic acid was repeated for the naphthalene sample. Results and Discussion: The heat of combustion of naphthalene was experimentally calculated to be -5025. 46 ± 78. 98 kJ/mol and the enthalpy of combustion of naphthalene was experimentally determined to be -5030. 44 ± 78. 8 kJ/mol which was a 2. 5% error from the literature value of -5160 ± 20kJ/mol. 2 In order to calculate these results, the heat capacity of the calorimeter had to be determined by combusting benzoic acid, which had a known heat of combustion value. The heat capacity of the calorimeter was 12362. 64 ± 0. 002 J/C. The uncertainty of each calculation in this experiment was determined by completing a propagation of error calculation. Figure 1 is the plot of the data obtained during the benzoic acid combustion. t60% t60% Figure 1: Temperature versus time for benzoic acid combustion
The data from Figure 1 was used to calculate the heat capacity of the calorimeter using equation (2). This step was necessary since the purpose of this experiment was to calculate the enthalpy of combustion of naphthalene, which could only be accomplished if the heat capacity of the vessel in which it was combusted was known. Figure 2 is a plot of the data obtained from the naphthalene combustion. Figure 2: Temperature versus time for naphthalene combustion To use the data in Figures 1 and 2, information needed to be known about the mass of substances being combusted during the process.
Also, in order to calculate the heat capacity of the calorimeter, the heats of combustion of both benzoic acid and the fuse wire were needed. Table 1 summarizes the data required to use the measured data from Figures 1 and 2. Table 1: Combustion calculation data Species| Benzoic acid| Naphthalene| Pellet mass (±0. 0001 g)| 1. 03| 0. 4880| Wire mass initial (±0. 0001 g)| 0. 0160| 0. 0163| Wire mass final (±0. 0001 g)| 0. 0034| 0. 0073| ?mfuse wire (g)| 0. 0126 ± 0. 00014| 0. 0090 ± 0. 00014| Wire heat of combustion (cal/g)| 1400| 1400| Benzoic acid heat of combustion (MJ/kg)| 26. 54| —| The heat capacity of the calorimeter, as well as the heat of combustion of naphthalene were calculated using equation (2), and then the enthalpy of combustion of naphthalene was determined using equation (3). An important concept used in the analysis data for this experiment was the utilization of T60% and t60% values. These values represent the temperature and time of the combustion when it was 60 percent complete which helped account for the heat created from stirring as well as heat lost through the wall of the calorimeter, in order to produce more realistic results.
T60% was used in equation (3) and t60% was used to calculate the instantaneous temperatures of the pre-ignition and post-ignition steps, Ti and Tf. T60% and t60% were determined by equation (4) and were 26. 453 ± 0. 0866 C at 534s for benzoic acid and 26. 2582 ± 0. 0866 C at 390s for naphthalene. To calculate Ti and Tf, t60% was plugged back into the equations for the pre-ignition and post-ignition lines. Ti and Tf were 25. 138 C and 27. 337 C for benzoic acid and 25. 335 C and 26. 895 C for naphthalene. One goal of this experiment was to calculate a theoretical enthalpy of ombustion for naphthalene and then compare it to literature values.
The theoretical enthalpy of combustion of gaseous naphthalene was first calculated using equations (1) and (5), which was determined to be -5353 kJ/mol. However, it was apparent that this calculation was inaccurate since the naphthalene was in the solid phase and measures had been taken to ensure that the water would be completely produced in the liquid phase. A more accurate calculation was completed by using a literature value for the enthalpy of sublimation of naphthalene (72 ± 4 kJ/mol) as well as the given heat of vaporization of water. Equation (5) was altered by substituting the enthalpy of sublimation as well as the heat of vaporization of water into the equation for their corresponding terms, which resulted in a theoretical enthalpy of combustion of solid naphthalene of -6862. 68 kJ/mol. The absolute value of the difference between the theoretical enthalpy of combustion of solid naphthalene and the literature value for the enthalpy of combustion of solid naphthalene (-5160 ± 20 kJ/mol) was 1702. 68 ± 20 kJ/mol. This difference gives insight into both the conditions of the experiment and the resonance energy of naphthalene.
One of the reasons that the difference is so large is that the theoretical value was calculated using values at standard temperature and pressure, which is 25 C and 1 atm. However, the experiment was carried out under a pressure that was 30 times larger than standard pressure, which would cause the enthalpy of combustion to become much more positive. The other reason that the difference is so large is likely due to the fact that the resonance energy of naphthalene is very large, characterized by a significant difference in the actual structure of naphthalene from its lowest energy structure.
It would then be reasonable to say that solid naphthalene is a fairly unstable molecule that will readily combust when a heat source is applied to it. This experiment was a success as each objective of the experiment was completed and the experimentally determined enthalpy of combustion of solid naphthalene was very close to its literature value. There was a large difference between the theoretical enthalpy of combustion of solid naphthalene and the literature value, however the reasons for this difference were explainable and characterized the resonance energy of naphthalene.
There were two main sources of error in this experiment associated with the bomb calorimeter. One source of error was that exactly the same amount of water needed to be used to immerse the bomb in both trials for the best results. The water was measured out using the human eye and therefore, the accuracy of its measurement is not reliable. The other main source of error is that the actual combustion of either solid was not the only source of heat transfer in the calorimeter. Stirring in the calorimeter produced a certain amount of heat and then another certain amount of heat was lost through the walls of the calorimeter.
Neither of these heats could be calculated, and their occurrence was only accounted for by arbitrarily deciding to use the temperature and time of the combustion when it was 60 percent complete for certain calculations. It was impossible to tell how much of the heat measured was solely due to the combustion of each solid but it is very unlikely that it was exactly 60 percent of the total heat variation measured. The results of this experiment would be useful for calculating other thermodynamic properties of naphthalene, since many of those calculations would require that the enthalpy of combustion of naphthalene be known. One way that this experiment could be improved would be to have a more efficient nickel alloy fuse wire system. It was difficult to get the wire to be both connected to each electrode and laying firmly against the sample. It was also difficult to determine if the wire was even in contact with the sample. One solution to this problem would be to have a raised groove or stand on the combustion pan made of a material that would not short out the fuse that the fuse could sit in. Then the sample could be placed on top of the fuse, eliminating any doubt that the contact between fuse and sample was not adequate.
Conclusion: The heat of combustion of naphthalene was experimentally calculated to be -5025. 46 ± 78. 98 kJ/mol and the enthalpy of combustion of naphthalene was experimentally determined to be -5030. 44 ± 78. 98 kJ/mol which was a 2. 5% error from the literature value of -5160 ± 20kJ/mol. The heat capacity of the calorimeter was 12362. 64 ± 0. 002 J/C. The theoretical enthalpy of combustion of solid naphthalene was calculated to be -6862. 68 kJ/mol and the absolute value of the difference between the theoretical heat of combustion and the literature value heat of combustion of naphthalene was 1702. 8 ± 20 kJ/mol. The results of this experiment indicate that naphthalene has a large resonance energy due to the instability of its actual configuration. The use of bomb calorimetry to calculate the heat of combustion of naphthalene was successful due to the low error between the experimental results and literature values. Acknowledgements: The author of this study would like to thank the members of Group #3 listed on the title page as well as teaching assistants Kyle Banyas and Eric Popczun who were responsible for teaching all the necessary data collection techniques. References: (1) Milosavljevic, B. H.
Lab Packet for CHEM 457: Experimental Physical Chemistry, Resonance Energy of Naphthalene by Bomb Calorimetry. University Press: University Park, July 2010. (2) NIST Chemistry WebBook. NIST Standard Reference Database Number 69. Material Measurement Laboratory. United States Secretary of Commerce. 2008. (3) McMurry, J. Organic Chemistry, 5th ed. , Brooks/Cole Publishing Co. : CA, 2000, p. 564-566. Appendix: Sample Calculation 1: Ccalorimeter Ccalorimeter=-?Usamplemsample?Ufuse?mfuse?T Ccalorimeter=-26452J/g*1. 03g5857. 6J/g*0. 0126g-2. 21C Ccalorimeter=12362. 64J/C Sample Calculation 2: ?H (naphthalene) ?H=?U+?ngRT ?H=5025. 6kJ/mol+(10-12)*8. 314J/molK*299. 4K/1000 ?H=5030. 44kJ/mol Sample Calculation 3: T60% (naphthalene) T60%=Ti+0. 6*Tf-Ti T60%=25. 327C+0. 6*26. 879C-25. 327C T60%=26. 2582C Sample Calculation 4: theoretical ?Hcombustion,naph(s) ?Hcombustion,naphs=ni+mi*?Hireactants-ni+mi*?Hiproducts ?H=8*414+5*610+6*347+12*146-20*803+8*464kJ/mol ?Hcombustion,naphs=-5353kJ/mol Uncertainty Analysis: Determining the uncertainty in a measurement is extremely important in order to understand how meaningful that measurement is. Two different instruments were used to make the measurements that went into calculating the heat of combustion of naphthalene.
Through a propagation of error calculation, which is demonstrated in Sample Calculation 5, the uncertainty in calculating each value was determined. The error associated with each instrument was known and is tabulated below: Table 2: Measurement device tolerances Measurement Device| Tolerance| Digital Balane| 0. 0001 g| Thermometer| 0. 05 C| Using the data from Table 2, the uncertainties of literature values, and a propagation of error equation, the uncertainty for the calculation of the thermodynamic properties were calculated. Sample Calculation 5: Error Propagation (?mfuse for naphthalene combustion) ?mfuse=?minitial2+?mfinal2 mfuse=0. 0001g2+0. 0001g2 ?mfuse=0. 000141g Sample Calculation 7: Percent Error (?Hcombustion,naph(s)) %Error=1-experimental valueliterature value*100% %E?H=1–5030. 44kJ/mol-5160kJ/mol*100% %E?H=2. 51% Report Questions: 1. To prove that the uncertainty in ?
T was the major source of error in this lab, as explained in the Discussion section, ?T was varied by 0. 1K and then the new ?H value was compared to the original value calculated in the lab. In the lab, ?Hcombustion,naph(s) was -5030. 44kJ/mol. When ?T was increased by 0. 1K, the value for ?Hcombustion,naph(s) became -5355. 4kJ/mol which is an decrease of 324. 7kJ/mol from the original and a 6. 45% difference from the original. 2. a. The large difference between the theoretical value and the literature value indicates that the resonance energy of naphthalene is very large, due to a significant difference in the energy of the actual structure of naphthalene from the energy of its most stable structure. This indication means that naphthalene is a fairly unstable molecule that will readily combust when a heat source is applied to it in the presence of oxygen. b.
The literature value for the resonance energy of benzene was 150 kJ, which is much smaller than the resonance energy of naphthalene which was experimentally determined to be 1702. 68 kJ. This difference in resonance energies is expected because a low resonance energy indicates a stable molecule, while a high resonance energy indicates an unstable molecule. Benzene’s six membered aromatic ring is very stable since its configuration puts minimal strain on each of its bonds. When the instability of naphthalene is compared to the stability of benzene, a large difference in their respective resonance energies is expected.
