CHEM 457: Lab Section 4
Lab Group #3:
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.2 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), ΔU was the heat of combustion, msample was the mass of the benzoic acid sample and Δmfuse 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), Δn 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...