A Novel Bomb Calorimetric Determination of the Resonance Energy of Benzene The bomb calorimetric determination of the resonance energy of benzene by comhustion of 1,3,5,7-cyelooctatetraene and benzene has been described previously in THIS JOURNAL.' There are numerous problems with this classic lab experiment, since benzene has been discovered to be a carcinogen, and 1,3,5,7-cyclooctatetraeneis expensive, given to spontaneous polymerization, volatile, and foul-smelling. Also, the latter compound rarely burned cleanly and completely under our conditions. However, the comhustion of 1,5,9-tram-,tram-,cis-eyelododecatriene is usually complete under conditions in our homb calorimeter. Furthermore, the material is sufficiently nonvolatile that it can easily he weighed in an open sample pan. The odor is very faint, and the material is cheap and stable.2 The material should be kept dry over CaCL2. In the absence of resonance energy the AH0 of the reaction below
would he the sum of the AHoof combustion of a mole of benzene (CsHs) and a mole of eyelohexane
(Calllz). Since the heat of combustion of both of these compounds is listed in the H a n d h o ~ kit, ~need not be measured, merely
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corrected for the heat of vaoorization (also known). t o eet the eas-ohase enthalov ."of combustion. For the hvdrocarbon. the measured enthalpy of comhustion can he corrected using a heat of vaporization derived from Trouton's rule. The difference between the corrected heats of combustion is a good measure of the resonance energy. Using the median student value shown in the table below, this is found to be 31 keal. Combustion of the hydrocarbon and calibration senlene cyclohexane cyclododeca~iene of the calorimeter with benzoic acid can easily be done in a three-hour period by a single student Heat of combustion 780.98 936.88 n.8. working alone, and i t gives experience doing bomb (liq)kcal/mol calorimetry on both solid and liquid samples. Heat of vaporization 7.895 8.090 10.584' While one could tell the student that he is kcal/mol measuring the resonance energy of benzene, the Heat of comb~stion ".a. ma. 1764b problem can also be stated as a discovery exercise. (student value) To do this, one stresses in the prelab handout that the bond energies and number of bonds formed and ~stimatedm m Trouton'o rule broken should he the same. When the student gets b ~ e d i a nstvaent valve a result, an uncertainty analysis is needed to show either agreement or disagreement with the stated hypothesis. The value for the heat of combustion of this new hydrocarbon is not readily available, so the student has to puzzle about whether the measurement is "wrong," or whether the fault lies with the original hypothesis. 'Stevenson, G.R., J. CHEM.EDUC., 49,781 (1972). Readily available from Aldrich Chemical Company. "Handbook of Chemistry and Physics," 57th Ed., (Editor: Weast, R. C.) Chemical Rubber Publishing Co., Cleveland, Ohio, p D274 Miles Pickering Princeton University Princeton. NJ 08544
318
Journal of Chemical Education
