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ERIC Number: EJ1084317
Record Type: Journal
Publication Date: 2015-Dec
Pages: 6
Abstractor: As Provided
ISBN: N/A
ISSN: ISSN-0021-9584
EISSN: N/A
Why Combustions Are Always Exothermic, Yielding about 418 kJ per Mole of O[subscript 2]
Schmidt-Rohr, Klaus
Journal of Chemical Education, v92 n12 p2094-2099 Dec 2015
The strongly exothermic nature of reactions between molecular oxygen and all organic molecules as well as many other substances is explained in simple, general terms. The double bond in O[subscript 2] is much weaker than other double bonds or pairs of single bonds, and therefore the formation of the stronger bonds in CO[subscript 2] and H[subscript 2]0 results in the release of energy, which is given off as heat or increases thermal motion. This explains why fire is hot regardless of fuel composition. The bond energies in the fuel play only a minor role; for example, the total bond energy of CH[subscript 4] is nearly the same as that of CO[subscript 2]. A careful analysis in terms of bond enthalpies, counting double bonds as two bonds to keep the total number of bonds unchanged, gives the heat of combustion close to -418 kJ/mol (i.e., -100 kcal/mol) for each mole of O[subscript 2], in good agreement (±3.1%) with data for >500 organic compounds; the heat of condensation of H[subscript 2]O, -44 kJ/mol, is also included in the analysis. For 268 molecules with = 8 carbon atoms, the standard deviation from the predicted value is even smaller, 2.1%. This enables an instant estimate of the heat of combustion simply from the elemental composition of the fuel, even for a complex mixture or unknown molecular structure, and explains principles of biofuels production. The analysis indicates that O[subscript 2], rather than fuels like octane, H[subscript 2], ethanol, or glucose, is the crucial "energy-rich" molecule; we briefly explain why O[subscript 2] is abundant in air despite its high enthalpy.
Division of Chemical Education, Inc and ACS Publications Division of the American Chemical Society. 1155 Sixteenth Street NW, Washington, DC 20036. Tel: 800-227-5558; Tel: 202-872-4600; e-mail: eic@jce.acs.org; Web site: http://pubs.acs.org/jchemeduc
Publication Type: Journal Articles; Reports - Research
Education Level: Higher Education; Postsecondary Education
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A
Grant or Contract Numbers: N/A