Why Fire Is Hot

A recent paper by K. Schmidt-Rohr (Chemistry) answered the question why combustion reactions are always exothermic.  Every scientist should be able to explain what makes fire hot, but neither chemistry nor combustion textbooks have provided a valid answer. Schmidt-Rohr’s analysis shows that the reason lies in the double bond in O2, which is much weaker than other double bonds or pairs of single bonds in the biosphere, so that the formation of the stronger bonds in CO2 and H2O results in the release of heat. The bond energies in the fuel play only a minor role; e.g., the total bond energy of CH4 is nearly the same as that of CO2. A systematic analysis of bond energies gives the heat of combustion close to –418 kJ (i.e., –100 kcal) per mol O2, in good agreement (±3%) with data for >500 organic compounds; the heat of condensation of H2O (–44 kJ per mol H2O(l)) is also included in the analysis. For 268 molecules with ≥8 carbon atoms, the standard deviation is even smaller, 2.1%. For a fuel of composition CcHhOoNn, this gives DcH ≈ -418 kJ (c + 0.3 h – 0.5 o), which enables instant estimates of heats of combustion simply from the elemental composition, even for complex mixtures of unknown molecular composition, and explains principles of biofuels production. The analysis indicates that O2, rather than fuels like octane, H2, ethanol, or glucose, is the crucial “energy-rich” molecule. It also challenges common notions about a relation between the oxidation state and the energy content of biomolecules.


One then needs to explain why O2 is abundant in air despite its high enthalpy: All the O2 in the earth’s atmosphere has been produced by photosynthesis in cyanobacteria, algae, and higher plants, as a by-product of photosynthetic proton and electron production from H2O. The “price” of the production of O2, which is energetically so unfavorable, is paid by plants and algae (with “cheap” energy from the sun) in order to be able to live wherever H2O is present. So one can conclude that atmospheric O2 stores solar energy that sustains us with every breath we take.

Schmidt-Rohr K. Why Combustions Are Always Exothermic, Yielding About 418 kJ per Mole of O-2. J Chem Educ. 2015;92(12):2094-9.

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