ENERGIES OF COMBUSTION, DISMUTATION, HYDRATION 



215 



Table 9.II 

 Bond Energies in kcal/mole at 291° K." 



» Calculated with Vc = 16S, D (H2) = 103, D (O2) = 118 and D (N2) = 170 kcal/mole, from heats 

 of formation given by Bichowsky and Rossini (1936). 



'' The energy of dissociation of OH into O and H is 117 kcal; the same value is obtained for the OH 

 bonds in H2O2, if one assumes O — O = 22 kcal (c/. footnote d). 



' If one assumes C — C = 86 kcal (c/. footnote g), one obtains 96 kcal for the C — H bond in ethane. 



"* The actual energy of dissociation of H202 into two OH radicals in 22 kcal. The value in the table 

 is obtained by assigning the standard value (110 kcal) to the O — H bonds in hydrogen peroxide. 



• Stabilized by resonance, mainly between — C:^ ^H and 



A. 



^H. 



^O^ ^"O 



/ Resonance mainly between 0=C=0, 0+=C0- and O"— C=0+. 



» The actual energy of dissociation of CzHs into 2 CH3 is 86 kcal; the value in the table is based on 

 C— H = 98 kcal. 



* One-third of the energy of the ring system in benzene, calculated with C — H = 98 kcal. 



Footnotes b, c, d, and g illustrate the discrepancies between the "standard" bond 

 energies and the actual energies of dissociation. There is, however, no way of improving 

 table 9.II, without assigning different values to the same bond in different compounds. 

 For example, if the energy of dissociation of H2O2 into two OH radicals (22 kcal) were 

 adopted as the strength of the O — O bond, one would obtain 117 kcal for the strength 

 of the average O — H bond in hydrogen peroxide, while the strength of the same H bond 

 in water is only 110 kcal. Similarly, if one would adopt the dissociation energy of 

 C2H6 into two OH3 radicals (86 kcal) for the strength of the — bond, this would 

 give 96 kcal for the average strength of a O — H bond in ethane, as against 98 kcal in 

 methane. MultipUcity of bond values would defeat the purpose of a standard bond 

 table, which is to provide a means for rapid — even if approximate — evaluation of the 

 energy content of dififerent molecules. 



O — O bond, which is among the weakest ones. The strength of O — H 

 bonds and the weakness of — bonds are the two reasons why photo- 

 synthesis is strongly endothermal: in the synthesis of a — CHOH group 

 and of an oxygen molecule from CO2 and HoO, one — H bond disappears 

 and one 0=0 double bond is created. 



According to table 9. II, the oxidation of a C — H bond by molecular 

 oxygen to carbon dioxide and water should liberate 48 kcal; and that of 



