214 REDUCTION OF CARBON DIOXIDE CHAP. 9 



Table 9.1 

 Energy of Dissociation of RH into R + H at Room Temperature 



The energy of the last carbon-hydrogen bond in saturated hydro- 

 carbons has been derived from experimental results (Table 9.1) and is 

 close to 100 kcal. The energies of the other C — H bonds are not known 

 separately, but their average strength can be calculated from the heat 

 of formation of methane — if an assumption is made concerning the heat 

 of sublimation of carbon, Fc. From spectroscopic considerations, the 

 two values, Fc = 123 and 168 kcal per mole, have been derived as possible 

 alternatives. Pauling (1940) thought the smaller value more plausible, 

 while Baughan (1941) and Kynch and Penney (1941) argued in favor of 

 the larger one. In Pauling's system, the average strength of the four 

 C — H bonds in methane is 87 kcal, and thus considerably smaller than 

 the energy of dissociation of methane into CH3 and H; with Fc = 168 

 kcal, it becomes equal to 98 kcal, and thus practically identical with the 

 methyl — hydrogen bond energy. However, even if the larger value is 

 correct (Herzberg, in 1942, again advocated the alternative Fc = 123 

 kcal, and C — H = 87 kcal) this does not mean that all four stages in the 

 dissociation of CH4 require the same energy; rather, the equality of the 

 strength of the last bond and of the average of all four of them must 

 be considered as coincidental. 



In the estimation of the heats of reactions, any consistent set of bond 

 energies can be used, since the value of Fc cancels out in the calculation. 



The greatest deviations from additivity are caused by double bonds, 

 which may be either weaker or stronger than two single bonds, and whose 

 strength is affected by conjugation and other resonance phenomena. 



Table 9. II contains bond energies, based on Fc = 168 kcal, which may 

 be useful in the discussion of the chemical mechanism of photosynthesis. 



2. Reduction Level and Energies of Combustion, Dismutation, 



and Hydration 



Table 9. II shows that all bonds directl}^ involved in photosynthesis 

 have strengths between 75 and 100 kcal, with the exception of the 

 O — H bond, which is the strongest single bond in existence, and the 



