216 



REDUCTION OF CARBON DIOXIDE 



CHAP. 9 



a C — C bond, 53 kcal. In a first approximation, we may thus expect 100 

 kcal to be released by the complete combustion of carbon — oxygen — 

 hydrogen compounds (excepting the peroxides) for each oxygen molecule 

 consumed in this process. 



The ratio of the number of oxygen molecules consumed in the com- 

 bustion of an organic molecule to the number of carbon atoms in it was 

 designated on page 109 as the "reduction level," and the following 

 equation was given for its calculation: 



2 nc + 0.5 riH — no 



(9.1) 



L = 



2nc 



nc, nn, and no being the numbers of carbon, hydrogen, and oxygen 

 atoms, respectively. We now see that the heat of combustion of an 

 organic compound is determined, in the first approximation, by its 

 reduction level. If the heats of combustion of a number of organic com- 

 pounds, reduced to one gram atom of carbon, are plotted against L, the 

 points fall near a straight hne, with a slope of about 110 kcal (c/. Fig. 23), 



1.0 L 1-5 



Fig. 23. — Heats of combustion of carbon-hydro- 

 gen-oxygen compounds per gram atom of carbon as 

 a function of reduction level L. 



The difference between this value and the expected slope of 100 kcal is caused by 

 the fact that, in the second approximation, C — O bonds contribute up to 20 kcal to the 

 heat of combustion, because of the extent to which they are stabiUzed in carbon dioxide 

 (the average strength of a C — O bond in CO2 is 95 kcal, as against only 77 kcal in alcohols, 

 82 kcal in aldehydes, and 90 kcal in carboxylic acids). 



