THERMAL VALUE OF CARBON LINKAGES 101 



hydrogen causes an evolution of 17-2 calories. In the same 

 way for the whole series we have 



CH 4 C 2 H 6 C 3 H 8 C^j C 5 H 12 



17-2 20-4 23*3 26-0 28-8 



14-3 + 2-9 n = 17-2 20-1 23-0 25-9 28-8. 



We can now give a purely empirical expression to the 

 additive character by deducing from these numbers a value 

 for carbon and one for hydrogen. Since the mean dif- 

 ference is 



CH 2 = 2-9, 

 we have 



H 2 = CH 4 -CH 2 = 17-2-2-9 = 14-3 



C = CH 2 -H 2 = 2-9 - 14-3 = - 1 1-4, 

 so that 



C w H 2w+2 = -i 1-471 + (71+1)14-3 = 14-3 + 2-971. 



In this expression, however, although it reproduces the 

 above data, a very important point is overlooked, since 

 a priori the heat of formation cannot be expected to depend 

 simply on the components but rather on their mode of 

 combination, as is indicated by any heat of reaction, since 

 according to our notions only the mutual linkages are 

 changed. 



Taking this into account, the formation of the above 

 hydrocarbons is to be regarded as a combination of carbon 

 with carbon (C C) and carbon with hydrogen (C H), and 

 assuming a constant value for the latter we get from the 

 heat of formation of methane 



4(0 H) = 17-2, so that (C H) = 4-3. 

 Hence for pentane 



4 (C C) + 12 (C H) = 28-8 so that (C C)= -5-7, 

 whence for C w H 2w+2 : 



(n-i)(C C) + (271 + 2) (C H)= -5.7(71-1) 



+ 4-3 (271 + 2) = 14-3 + 2-971. 



Although the previous expression and the experiments 

 agree with this, it needs modification in case of combina- 



