HEAT OF FOKMATION OF NITROBENZENE. 271 



a special series of experiments was made for the purpose of 

 measuring the heat disengaged by the same acid, when treated 

 with certain proportions of water, which are increased from a 

 limit below that produced in the experiments given to one a 

 little above it. These experiments were carried out at the 

 same temperature, under the same conditions, and on the same 

 day each trial being twice repeated. In this way were 

 obtained two pairs of results, from which it was easy to trace 

 the curve representing the heats of hydration of the acid, be- 

 tween limits which comprised the hydration in the preparation 

 of the nitrobenzene. The heat, q, corresponding to the propor- 

 tion of water formed at the same time as the nitrobenzene may 

 thus be calculated. 



2. Lastly, there was selected from amongst these mixtures the 

 one that agreed best with the final data of the experiment re- 

 lating to the formation of the nitrobenzene. In it was dissolved 

 pure nitrobenzene, in the same relative proportions ; the heat of 

 solution was very small. It was brought, by calculation, to the 

 data of the experiment relating to the formation of nitrobenzene, 

 which gave a value, q lt for the weight, C 6 H 5 N0 2 . In short, the 

 number, 



Q - 2 - <?i> 

 represents the heat disengaged in the following reaction : 



C 6 H 6 + (HN0 3 + -335H 2 0) 

 = C 9 H 5 N0 2 + H 2 + -335H 2 0. 



The numbers found in the two experiments were, + 35 and 

 4- 35*2, average 35*10. In order to make this number apply to 

 true monohydrated acid, HN0 3 , we must add to it the heat 

 given off in the reaction of '335H 2 upon this last acid ; or 

 -h 1*5, according to the author's experiments. 1 



3. We get then, finally 



C 6 H 6 (pure) + HN0 3 (pure) + H 2 disengages + 36'6 Cal. 



4. It is easy to deduce from this the heat of formation of 

 nitrobenzene from its elements 



C 6 (diamond) + H 5 + N + 2 disengages + 4'2. 

 In short, it was found that 



Benzene, C 6 (diamond) + H 6 = C 6 H 6 (liquid) ... +5-0 

 Nitric acid, H + N + 3 = HN0 3 (liquid) ... + 41-6 



Reaction ... ... ... ... ... -|- 36-6 



Sum + 73-2 



1 " Annales de Chimie et de Physique," 5 e seVie, torn. iv. p. 448. 



