HEAT OF VAPORISATION OF NITROGEN PENTOXIDE. 183 



This value is very high and equal to about six times the heat 

 of solidification of water (+ 072 for H 2 = 9 grms., according 

 to Desains). 



Therefore 



N 2 6 liq. + H 2 liq. = 2HNO 3 liq. and pure liberates + 5-3 Cal. 

 N 2 6 liq. + H 2 liq. + water = dilute acid -f 12'48 Cal. 



The first value is nearly but not quite equal to that of the 

 heat of hydration of acetic anhydride (C 4 H 6 3 liquid + H 2 

 liquid = C 4 H 8 4 liquid liberates + 6'9), but the second is much 

 greater than the heat of hydration of anhydrous acetic acid 

 referred to the dilute acid ( -f 7'3). Hence the action of the 

 liquid nitrogen pentoxide on water is extremely violent in con- 

 trast with the very much weaker reaction which water exercises 

 on the solid acid. 



Gaseous state. Heat of vaporisation. N 2 5 gas, changed into 

 liquid, liberates + 242, and into solid, -f 6'56. 



This quantity was determined by introducing dry air charged 

 with nitrogen pentoxide vapour into the water of the calori- 

 meter, at a temperature of 43. The preliminary vaporisation 

 of the pentoxide in the current of air was produced by means 

 of a small air bath. 



The decomposition of the pentoxide into the tetroxide and 

 oxygen is not appreciable under these conditions of vaporisation. 



Known weights of pentoxide, previously weighed in a sealed 

 tube, have been operated on, the result being checked by the 

 acidimetric test of the aqueous solution. 



By this means is obtained the heat liberated when nitrogen 

 pentoxide is changed into dilute acid, viz. 



N 2 5 gas + water = dilute acid, at -f- 10 liberates + 14-9. 



The heat of vaporisation of the liquid acid is therefore for 

 the weight 



N 2 5 = 54 grms. 

 14-9 -12-48 = 2-42, 



or, for (N 2 5 =108), 4'84 Cal. 



That of the solid, for (N 2 5 = 54 grms.) 



14-90 -8-34 = 6-56, 

 or, for (N 2 5 = 108 grms.), 13'12. 



According to the above figures, the heat of vaporisation of the 

 liquid nitrogen pentoxide (admitting N 2 3 = 2 vols.) will be for 

 N 2 5 , 4-84. It is nearly the same as that of nitric peroxide 

 at the same volume, or 4*3 for NO 2 . It is also nearly the same 

 as the heat of vaporisation of nitrogen monoxide, viz. 4'42 for 

 N 2 0, according to Favre. 



The thermal formation of nitrogen pentoxide from the elements 



