312 



SMITH S INTERMEDIATE CHEMISTRY 



The most interesting property of nitrogen tetroxide is its action 

 upon water, whereby nitric acid is formed, and nitric oxide escapes : 



3N0 2 + H 2 -> 2HN0 3 + NO. 



When oxygen is present also, then the NO gives more N0 2 , and 

 this in turn gives more nitric acid. This action plays an important 

 part in the making of nitric acid from the nitrogen of the air (see 

 next section). 



vThe gas is sometimes used for bleaching flour, but traces of the 



**'% 

 oxide remain in the bread. ) 



Fixation of Atmospheric Nitrogen. Oxygen and nitrogen 

 have no natural tendency to combine at the ordinary temperature, 

 but rather the reverse the compounds tend to decompose with 



evolution of heat. But a high tem- 

 perature will supply the necessary 

 energy. Even so, however, the 

 union extends to only 1 per cent of 

 the mixture at 2000 and 5 per cent 

 at 3000: 



N 2 



2 < 2NO. 



FIG. 83 



Note that, since the formation of 

 NO is an endothermic reaction, the 

 yield of NO is increased by raising 

 the temperature (van't Hoff's law, 

 p. 242). In spite of the poor yield 

 obtainable under the best condi- 

 tions, the supply of natural nitrates is so limited that machinery 

 has been devised, and is now in successful use, for carrying on 

 the combination on a commercial scale. Three devices are in use, 

 and all employ hydro-electric power. 



In the Birkeland-Eyde process (Fig. 83), used at Notodden and 

 elsewhere in Norway, an arc discharge between rods of carbon is 

 spread, by the influence of powerful electromagnets, into a circular 







AsJ*k/\r 



f 



