AMMONIA 301 



elements as the temperature is raised, makes it impossible to 

 obtain high yields of ammonia by the Haber process at high 

 temperatures, while at lower temperatures the combination is too 

 tardy, even in the presence of a catalyst. Fortunately we are 

 able to make use of another of our general laws, the principle 

 of Le Chatelier (p. 244), to improve matters. It will be noted 

 from the equation given above that_ihe union of hydrogen and 

 nitrogen to form ammonia is accompanied with diminution of 

 .volume, 1 volume of nitrogen + 3 volumes of hydrogen = 2 vol- 

 umes of ammonia. Consequently the forward action will be 

 fovored-hujincrease of pressure. In fact, under 200 atmospheres 

 pressure the yield of ammonia in the equilibrium mixture is as 

 follows: at 200, 86 per cent; at 500, 17.6 per cent; at 600, 

 8.2 per cent; at 1000, 0.9 per cent. 



There still remains the question of spegsLof combination. This 

 decreases very rapidly as the temperature is lowered (compare 

 p. 241), and no catalyst has yet been prepared which is sufficiently 

 active to make the combination of nitrogen and hydrogen speedy 

 enough to permit the process to be operated on an industrial 

 scale much below 600. A yield of about 8 per cent ammonia, 

 therefore, is the best that can be obtained. 



/Details of the Haber Process. The hydrogen may be 

 obtained either as a by-product in an electrolytic process (p. 166), 

 or by the action of steam on iron (p. 51), or by careful purifica- 

 tion of water-gas (see p. 338). The preparation of pure hydrogen, 

 it may be noted, is the most costly feature of the whole process. 

 The nitrogen is obtained from liquid air. After removal of all 

 impurities injurious to the catalyst, the mixed gases -re pflifif* p ^ 

 under high pressure into the vessel containing the catalyst. This 

 consists of a steel " bomb " specially adapted to withstand the 

 enormous pressure. Very serious disasters have taken place 

 owing to the explosion of such bombs. After passing over the 

 catalyst, the reaction mixture is cooled and its ammonia content 



