OXIDES AND OXYGEN ACIDS OF SULPHUR 261 



SULPHUR TRIOXIDE AND SULPHURIC ACID 



Sulphur Trioxide. Sulphur dioxide and oxygen, when 

 heated together to 400, unite very slowly with evolution of heat 

 to give sulphur trioxide: 



2S0 2 H- O 2 -> 2SO 3 + 45,200 calories. 



This reaction, however, cannot be utilized for the manufacture 

 of sulphur trioxide except under special conditions, for at 400 

 the union takes place far too slowly for use in industrial work, 

 while at higher temperatures the reverse action becomes appre- 

 ciable and poor yields are obtained. If we apply van't Hoff's 

 law to the reversible reaction : 2S02 -f- 02 ^ 2SOs we see that, 

 since the forward change is exothermic, raising the temperature 

 will favor the backward change. In actual practice it is found 

 that at 400, 98-99 per cent of the materials unite; at 700, only 

 60 per cent; at 900, practically none. 



Sulphur trioxide is a white solid which exists in two allotropic 

 crystalline forms. One melts at 15, and is therefore fluid at 

 ordinary temperatures. The other vaporizes without melting 

 at 50. Both forms react vigorously with water, causing a hissing 

 noise due to the steam produced by the heat of the union: 



Sulphur trioxide combines also with sulphuric acid to give oleum, 

 or fuming sulphuric acid, H 2 S 2 7 : 



H 2 S0 4 -H 2 S20 7 . 



ft Ari^. The interac- 



tion of sulphur dioxide and oxygen is hastened by many substances, 

 such as glass, porcelain, ferric oxide and, more especially, finely- 

 divided platinum, which remain themselves unchanged and 

 simply act as contact or catalytic agents. The contact process, 

 as this is called, is now very extensively used in the manufacture 

 of sulphuric acid. 



