I •_'•'> II'. <•'. Mixter — Thermochemistry of Silicon. 



water were made either before and after the calorimetric tests 

 of a preparation or with a portion weighed at the time of a test. 

 >inee silicic acid witli small water content gains weight rapidly 

 in the air while that having much water ina\ have lost some 

 combined water in a closed vessel if the room is hot. For 

 example, preparation A (Table II) contained 0-3 per cent of 

 combined water and 0'2 per cent absorbed during the neces- 

 sary exposure to the air. The term " silicic acid " is applied 

 to all preparation- having combined water. 



The following are the results obtained with fusions of mix- 

 tures of silica or silicic acid, sodium peroxide and lampblack : 



Table I. ' 



Amorphous silica which was heated to a constant weight over 

 a blast lamp: 121S, 1217, 1226 : mean 1220 cal. 



Silicic acid having 



6-9 per cent of H 2 1316, 1307, 1301: mean 1308 cal. 



8-0 " " 1302, 1341 " 1322 " 



12-5 " " 1386, 1390 " 13S8 " 



21-5 " " 1230, 1402, 1310, 1204 " 1236 " 



The reason for the wide variation in the last results is this 

 The silicic acid in the dry air of the mixture gives off water 

 which reacts with the sodium peroxide. A thermometer 

 placed in the last mixture before closing the bomb showed a 

 slight rise in temperature. The silicic acids with 6*9 and 8 # 

 per cent of water have vapor pressures too low to affect the 

 determinations. Any of the results may be low owing to a 

 little silica left unchanged in a fusion winch can not be deter- 

 mined since it would dissolve in the water solution of the 

 fusion. 



From the results with the silicic acids containing 6 - 9 and 

 8 - per cent of water we have the equations 



0-931 x + 0-069 y = 1308 

 0-92a; + 0-08y = 1322 



in which x = 1250 and y = 2100 cal., in which x equals the 

 heat effect for 1 gm. SiO, and y is the heat effect for 1 gm. of 

 water. The value for y is 200 cal. higher than the heat of the 

 reaction of solid water, ice, with sodium oxide. These derived 

 values are only approximations. They indicate, however, that 

 little energy is required to separate water from silica. For the 

 heat of polymerization of silica we have 1250-1220 X 60'4 

 = 1800 cal. or about 2-0 Cal. This small value accords with 

 the fact that silica does not glow when heated. 





