W. G. Mixter — Thermochemistry of Silicon. 125 



Art. XIY. — The Thermochemistry of Silicon; Heat of 

 Combination of Silica with Water; by W. G. Mixter. 



[Contributions from the Sheffield Chemical Laboratory of Yale University. J 



This article contains experimental work by the writer and a 

 review of the thermochemistry of silicon, which has been 

 regarded as veiw uncertain. Some of it is undoubtedly good 

 and some of the fundamental values from which others are 

 derived are far from accurate. The writer's results are given 

 first as they are used in discussing the work of others. 



There is nothing in the literature bearing on the heat effect 

 of SiO, + Aq except the heat of formation of Si0 2 .Aq, 179 - 6 

 Cal. and of SiO„, 19 1 Cal. These indicate that it is endothermic. 

 The problem of finding the heat of union of silica with water 

 is complicated since they do not combine directly and because 

 molecules of silica are complex at temperatures required to 

 dehydrate silicic acid. It is impossible to determine with an 

 approximation to accuracy the quantity of heat required to 

 separate a small quantity of water firmly held, but it appears 

 practicable with silicic acid containing much water. Two 

 methods have been tried, one by fusion with sodinm peroxide 

 with unsatisfactory results and the other by solution in hydro- 

 fluoric acid. 



In order to learn whether or not silicic acid prepared at 

 room temperature differs from that made at 100°, two prepara- 

 tions were made as follows : A solution of sodium silicate was 

 added gradually to hydrochloric acid and the silicic acid which 

 separated after a time was washed with water at room tempera- 

 ture. It was dried in the air, then by a current of dry air 

 under diminished pressure passing through the powder, and in 

 vacuo over oil of vitrol until the water content was reduced to 

 6"9 per cent.* Another preparation was made by adding a 

 solution of sodium silicate to an excess of hydrochloric acid and 

 evaporating on a steam bath. The dry residue was moistened 

 with acid and the silicic acid was washed with hot water. It 

 was then left some time in a steam drying oven and then over 

 oil of vitriol. It contained 8*0 per cent of water. The results 

 on p. 126 show that the silica of silicic acid prepared at 100° 

 js in the same molecular condition as that made at room 

 temperature. Hence other lots of silicic acid were made by 

 the second method, as it is somewhat simpler. The water con- 

 tent was found by the common method, heating finally over 

 a large blast lamp to constant weight. The determinations of 



* Since gelatinous silicic acid dries to hard tough lumps it should be pul- 

 verized from time to time during the drying process in order to obtain a 

 uniform product. 



