L80 II'. <>■ Miaier — Thermochemistry of Silicon. 



the two forms of silicon in a mixture of hydrofluoric! and nitric 

 or chromic acids in a calorimeter that Si (amor.) — >Si(cryst.) 

 = less than ± 2 Cal. The writer* burned a mixture of crys- 

 talline silicon and carbon in oxygen in a bomb. As the 

 results varied widely they were disregarded at the time. Seven 

 experiments in all were made with an average of 187*2 

 Cal. But the average of live, leaving out the two lowest 

 results, is 191*8 Cal. for 28*4 g. of crystalline silicon. When 

 the products of a combustion were treated with hydrofluoric 

 acid to dissolve the silica a little gas came off, presumably 

 hydrogen, indicating the presence of SiO or a soluble form of 

 silicon. In either case this would make the result a little high 

 for Si + 20. It is impossible to say which is the most accu- 

 rate of the values found, but it seems best to use 191 cal. in 

 deriving other values from results obtained by the writer. 



It has beeu shown that »SiO, (unpotymerized) = (Si0 2 ),T 

 (polymerized) + about 2 Cal. Then 191- 2 = 189 Cal. for 

 the heat of formation of silicon dioxide as it exists in silicic 

 acid. And it has also been shown that silica combines with 

 water with very small or no heat effect. Hence the conclusion 

 that 189 Cal. is the best value at present for the heat of 

 formation of silicic acid from crystalline silicon, oxygen and 

 water. 



If we accept the above value for the heat of formation of 

 silicic acid, 35*5 for SiO„ + 6IIF,Aq, Berthelot and Moissan's 

 for H + F + Aq = 50*3, then Si0 2 ,Aq + 6HFAq = 35*5 

 = 2H + Si + 6F + Aq + 2(2H + O) = (Si + 20 + Aq) 



x 139 189 



- 6 (H 4- F Aq) 



301*8 

 in which x= 387*1 Cal. the heat of formation of hydrofluosilic 

 acid in water from crystalline silicon. From the heat effects, 

 of a number of reactions, using Berthelot's value 179*6 for 

 silicic acid, Guntzf derived in a different way 374*3 (Si = 28° 

 and 380 Cal. (Si = 28*4). Berthelot:}: gives 374*4 Cal. and 

 gives with the reactions measured and used in the calculation 

 Truchot's for SiF 4 + 2HF = 34 Cal. But Truchot's num- 

 ber given in the Physikalisch Chemische Tabellen for hydro- 

 fluosilicic acid is 375*1 Cal. G-untz's value given in the 

 Tabellen for Si (cry's.) + 4F = 239*8 Cal. the same as Ber- 

 thelot gives. This is obviously 



SiF < =(2lI + Si + 6F + Aq) 



374-4 -2(H+F + Aq)-(SiF + 2HF, Aq) = 239-8 

 100-6 34 



* Loc. cit. t Loe. cit. 



X Theimochemie II, 152. 



