ir. G. Mixter — Thermochemistry of Silicon. 129 



Sn + 20 + Aq = 133 Cal. 



2Sb + 30 + Aq = 167 Cal. 

 Sn + 20 = S11O (crys.) = 137 



2Sb + 30 = 163 



2Bi + 30 + Aq = 137-7 



2Sb + 50 + 3H.0 = 228-8 

 2Bi + 30 = 137-5 



2Sb + 50 = 229-6 



The heat of solution of 1 g. of silica in hydrofluoric acid 

 found in experiments E, F and G, Table II, is 588 cal. or 

 35-5 Cal. for 1 gram molecule. (Si = 28-4.) Guntz* obtained 

 33 - 6 Cal. (Si = 28.) Thomsenf found in the reaction of 

 hydrofluoric acid on a solution of silicic acid that the heat 

 effect rises regularly until eight molecules of acid are added 

 and ceases with the" tenth. With 6HF it is 32-3 cal. In the 

 writer's experiments 12 to 17 HF were used to lSiO„. It 

 may be stated that anhydrous silica dissolves too slowly in 

 hydrofluoric acid for' a calorimetric test. 



The heat of formation of silicic acid is most important, as 

 other values may be obtained from it. BerthelotJ derived 

 from the heat of the reaction of SiCl 4 with water 



Si (crys.) + 2 + Aq = SiO a Aq + 179-6 Cal. 



The only uncertain value used is that for the heat of formation 

 of SiCl 4 . Troost and Hautefeuille§ burned impure amorphous 

 silicon in chlorine and gave as the result 157'6. B corrected 

 for an error in the water equivalent given by T and H for the 



mercury calorimeter used, and for Si (amor.) > Si (crys.) and 



states that 



Si (cryst.) + 2Cl a = SiCl 4 (liq.) + about 128-7 Cal. 



The heat effect of SiCl, (liq.) + Aq is, according to Berth el ot, 

 69-0 cal. : to Thomsen, 69-3 Cal. 



The heat of oxidation of silicon has been determined by two 

 methods with fairly concordant results. The writer | burned 

 crystalline silicon, 99 - 95 per cent pure, carbon and silicon car- 

 bide in sodium peroxide, and the last two in oxygen, and from 

 the results derived 



Si (crys.) + 2 = SiO (polymerized) + 191-0 Cal. 



von WartenburgT burned amorphous silicon, containing 2*5 

 per cent of Si0 2 , in oxygen and from nine experiments obtained 

 a mean of 194-9 Cal. ± 4-1 Cal. He also found by dissolving 



* Ann. Chim. Phys. (6). iii, 60. 



\ Ber. deutsch. Gesellsch., iii, 575. 



i Thermocheinie, II, 123 and 151. 



S Ann. Chim. Pbvs. (5), ix. 77. 



|| This Journal, xxiv, 120, 1907. 



IT Nernst Festschrift, 459 ; Chem. Zentralblatt, 1912, II, 1095. 



