COMMUNICATIONS TO THE EDITOR
Feb., 1962
381
+
dentally err in the same sense and amount, Chipman's observation implies either that the standard heat of formation of quartz at 25' is about 5 kcal. -+ mole-' more negative than the currently accepted value4 of -209.9 f 1.0 kcal. mole-' or that the ac?65HP~3376Ha0(liq.)= 71SHF~3204H~O(Sq.) + cepted entropies of crystalline silicon or quartz a t AH = 0.5 k 0.2 (IV) 47HF.l72€IzO(liq.) 25" are in error by absurdly large amounts. In a Si(c) + Oz( g) = SiOn(c, quartz) paper which apparently has gone unnoticed in this AH = -217.5 i 0.5 (V) country, a Russian thermochemist, Golutvin,6 has The heats of reaction are expressed in kilocalories presented evidence that Humphrey and King4 a t 25". Equation I gives the result of the measure- erred in their analytical determination of the ments of this laboratory. Equation I1 gives the quantity of unburned silicon remaining after oxyresult of the heat of solution measurements of gen-bomb combustions, and that the true value of King,6 extrapolated to 25" and corrected for the the standard heat of formation of silica lies between change in the atomic weight of silicon from 28.06 to -215 and -219 kcal. mole-l. To resolve this 28.09. Equation I11 represents a dilution reaction problem we sought the simplest combination of studied in this Laboratory by a somewhat crude reactions that would yield an unequivocal value method already described6 The heat of eq. I11 for the heat of formation of silica. The method we was found to be very small and probably is ther- chose is based on fluorine-bomb calorimetrya; it mally insignificant, but this result should be veri- consists of combining the thermochemical equations fied by more sensitive dilution calorimetry. The Si(4 2Fdg) = SiF4g); m l (1) heat of dilution for eq. IV was computed from values in Circular 500.' Addition of eq. I, 11, I11 and and IV results in eq. V, the equation for the formaSiOz(cu,c) -I- 2Fz(d - SiFdg) 02; AHz (2) tion of quartz from the elements and the heat of formation. t o give The previously accepted value for the heat of Si(c) + Oz(g) = SiOz(or,c); formation of auartz3is -209.9 1.0 kcal., about 8 AHf = AH1 - AH2 (3) kcal. less negative than the present value. Thus, the free energy of formation of silica at 25" is For the measurement of AH1 and AHz weighed about, 8 kca.1. mole-1 more negative than the earlier amounts of crushed crystals of the high purity matevalue, a in agreement with Chipman's conclusion.2 rials, silicon or alpha quartz, were mixed with a Full details of this investligationwill be given in a small known amount of powdered silicon and placed paper in preparation. The results presented here on a nickel support dish. On exposure to four are confirmed by those in the accompanying letters, atmospheres of fluorine in a two-chamber nickel which were determined by entirely different ther- bomb and tank system (described elsewhere'), the mochemical methods. powdered silicon ignited spontaneously and fired the entire sample. Because the products of comCONTRIBUTION NO. 112 FROM THE bustion were solely the gases indicated in (1) and THERMODYNAMICS LABORATORY BARTLESVILLE PETROLEUM RESEARCH CENTERW. D. GOOD (a), the extent of the reactions was determined by BUREAU OF MINES,U. S.DEPARTMENT OB weighing the residues. AH1 and AHz, at 25", INTERIOR, BARTLESVILLE, OKLAHOMA were found to be -386.02 i 0.248 and -168.27 RECEIVED DECEMBER 2, 1961 k 0.24 kcal. mole-l, respectively. Hence, €or a-quartz AHf'298.15 = -217.75 k 0.34 kcal. mole-'. (6) W. D. Good, D. W. Scott, J. L. Lacina and J. P. MoCullough, The agreement between this determination and J . Phya. Chem., 6 8 , 1139 (1969). (7) F. D. Rowaini, D. D. Wagman, W. H. Evans, 8. bcvine and I. that of Goodg (preceding communication) is exJaffe, "Selected Values of Chemical Thermodynarnio Properties," cellent; the conclusions of Chipman3and Golutvin6 Natl. Bur. Standards Circular 500 (1952). are substantiated. Full details of this investigation and of its extension to other forms of silica will be presented THE HEAT OF FORMATION OF SZLICA AND later. SILICON TETRAFLUORIDE'P' UNIVERSITY OF WISCONSIN S. S.WISE
HzSiF~.759HB1*3378HzO(liq.) = SiOe(c, quartz)
32.3 f 0.1 (11) 718HFm3204HpO(liq.) = HzSiFa.759HF.3378H20(liq.) AH = 0.0 t 0.3 (111)
765HF-3376H*O(liq.) HgSiFn.41HFs174H20(1;s.)
AH
=
+
+
Sir:
Chipmana has presented evidence, based on a number of observed equilibria, that the standard free energy of formation of silica is about 5 kcal. molee1 more negative than the currently accepted value. Unless all the observed equilibria coinci(1) This work wa8 performed under the auspices of the U. S. Atomic Energy Commission. (2) Abstracted from a thesis submitted by S. S. Wise to the faculty of the University of Wisconsin in partial fulfillment of the requirements for the Ph.D. degree. (3) J. Chipman, J . A m . Chena. Soe., 88, 1762 (1981). (4) J. P. Coughlin, U. S. Bur. Mines Bull. 642 (1954),based on the work of C. L. Humphrey and E. G. King, J. Am. Chern. SOC.,74,2041 (1952).
J. L. MARQRAVE MADISON, WISCONSIN ARGONNE NATIONAL LABORATORY H. M. FEDER ARCONNE, ILLINOIS W. N. HUBBARD RECEIVED DECEMBER 2, 1961
(5) Yu. M.Golutvin, Zhur. Fka. R h i n . , 80, 2251 (1958). (6) Previous papers dealing with thia subject are (a) E. Greenberg, J. L. Settle, H. M. Feder and W. N. Hubbard, J . Phys. Chena., 66, 1168 (1961): (b) J. L. Settle, IH. M. Feder and W. N. Hubbard, ibid., 66, 1337 (1961); (c) S. 8. Wise, J. L. Margrave, H. M. Feder and W. N. Hubbard, ibid., 66, 2157 (1961). (7) R. L. Nuttall, S. S. Wise and W. N. Hubbard, Reu. Sci. Inatr., in press. (8) This value for the heat of formation of SiFd(g) differs considerably from literature values; the latter, however, either are baaed on AH/ (SiOS or depend on unreliable experimental methods. (9) W. D. Good, J , Phya. Cbem., 66, 380 (1962).
