COMMUNICATIONS TO THE EDITOR
380
Vol. 66
THE STABILITY OF SILICA
listed in most of the recent thermodynamic compilations could not be employed because it was derived Sir: using the hcat of formation of silica that is in quesChipman' recently reviewed five lines of evidence tion. that indicate that the presently acceptcd heat of The heats for rcactions V, VI, and VI1 were formation of silica (-209.9 kcal./molc)2 calciilstcd tnkcn in each case from the most recent thcrmodyfrom the heat of combustion of silicon in an oxygen namic compilations in which they appeared. bomb may bc as much as 5 kcal./mole too positive. The derived value of -215.8 kcal./mole for Still more recently, Good3 measured tJhc heat of AHf0298(c,quartz) indicates that quartz is about 6 formation of aqueous fluosilicic acid and calculated kcal. more stable I han previously thought, in good a value of -217.5 0.5 kcal./mole for AHfoZmagreement with the values given by Chipman and (c, quartz) which is 7.6 kcal./molc more negative Good. than the litcmture value. Somc recent effusion COMPANY OF AMERICA measurements that the authors have made of the ALUMINUM ALCOA RESEARCH LABORATORIES C. N. COCHRAN reactions between gallium and quartz and gallium PHYSICAL CHEMISTRY DIVISION L. M. FOSTER PENNSYLVANIA and magnesium oxide in an alumina cell4provide an NEWKENSINGTON, RECEIVED NOVEMBER 22, 1061 independent basis for calculating this value. The heats listed for reactions I and 11 in the table were obtained by third law treatment of the T H E HEAT O F FORMATION OF SILICA' effusion results. These two heats are cornbincd with other thcrmochcmical data in the table to Sir: The free energy of formation of silica is a subject drrive the heat of formation for crystalline quartz. (The heat for reaction I has been decreased by 0.3 of current interest. Chipman* has discussed evikcal. to correct for the fact that vitreous quartz was dence that the free energy of formation of silica (in employed in the effusion experiments whrreas any of its various forms) is about 5 kcal. mole-' thermodynamic functions for crystalline quartz more negative than the presently accepted value. were used in deriving the former heat for reaction Recent work in this Laboratory has shown that the I from the effusion data. Values from JAXAF thermochemical discrepancies noted by Chipman Thermochemical Tables6 were employed in making are the result of an m o r in the reported heat of formation d a h 3 The earlier heat of formation this correction.) value was based on measurements of the heat of TABLEI combustion of silicon in an oxygen bomb. The Reaction AHOm value reported here was obtained by a different I 2Ga(l) + Si02(vitrcous) )r thermochemical process, which avoided uncertain4-170.8 f 0 . 7 SiO(v) + Gs/rO(v) ties that are inherent in t he earlier method. In the present experiments, the heat of formation I1 2Ga(l) + MgO(c) )r of aqueous ffuosilicic acid was determined in a 1-159.3 It 0 . 7 Mdv) Gs20(v) rotating-bomb calorimeter. Mixtures of silicon I11 SiOz(d, quartz) SiOr and vinylidene fluoride polymer were burned i n $0.546 0.10 (vitreous) oxygen in the presence of aqueous HF, the product IV I/k3i(c) + l/zSiOn(c,triof combustion being fluosilicic acid in excess Hl? $82.98 f 0.23'J dymite) SiO(v) solution. Experimental procedures werc similar to V 1/,SiOl(c, quartz) Jr 1/2SiOsthose already dc~cribcd.~A sample of high purity (c, tridymite) $0.258 f 0.19 silicon (99.96% Si, 0.040/, SiOz; 50 to 75 micron VI Mg(c) f 1/202(v) z2MgO(c) -1143.7 zk 0.V particle size) was obtained through the courtesy of $35.3S6 f O . Z g Dr. J. E. Kunzler, Bell Telephone Laboratories, VI1 Mg(c) nfdv) Murray Hill, N. J. The combustion samples mere VI11 = I - I1 + I11 - IV - v prepared by mixing the silicon and powdered vinyliVI + VII l/zSiOl(c, quarts) I_ dene fluoride polymer in scaled polyester bags, +107.9 f 1 . 1 1/2Si(c) 1 / ~ 0 2 ( v ) which then were rolled and pelleted. The samples Si(c) + Oz(v) SiO?(c,quartz) -215.8 f 2 . 2 burned completely. and all of the silicon was conThe heat of formation of silicon monoxide from verted to aqueous fluosilicic acid. The result for the heat of formation of fluosilicic tridymite and silicon in reaction IV was calculated from data given by Brewer and Edwardss in their acid and those for the heat of solution of silica in analysis of the effusion data of Schafer and H o r n l ~ . ~aqueous HF reported by Kings permit calculation The heat of formation of silicon monoxide that is of the heat of formation of silica. The thermochemical equations are (1) J. Chipman, J . Am. C k m . Soc., 83, 1782 (1981).
*
+
*
+
(2) J. P. Coughlin. U. 9.Bureau of Mines Bull. 542 (1954): 0.L. Humphrey and E. G . King, J . Am. C k m . Soc.. 74, 2041 (1952). (3) W. D. Good, presented st the International Calorimetry Conference, Ottawa, Ontario, Canada, August 14-18, 1861. (4) Accepted for publication in J . h7erlrochem. SOC. ( 5 ) Joint Army-Navy-Air Force Interim Thermochemical Tablea. Thermal Laboratory, Dow Chemical Co.. hiidlaad, Michigan. (8) L. Brewer and R. K. Edwards, J . Phys. Ckrn., 68, 351 (1954). (7) I€ Schtlfer and R. Htirnle, 2. anorg. u. allgem. Chem., 263, 281 (1950). (8) J. P. Coughiin, U. 8. Bureau of Mines Bull. 542 (1954). (B) Estimated by the suthora
+
+
Si(c) O&) 47HF.l72IJ~O(liq.)= HnSiF~.41HF~17411~O(liq,) AH = -250.3 f 0.3 (I)
(1) Presented at the International Calorimetry Conference, Ottawa, Ontario, Canada, August 14-18, 1981. (2) J. Chipman, J . Am. Chem. Soc.. 88, 1782 (1981). (3) J. P. Coughlin, U. S. Bur. Minee Bull. 542 (1954); G. L. IIurnphrey and E. G. Icing, J . Am. Chem. Soc., 74, 2041 (1952). (4) W.D. Good, D. R. Douslin, D. W. Scott, A. George, J. L. Lacina, J. P. Dawson and G. Waddington, J . Phyr. C k m . , 63, 1133 (1959); W. D. Good, D. W. Scott and G. Waddington, ibid., 60, 1080 (1960). (5) E. 0.King, J . Am. Chem. SOC.,73, 868 (1851).
