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J. E. MCDONALD A N D J. W. COBBLE
activation energies for the (311) and (111) planes were found to be identical and larger than that of the (1 10) plane. The assumption that the oxidation rate is the rate of reaction between water molecules and silicon bonds a t the silica-silicon interface is substantiated by the data in Table IT’. The calculated order for the pre-exponential constants in the series (110) : (311) : (111) is 1.000: 1.707: 1.227 (Table 111) and the experimental order is 1.00:1.7:1.3 (Table ITT). Although the argument for the activation energies is greatly oversimplified, the predicted order (111) > (311) > (110) is close to the experimental order (111) = (311) > (110)
Vol. 65
The calculation in Table I11 shows that N for the (100) plane is the smallest of the four planes listed and would thereby have the smallest pre-exponential constant. The analysis for the activation energy given above predicts that this plane wou!d also have the largest activation energy. It is expected therefore that the oxidation rate of the (100) plane would be smaller than those of the (111), (311) and (110) planes. Acknowledgments.-The author wishes to express his appreciation to Miss R. E. Cox for carrying out the oxidations, to L. E. Howarth for providing the oriented wafers, and to W. G. Spitzer for his interest and helpful discussions.
THE HEAT OF FORMATION OF THE HYPOBROMITE ION1 BY J. E. MCDONALD~ AND J. W. COBBLE Department of Chemistry, Purdue University, Lafayette, Indiana Recezted May 3, 1961
The heat of formation o,f the hypobromite ion has been determined by measurement of the heat of hydrolysis of bromine in alkaline solutions a t 25 . The value so obtained was AHrO = -23.05 f 0.2 kcal./mole. Withoa free energy of AFra = -8.2 kcal./mole, the partial molal entropy has been determined to be 8.5 f 0.7 gbs./mole a t 25 .
Introduction As a coritinuation of the work on the hypohalite ions as chemical calorimetric oxidants, the heat of formation of aqueous hypobromite has been redetermined. All previously listed value^^^^ have been based upon the older work of Thomsen5 and Berthelot.6 As a result of certain inconsistencies noted between these thermochemical data and more recent work,’ it was of interest to fix the thermodynamic functions of hypobromite more precisely. Experimental The calorimeter and associated equipment have been deRcrihed in detail elsewhere.8 All heats of reaction were carried out a t 25.0 5 0.05’. Chemicals.-The bromine was obtained from the Great Lakes Oil Comoanv and certified to be 99.9%. ilnalvsis for water showed le& than 50 p.p.m. Sodium hydroxide solutions were prepared by dilution of a saturated stock solution containing J. T. Bakers Analyzed Reagent with distilled water. Procedure.-The experimental procedure was essentially the same as that described previously8 and involved releasing known amounts of bromine sealed in small glass bulbs into various concentrations of sodium hydroxide solu(1) Tliis research x a s supported by the United States Air Forre through the Air Force Office of Scientific Research of the Air Research and Developnient Command under Contract AF 18(6@0)-1525. Reproduction in whole or part is permitted for any purpose of the United States Government. (2) From the Ph.D. thesis of J. E. McDonald, Purdue University, 1901; Dow Chemical Pellow, 1959-1960. (3) “International Critical Tables.” Vol. 5 , XlcGran-Hill Book Co.. New Yolk, h-.Y., 1933, p. 169. (4) F. R . Bicliowskg and F. D. Rossini, “Thermochemistry of the Chemical Substances.” Reinhold Publ. Corp., New York, K. Y.,1936, p. 188. ( 5 ) J. Thornsen, “Thermochemische Untersuchungen,” Bartli, Leipzig, 1882-1886. (6) 11.Berthelot, Ann. chin. phys., 7, 410 (1886). (7) J. P. King and J. W. Cobble, J . Am. Chem. Soe.. 82, 2111 (1960). (8) J. E. McDonald, J. P. King and J. W. Cobble, J. Phys. Chsm., 64, 1345 (1960).
tion. The hypobromite formed was determined immediately on removal from the ealorimeter by addition of excess solid KI and titration of the 1 3 - so formed with standard thiosulfate. Bromate formed by the disproportionation of the hypobromite does not’ react with the iodide under the conditions of this titration; therefore a correction must be made for this loss of hypobromite. The rate of disproportionation of hypobromite under the conditions of the calorimetric experiments has been reported to be slow.9 This fact was verified by a set of separate experiments which established the rate of this reaction under the experimental conditions existing in this research. The magnitude of this correction varied between 1 and 3y0 of the number of milliequivalents of OBr- formed. S o other decomposition reaction was considered since the disproportionation has been found to be the predominate mode of hypobromite decomposition. The alternate reaction 20Br- = 2BrO2 (1) has been reported to account for less than 27, of the total decomposition under the conditions of the present experiments .9
+
Results The stoichiometry of the reaction Br,(l)
+ 20H-(aq)
=
OBr-(aq)
+ Br-(aq) + H2O
(2)
was checked by comparison of the amount of bromine used to the quantity of hypobromit’e formed. The results of the hydrolysis experiments are summarized in Table I. LTsingt’he ext’rapolated heat of react’ion, AH = - 10.35 f 0.20 kcal./mole, and auxiliary thermodynaniic data,lOionof pH about its isoelectric point, He introduced in his theory the interactioll of the ampholyte with a small ion at one site jn an (1) This investigation was supported b y a Public Health Servire research grant #RG-6730 a n d a training grant k2G-718 from the Division of General Medical Sciences, Public Health Service. (2) (a) K. Linderstrom-Lang, Arch. Bzochem , 11, 191 (1946). (b) R. Linderstrom-Lang a n d S 0.Nielsen, "Acid-Base Equilibria of Proteins," in "Electrophoresis," (Milan Bier, ed ) Academic Press, New York, N. Y., 1959, Chap. 2.
attempt to rationulize a discrepancy bctmeeii theory and experiment when the theorv waq applied in an estimation of the molecular. weight of P-lactoglobulin, utilizing the solubility data of Grijn\~all.~ The possibility of extending this theory to more general case suggested itself, and is herein developed. The results suggest a new method for the study Of interactions between an'pholytes and other molecules. Such interactions have receircd considerable attelltion hv BJerrum; (3) -4.Gronnall, C o n p t rend. trau. lab. Carlsberg Ser. chtm , 24 185 (1942).
