NOTES
domly with reproducible pressures (within the large scatt,er shown) obtained a t any given temperature. I t will be observed that effusion pressures are from l/lo,ooo to 1!600 as large as expected equilibrium values, extrapolated from the higher temperature diaphragm gage s t ~ d y . Whereas ~ the pressures increase as the orificcb diameters decrease, no simple systematic relationship could be found to correlate the pressures with effusion cell geometries or t'o predict from the various steady-state pressures the value of the equilibrium limit. It appears that the condensation coefficient is very small, of the order of similar in magnitude to the apparent value for the iron(II1) chloride decomposition reaction.2 The appearance of the sample after removal of considerable amounts of bromine suggests that decomposition occurs largely a t the edges of the plate-like crystals. Xo appreciable yellow coloration (IceRr2) could be seen on the large faces of P'eRra crystals. The effective surface area for the dccomposition may be limited to the edges of these crystals, similar to the behavior (loss of water) in the Alg(OH)2 system.' The condensation coefficient on these edges alone may be considerably larger; the average value of CY could appear very small because the reacting edges coiistitute only a small fraction of the total area of the sample. The large scatter of data may well be caused by variations in the effective reacting surface area as well as possible variations i n the coiidcnsatioii and vaporization coefficients as the decomposition progresses. The slopes of the least-squares lines i n Iig. 1, drawn through effusion data from the various cells, are different from that of the equilibrium line. From the variation in COI ide nsat ion cocfficieI i t with ti: mpera t urr suggested t).y these slopes, all apparent heat of activation for condeiisation of 12 kcal. mole-' of bromine, and for Vaporization of 28 kcal. mole-', is ~ b t a i n e d . ~ In view of the large difrererice t)etmcen st>eady-state and equilibrium pressures, these apparent values may have little significance. Itj is of interest, however, that the apparent heat of activation for vaporization of bromine is virtually the same as that indicated for re!ease of chlorine from I"eC13 (26).2 However, the value for condensation of chlorine appeared to tie near zero. Simultaneous vaporization of li'e2C16in the cliloride case provides a mechaiiism for renewing the I'eCIS solid surface which was not present in thc bromide systrm. 'I'hcse results cmphasize that caution must be exercised in the application of the effusion method t'o the study of decomposition reactions. The method is riot useful as a means for determlriation of relative
965
thermodynamic properties of iroii(II1) and iroii(I1) bromides and chlorides, respectively. Acknowledqmmt. l'inaiicial support, for this work was received from the Satiorial Scirnce lj'oundation. (7) N. W. Gregory, J . Phys. Chem., 67, 618 (1963).
The Vapor Pressure and h a t and Entropy of Sublimation of Solid ?Iagnesium Fluoride
by Alichael A. Greeribaum, Iion Chung KO, Madeline Wong, and lIilton Icarber Rocket Power, Inc., Rrseawh Laboratories, Pasadena, Cdifornul (Received October 3, 1963)
The first vapor pressure measurements of AIglcz were reported by Ruff and 1,eBoucher' in 1034. These investigators used a mariomct,ric method to obtain a vapor pressure curve for liquid AIgF2 between 1934 arid 2120'K. In 1945, Kaylor2 reported values for the melting point arid heat of fusion of lIgI('2. Using these values, a heat of sublimation a t 2 M 3 K . was calculated for llglc2.3 The value obtained in this mariner was 85.6 f 1.0 kcal./mole. In 1!)B2, Bcrkowitz and Marquart" reported the first' experiment>aldetermination of the vapor pressure of solid IlgIc, together with the heat of sublimation. 'I'hc solid was studied a t 1220 14d0'K. by means of a mass spectrometer. IIowwer, only nine points Ivere measured in this range. I'rom a plot of log I' us. l / Y the authors obtairied a second-law value of 8fi kcal./mole for the heat of sublimation of RIgI', a t 1B30'K. Their thirdlaw value for AHsob was found to be only 81 kcal./ mole, however. Hecause of the limited amount of experimental data available or1 the vapor pressure of solid JlgP', coupled with the lack of a definitive value for the heat, of suhlimation, it was decided to undertake a detailed measurement of the vapor pressure of MgF2(c)by means of the torsion effusion procedure.
Experimental The measurement of the vapor pressure of lrgIp2(c) was carried out over the temperature rangc 1278.(1) 0. llriff and 1,. LeBoucher, %. anorn. allgem. Chom., 219, 3 7 6 (1934). (2) 13. 1,'. Naylor, -1. A m . Chem.
&IC.,
67, 150 (1945).
(3) .IANAI
