Sept., 1962
S I M U L T A X E O U S ~iL4€'oRIZATION i l N D
DECOMPOSITIOK OF S O L I D
It provides a single peak (actually a poorly resolved triplet due to weak spin-spin interaction with El4)that can be detected readily a t low concentrations and apparently is chemically inert. An internal standard is especially valuable when small effects on chemical shifts must be measured accurately and bulk susceptibility corrections are too uncertain.
1705
I R O N (111) C H L O R I D E
Acknowledgments.-The authors are indebted to Prof. Paul Bender for assistance in obtaining n.m.r. spectra under optimum operating conditions. One of us (L.E.E.) wishes to express his appreciation to the Petroleum Research Fund for support and to Pennsylvania State University and Prof. R. W. Taft for use of facilities for the completion of this work.
AN EFFUSION STUDY OF THE SIMULTANEOUS VBPORIZATION AND DECORIPOSXTION OF SOLID IROX(II1) CHLORIDE BYR. R. HAMMER AND N. W. GREGORY Department of Chemistry at the liniversity of Washington, Seattle, Wash. Reeezned April 6 , 1968
and for the concomitant release of Estimates of the condensation coefficients for the vaporization of Fed& (CY = chlorine ( a =- 10-8) from solid PeC&between 120 and 150' have been made from effusion data.
Vaporiza,tion characteristics of solid FeC13 between 200 and 300' have been investigated by a number of workers.'-a These results have been compared and additional data provided, which also ext'eiicl down to 160°, in a recent paper from this L a b ~ r a t o r y . ~R e now wish t'o report an effusion study of Fe(& The vaporization processes of interest are and
2FeC13(s) = Fe2Cla(g) 2FeC13(s) = 2FeCl2(s)
(1)
+ Clz(g)
(2) (The work of Kangro and Bernstorff10 and of Schaferll indicates t'hat the partial pressure of monomeric FeCI3(g) is less than 1% of that of the dimer in the equilibrium vapor in the range of effusion experiments.) Extrapolation of results at higher temperature^^-^ suggests that at ca. 150°, (1) a,nd ( 2 ) can be studied simultaneously in the same effusion experiment. Inasmuch as equilibrium da t'a a,lready are a,vaila,ble, the principal object'ive of the work was to compare condensation coefficient,s, calculated from the dependence of steady stale effusion pressures on cell dimensions following 51. method described earlier. 12 Also, the applicability of the effusion met>hodfor the study of decomposition reactions such as (2) has been a qnestion of particular interest. in this Laborat,ory. It has been shown, for example, that effusion is not at, all suitable for determination of equilibrium (1) C. I l a i e r , 5. S. Bur. Mines Tech. Paper 360, 1925. (2) E. Stirnemann, Neues Jahrb. Mineral., Geol., Palaeont., S a l , 334 (1925). ( 3 ) K. Jelliriek a n d R. Xoop. Z. p h y s i k . Chem., 146A, 305 (1929). (4) K. Sano, J . Chem. Soc. Japan, 59, 1073 (1938). ( 5 ) H. F. Johnstone, H. C. Weingartner, and W. E. Winsohe, J . Am. Chem. Soc., 64, 241 (1942). (6) 0. E. Ringwald, Doctoral Dissertation, Princeton Univ., 1949. (7) W. Kangro a n d E. Petersen, 2. anorg. allgem. Chem., 261, 157 (1950). ( 8 ) H. SohaEer a n d E. Oehler, ibid., 271, 206 (1953). (9) L. E. Wilson and N. W. Gregory, J . Phys. Chem., 62,433 (1958). (10) 1 %'. Kangro a n d H. Bernstorff, 2. anorg. allgem. Chem., 263, 316 (1950). (11) TI. Rchafer, ibid.,269, 53 (1949). (12) J. H. !Stem and N;, W: Granary, .T" P h y s . Chtrmi 61, 1226 (J.967).
water vapor pregsnres in the system Mg(0H)Z 3 MgO Hz0.'3
+
Experimental Eastman practical anhydrous FeC& and, alternately, material prepared by reaction of analytical grade iron wire and commercial tank chlorine, were used. Samples were resublimed twice under high vacuum, with the sublimate finally condensed directly into the Pyrex effusion cells (described previouslyle). Cell characteristics are Cell
ao X 108,cm.2
3 5 6
24.2 16.7 4.23
UQ
x
1Oa/Us
2.5 13.5 0.6
K
0.99 .96 .97
where a0 is the orifice area, a, the cross-section area of the cell, and K the Clausing factor14for the cell orifice. Runs were initiated by placing a preheated furnace around the effusion cell which was attached to the high vacuum system. The temperature was measured with two ChromelAlumel thermocouples, one near the orifice and the other at the opposite end of the cell. A potentiometric controller held the temperature within 1 1 ' during the 8-30-hr. effusion periods. The effused ferric chloride vapor condensed on a glass insert tube, ca. 25 mm. o.d., which fitted snugly against the wall of the vacuum line and extended from the room temperature Eone into the furnace t o the glass wall containing the orifice. At the conclusion of the run, the cell was cooled, an inert gas (dry air was also found satisfactory) admitted, the collecting tube quickly replaced, and the system reevacuated. The ferric chloride washed from the collector was determined either by titration with standard potassium dichromate, sodium diphenylamine sulfonate indicator,15 or colorimetrically as the tris-( 1,lO-phenanthro1ine)-iron(11)complex ion.15 Effused chlorine was trapped either on a liquid nitrogen cooled finger which extended to within ea. 3 em. of the orifice or in a c&ventional trap which could be isolated from the effusion cell by closing a fluorocarbon-lubricated stopcock. Both methods gave similar results. The chlorine sample was dissolved in KI solution and the liberated iodine was determined by the amperometric dead-stop end-point, method .I7 (13) E. Kay and N. W. Gregory, ibid., 62, 1079 (1958). (14) P. Clausing, Ann. Physik, 12, 961 (1932). (16) I. M. Xolthoff and E. B. Sandell, "Textbook of Quantitative Inorganic Chemistry," Revised Ed., The Macrnillan Co., New York. N. Y., 1947, pp, 493-494, 600, 608-609. (16) W. B. Fortune and hl. G . Msllon, Ind. Eng. Chem. (Anal. Ed.), 10, 60 (1938). (17) Gi Wsmimcant and F. J L Hopkinson, i b i d , , 12, 308 (1940).
R. R. HAMMER AND N.
1706
w.GREGORY
Vol. 66
Log P e values predicted from effusion results are about 3% higher than those calculated from the W 0 Cell 6 equation given in ref. 9. a values calculated from eq. 3 and P e values from ref. 9 are about 25% e Cell 5 larger than those listed in Table I. 0 Cell 3 Results in Table I indicate dcr/dT to be positive .. I.__ pe for FezCls. The treatment described in ref. 12 leads to values for a standard enthalpy and standard entropy of activation for vaporization a t 135’ of 38 kcal. mole-l and 61 e.u., respectively; AH0 is 33 kcal. and AX0 56 e.u. for sublimation. In earlier work da/dT was found to be negative for iodine, l 2 which gave activation enthalpy and entropy less than the sublimation values. The different behavior may be associated with the fact that FezCle molecules must be removed from an 232 236 240 2 4 4 2 4 8 2 5 2 256 2 6 0 2 6 4 268 “infinite sandwich-layer-type crystal” of FeC4 1000/T°K. in which each ferric ion is surrounded by six Fig. l.-Fed&(g). halogen ions, whereas the iodine crystal is composed Steady state ressures of each species were calculated of molecular units of the same type as those in from the usual e8usion equation iodine vapor. Even though the condensation coefficient of Fe2C16is rather smell, steady state effusion preswhere n is the number of moles of effusate, M the molecular sures, when extrapolated to zero orifice area, are weight of the species in question, T the absolute temperature in reasonably good agreement with equilibrium of the effusion vessel, and t the effusion time in seconds. vapor pressures extrapolated from measurements Results and Discussion by other methods a t higher temperatures. ZFeCl,(s) = ZFeClz(s) C12(g).-Steady state 2FeC13(s) = Fe2C16(g) .--FezCls effusion pressures were dependent on cell geometry, Fig 1. chlorine pressures are shown in Fig. 2. These Steady state pressures, P,,, from cell 6 are not sig- points, with the exception of some runs with cell 5 nificantly different from values predicted by ex- for which insufficient chlorine for analysis was coltrapolation of the equation reported in ref. 9. lected, correspond to the same experiments from A least squares treatment of data from each cell which FezCls pressures plotted in Fig. 1 were dewas used to establish the lines shown; correspond- termined. Data shown for cell 6 were collected ing constants in equations of the form log P(mm.) from 10 independent samples; usually about six = - AT-I B are listed in Table 11. Data from successive measurements a t various temperatures were made with each sample. The first values, the t!hree cells were correlated by the equation12 indicated by symbols of the form 9 , in such a series P e = Pm (1 !/a) were invariably much lower than subsequent values; (3) the latter were used to fix the position of the line where P, is the equilibrium pressure, f the ratio drawn in Fig. 2. Tt mill be observed that pressures orifice area over cell cross-section area, and a the corresponding to this line are only ca. 1% of the condensation coefficient. At each of the four equilibrium values (dotted line) predicted by extemperatures listed, Table I, values of P,, for each trapolation of data quoted in ref. 9. cell were calculated from the smoothed line (Fig. The unusually low value in the initial run for 1) and a least squares treatment based on ( 3 ) was each series might be associated with the initial used to obtain values of P, and a. small particle size of FeC12. I n the equilibrium case a t least, chlorine pressures for (2) with FeTABLE I EQUILIBRIUM PRESSURES AND CONDENSATION COEFFICIENTSC12 in a microcrystalline state would be less than the value when FeC12 crystals are large enough for FOR FezC16 OVER SOLID FeCls FROM EFFUSION DATA surface free energy effects to be negligible. This t , “C. Pe X 108 (mm.) x 108 argument does not necessarily apply to the steady 120 1.0 5.2 state pressures, however. I n one series of measure130 2.7 6.4 ments, corresponding data labeled 0-,ca. 10 mole 140 7.1 7.7 yo of FeClz was introduced before the FeC4 was 150 18 9.3 sublimed into the cell; the initial point, as well as TABLEI1 subsequent values in this series, was in general EFFUSION STEADY STATEPRESSURES OF FezCls OVER SOLID agreement with the cell 6 line. Solubility of FeCl2 FeCls in FeC13 would be expected to make initial chlorine Cell I x 10‘ -A B pressures high; actually, no evidence for appre5 135 7790 16.27 ciable solid solution between FeClz and FeC4 has 3 25 7600 16.15 been observed.5 In a number of cases in the pres6 6.0 6902 14.51 ent work, effusion was continued until virtually all Predicted equil. pressure of the volatile material had sublimed from the cell; effusion data and (3) 6887 14.52 other than the effect discussed above, no dependRef. 9 7142 15. I I
+
+
+
(I
Sept., 1962
SIMULTANEOUS VAPORIZATION AKD DECOMPOSITION OF SOLIDIRON(III)CHLORIDE 1707
ence of pressures on the relative amounts of FeC4 and FeClz was observed. Fe2CI6pressures did not show a corresponding anomalous behavior for the initial measurements. One might expect that as cracks and crevices and roughening of the FeC18 crystals develop, the rate of the decomposition reaction would increase. The effect of such surface changes might be more readily obFerved in the case of the decomposition pressures, which are far below equilibrium values, than for the Fe2Clspressures, which are moderately close to equilibrium. Results from cell 3 are lower than those from cell 6 by another factor of ca. ten. Pressures shown, Fig. 2, were obtained from a single sample. Again the first measured value lies appreciably below the line drawn through the others. The order in which the temperature was changed between measurements was varied randomly. A satisfactory set of steady state pressures was not obtained from cell 5 . The capacity of this cell is small, which made it inconvenient to use large samples of FeC13; usually the sample was exhausted after three or four measurements; sufficient chlorine for analysis was obtained only a t the highest temperatures and these results were widely scattered. In contrast, FezC16 data from this cell were quite consistent, however (see Fig. 1 ) . If P, is extrapolated from data at higher temp e r a t u r e ~and ~ eq. 3 applied, both cell 3 and 6 data give a very small condensation coefficient, 7 X lo-' and 4 X respectively. Even though these values are in reasonable agreement, the effusion method does not seem well suited as an independelit means of establishing the equilibrium pressure in such a case. With a of the order of lo+, the assumption that the surface area can he approximated by the cross-section of the re11 is of questionable validity; some change in P,, as the reaction progresses is observed and variation in the effective value of a, from one sample to another is suggested by thc relatively large scatter of results. A number of Rdditional experiments with cell 6 were conducted in which chlorine pressures, but not ferric chloride pressures, were determined. This permitted a series of successive measurements to be made without exposing the sample in the cell to dry air or to an inert gas, as the collecting trap for chlorine was isolated from the cell by a fluorocarbon-lubricated stopcock. The size of the FeCL sample initially placed in the cell also was varied by a factor of the order of ten. Results of these experiments were similar in every respect to those shown in Fig. 2 ; steady state pressures showed no definite correlation with initial sample size but, as in earlier measurements, the first measured chlorine pressure was appreciably lower than subsequent values. The chlorine steady state pressure dependence on temperature is not sufficiently well defined to
2 3
.4 E
a!! QI
35 I
6
7
I
0
; 2.34
2.38
2.42 2.46 100O/To K.
2.50
Fig. 2.--C12 effusion pressures: 0, cell 6; 0 , cell 5 ; 0 , cell 3; - - - -, P,, Wilson and Gregory.
draw quantitative conclusions about the dependence of the coiideiisatioii coefficieiit 011 temperature. However, the slopes (Fig. 2 ) appear similar to that of the equilibrium line, Le., a! does not appear to change materially with temperature, which suggests the small value of 01 is due maiiily to probability rather than energetic factors. The value of a for reaction 1 is about ten thousand times larger than that for ( 2 ) . Since both processes originate on the FeG18 crystal surface, the large difference betlveen steady state pressures aiid equilibrium values must be attributed to a difference in the kiiietics of the two processes rather than some factor such as the temperature of the crystal surface or geometric characteristics of the efhsion cells or vacuum system. 113 earlier work the reaction 2FeCls(s) +C12(g) -+ 12enC16(g)was studied between 160 and 430' by the transpiration method. Equilibrium constants were obtained from data between 220 and 430' but the rate of combination of chlorine with FeClz(s) between 160 and 220' was so slow that equilibrium was not established even a t a small flow rate when approached from the high chlorine pressure side. Seither of the dynamic methods, effusion or transpiration, is well suited for the study of reaction 2 at temperatures of the order of 150O. A~knowliedgrnent.--T4~eare pleased to aclinowvledge financial assistance from the National Science Foundation.
