688
The Journal of Physical Chemistry, Vo/. 83, No. 6, 1979
N. W. Gregory
to the gas phase, was apparently responsible for this decrease in selectivity through the formation of alkylperoxy radicals. These results suggest that, in catalytic systems where 0- ions are responsible for dehydrogenation, nitrous oxide would be superior to molecular oxygen if maximum selectivity to the alkene is desirable. This assumes a sufficiently low temperature so that the equilibrium for the formation of ROO. (eq 8) is fav0rab1e.l~
(3) K. Aika and J. H. Lunsford, J . f h y s . Chem., 82, 1794 (1978). (4) A . J. Tench and T. Lawson, Chem. Phys. Lett., 7, 459 (1970); J . Chem. Soc., Faraday Trans. 1 , 68, 1169 (1972); A. J. Tench, ibid., 69, 1181 (1972). (5) N. B. Wong and J. H. Lunsford, J . Chem. Phys., 56, 2664 (1972). (6) V. V. Nikisha, S. A. Surin, B. N. Shelimov, and V. B. Kazansky, React. Kinet. Catal. Letf., 1, 141 (1974). (7) V. B. Kazansky, V. A. Shvets, M. Ya. Kon, V. V. Nikisha, and B. N. Shelimov, "Catalysis", J. Hightower, Ed., North Holland Publishing Go., Amsterdam, 1973, pp 1423-1435; B. N. Shelimov, C. Naccache, and M. Che, J. Catai., 37, 279 (1975). (8) D. A. Parkes, Trans. Faraday Soc., 68, 613 (1972). (9) C. Naccache and M. Che, "Catalysis", J. Hightower, Ed., North Holland Publishing Go., Amsterdam, 1973, pp 1389-1400. (10) N. 9.Wong and J. H. Lunsford, J . Chem. fhys., 55, 3007 (1971). (11) M. J. Lin and J. H. Lunsford, J . fhys. Chem., 79, 892 (1975). (12) C. F. Chacaty, C. R . Acad. Sci. Paris, Ser. C, 268, 300 (1969). (13) Y. Fukuda and K. Tanabe, Bull. Chem. SOC.Jpn., 46, 1616 (1973). (14) J. H. Lunsford and J. P. Jayne, J . Chem. fhys., 44, 1487 (1966). (15) S. W. Benson, J . Am. Chem. SOC., 87, 972 (1965).
Acknowledgment. This investigation was supported by the National Science Foundation under Grant No. CHE 75-15456. References and Notes (1) K. Aika and J. H. Lunsford, J . fhys. Chem., 81, 1393 (1977). (2) M. B. Ward, M. J. Lin, and J. H. Lunsford, J. Catal., 50, 306 (1977).
Interaction of Bromine with Iron(I1) Chloride N. W. Gregory Deparfment of Chemistty, University of Washington, Seattle, Washington 98 195 (Received September 8, 1978)
An absorbance study between 260 and 600 "Cof the vaporization reaction between bromine and FeClz(s)indicates that mixed halide iron(II1) molecules, FezBr,C16-xand FeBr,Cl,-,, are major constituents in the equilibrium vapor. The equilibrium solid phase appears to be a solid solution containing substantial amounts of bromide ion and small amounts (up to mole fraction 0.1, as estimated on the basis of an ideal ionic solution, depending on the temperature and the concentration of free bromine) of ferric ion. Thermodynamic properties of these mixtures are discussed.
Introduction The interaction of bromine with iron(I1) chloride has been studied between 260 and 600 "C by measuring the absorbance, attributed to bromine gas and iron(II1) halide molecules, of equilibrium vapors in the UV-visible region of the spectrum. The generation of iron(II1) mixed halide molecules in the vapor phase was anticipated, and recent work1 has shown formation of solid solutions between FeC12and FeBrz in this temperature range. Formation of such solid solutions, by a halogen exchange reaction between bromine and iron(I1) chloride, will have a significant effect on the concentrations of various molecules in the equilibrium vapor. Samples with substantially different relative amounts of bromine and iron chloride have been examined and a model, based on predictions from the thermodynamic properties of the simple iron(I1) and iron(II1) halides, proposed which appears consistent with the experimental observations. Experimental Section A Cary 14H spectrophotometer was used to measure vapor phase absorbances of 11different mixtures at various temperatures. FeC1, was prepared either by vacuum dehydration of FeCl2.4Hz0 (Baker's Analyzed Reagent Grade), and subsequent sublimation of FeC1, away from the residual oxide-chloride mixture, or by reaction of chlorine, released by heating CuCl,, with analytical grade iron wire. Baker's Analyzed Reagent Grade bromine was dried in the Pyrex vacuum system and the reactants sublimed (separately) directly into quartz absorption cells. The mixtures were isolated by vacuum seal-off of the connecting cell side arm. During the absorbance mea0022-3~54/79/20a3-0~ 1.oo/o ~~~0
surements cells were heated in a clam shell furnace; temperatures were measured with chromel-alumel thermocouples placed at the center of the cell body (TI),at the windows (T2),and at the tip of the side arm (T,). In the study of heterogeneous systems, the condensed phase was held at T,, a few degrees cooler than T z (and Tl), to prevent condensation of a solid on the windows. In most instances the amount of FeCl, originally introduced was found to be in slight excess of that which could be completely vaporized by bromine oxidation. The attached side arm was left long enough so, after an initial series of absorbance measurements, some of the solid phase could be removed by sealing off a portion of the tip. In this way the amount of iron halide in the cell was reduced until complete vaporization at higher temperatures could be achieved with absorbances remaining in the observable range. Predicted concentrations of iron(I1)vapor molecules were not high enough to contribute significantly to the measured a b s o r b a n ~ e s . ~ ~ ~ In Table I each independently prepared sample i s identified with a number; a prime designation indicates that the sample was reexamined after sealing off some of the solid phase. The letter E denotes an "excess" of solid phase was present; V indicates that, in the temperature range designated, all of the solid phase had been vaporized. C,(Fe) represents the concentration (mol L-I) equivalent of the total number of iron atoms in the cell, as determined by atomic absorption analysis after completion of the absorbance measurements; Co(Br2) represents the equivalent of the total concentration of bromine (as Br2) introduced into the cell. The latter was calculated from the absorbance measured at the maximum of the bromine 0 1979 American
Chemical Society
Interaction of Bromine with Iron(I1) Chloride
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
TABLE I: Observed and Calculated Characteristics of Mixtures Studied I _ _ _ _ _
689
-
-
mol fraction sample, length, vol'2 -1(2) 1' ( 2 ) 2 ( 5 ) (14.2) 3 (1)(3.22) 5 (2.5) (8.32) 6 (1) 6' (1) 6" (1)(3.14) 8 (5) 8' ( 5 ) 8" ( 5 ) (13.4) a
C,(Fe) X 105
5.422 65.61 7.166
26.12
5.383
C,(Bra) X 10
118 130 58.0 225 218 420 422 422 ? ? ?
E E E V E V E V E E E V E E E V
temp range,"C
hmax, nm
335-429 371-434 364-411 467-524 358-446 479-550 330-360 385-457 260-343 375-380 280-440 389-474 382-474 433-518 439-560 595-621
260-265 265-270 275-280 285 270-275 280 280-285 288 250-253 260 255-260 270 285-288 288 285-290 285
UVpeak
solid sol vapor phase X F ~ x ~ l o+3 X B ~ - X10 C1 x 10
-
13-5 8-4 4-3
1.2-2.1 2.1-3.1 4.3-4.7
12-5
2.1-3.6
12-9
3.1-4.5
98-32 15 58-20
0.4-0.7 1.8 0.6-1.4
2.0-1.5 0.9-0.5 0.6-0.2
2.1-5.0 4.3-4.9 1.9-2.0
7.6-6.5 6.4-5.3 3.9-3.6 3.6 6.5-4.7 4.6 5.0-3.7 3.5 9.1-8.5 6.9 8.6-7.4 7.1
Cell path length in cm, volume in mL.
absorbance band near 420 nm and published molar absorptivities of b r ~ r n i n e . Above ~ 100 "C solid iron(II1) halide phases containing significant amounts of bromine are not expected to be stable at the bromine concentrations u ~ e d , and ~ J ~the concentration of iron(II1) halide molecules in the vapor phase below 200 "C was not sufficient to be detectable by the Cary 14H instrument or to significantly change the bromine concentration. In addition, as will be discussed below, the amount of bromine entering the iron(I1) chloride solid phase (by an exchange reaction) is not expected to significantly affect the bromine vapor concentration at temperatures around 100 O C 6 A, values indicate the wavelength (range) a t which the peak maximum in the UV region was observed. Also listed are the ranges of compositions calculated for the vapor and solid equilibrium phases in the indicated temperature interval.
Results and Discussion Previous Work. In earlier studies5J bromine was shown to react with solid iron(I1) chloride near room temperature to form an iron(II1) mixed halide solid phase with composition FeBrCl,. Three mixtures formed from widely differing relative amounts of FeC12 and bromine gave similar bromine equilibrium decomposition pressures, ranging from ca. 50 torr at 30 "C to ca. 1200 torr at 98 Considering the reaction 2FeC12(s)+ Br2(g) = 2FeBrC12(s), it was concluded that the respective solid phases in the three independently prepared mixtures must have virtually the same composition. X-ray powder pattern lines of the iron(I1) phase showed no detectable shift from those of pure FeClz(s) and, unless significant and variable amounts of bromide ion are introduced into the iron(I1) phase by halogen exchange, the iron(II1) phase is expected from the stoichiometry of the reaction to have a fixed 1:2 Br:C1 ratio. X-ray powder pattern lines attributed to the iron(II1) mixed halide phase indicated the same structural form as reported for FeC13 and FeBr3, but with intermediate cell parameters.13 The patterns did not give sufficient information to distinguish between random and ordered arrangements of the halogen atoms; the composition FeBrC1, may be only one of a series of possible solid solution mixtures of FeC13 and FeBr,. The thermal instability of the latter (relative to FeBr,(s) and bromine) indicates that such mixtures will only be stable in the presence of substantial partial pressures of bromine. It was reported that samples held around 150-250 "C in a pressure of 500 torr of Br2 showed a slow vapor transport of FeCl,, presumably by sublimation as a mixed halide
iron(II1) vapor phase. However no further study of the vapor was undertaken at that time. More recently1 X-ray powder pattern analysis and a study of the hydrogen halide exchange equilibrium has shown that FeCl, and FeBr, form a series of solid solutions at temperatures around 400 "C. Values of K1 for the reaction FeC12(s) + Brz(g) = FeBrz(s) + C12(g) (1) AGOl = 15189 - 0.672' (ref 6) have now been used with the equilibrium data reported in the earlier study5 to calculate the mole fraction of C1expected in the solid iron(I1) phases in those samples if ideal mixing of C1- and Br- is assumed. Values in all mixtures were found to remain near unity, >0.995, with equilibrium bromine partial pressures from 500 to 5000 times larger than the chlorine partial pressures. However a similar calculation at the higher temperatures and at the bromine concentrations involved in the present work indicates that substantial amounts of bromide ion may be expected in the equilibrium iron(I1) chloride phases. Present Work. Qualitative Observations. The TJVvisible absorbance spectrum of vapors generated by interaction of bromine with FeC1, is observed to be very similar to those of iron(II1) c h l ~ r i d eand ~ ~of~ iron(II1) bromide.1° In general peak maxima for the various samples were found to lie at intermediate wavelengths (for ferric chloride, 250-260 nm (UV) and 365 nm (visible); ferric bromide, 305 and 460 nm), the exact location depending on the relative amounts of bromine and FeC1, and whether or not the sample had been fractionated by sublimation. The absorbance in the visible region overlaps strongly with that of free b r ~ m i n ehence ; ~ quantitative conclusions about the composition of the vapor were based on the magnitude of the absorbance and the location of the UV peak. Fifty absorbances for the various samples at various temperatures were measured with vapors in equilibrium with a solid phase; an additional sixteen were measured with all the iron halide in the vapor phase (in the presence of excess bromine). Results are displayed graphically in Figure 1. The regions (Figure 1)of marked increase of absorbance with increasing temperature correspond to the behavior of the samples when a solid iron(I1) phase is present. A discontinuity may be expected at the temperature at which all of the solid is vaporized. After complete vaporization a small decrease in absorbance as the temperature is increased is observed, attributable to the dissociation of dimer molecules to monomer molecules.6-10 The lines
N. W. Gregory
The Journal of Physical Chemistry, Vo/. 83, No. 6, 1979
690
3.0
1
ferric ion. A separate predominantly iron(II1) bromide phase would be expected to generate measurable amounts of free bromine.jJl Furthermore, in light of this behavior, it is possible, particularly in view of the sizable quantity of iron(I1) halide phase in the system, that significant amounts of ferric ion (and bromide ion) may have been incorporated in the solid during the bromination treatment to form FeBr2;hence the correct value of Co(Br2)could not be independently fixed for samples 8, 8', and 8". This effect was not observed with any of the other samples as the initial amounts of iron(I1) chloride were small, very near values corresponding to Co(Fe) in Table I, and the amounts of bromine relatively large. Quantitative Considerations. These observations and the temperature dependence of the absorbances have been compared with predicted compositions of the equilibrium vapors based on the following thermodynamic constants, in addition to those indicated above for reaction 1:
'- -7 - -
3--
. d
P 0.05
-
0.03
-
0.01 L 1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
I
2FeBr2(s)+ Br2(g) = Fe2Br6(g) AGO2 = 16300 - 187' (ref 12)
(2)
2FeC12(s)+ Cl2(g) = Fe2C16(g) AGO3 = 5122 - 14.85T (ref 6)
(3)
1000 ( T K ) - I
Figure 1. Plot of absorbances ( A ) at A ,, for the UV peak (250-290 nm) observed for vapors generated by mixtures of Br, and FeCI, at 2, X; 3,A; 5, * ; 6, 0 ;6', various temperatures: sample 1, 0 ; l', 0; 8 ;6 I r , m; a, A;a', 4; a", ,A.
drawn will be discussed after a model is proposed to explain the behavior of the system. It will be observed that the curves for the various samples are displaced from each other. In the homogeneous vapor region the total absorbances correlate well with the total amounts of iron in the cells. When a condensed phase is present, one would expect, on the basis of a simple reaction such as eq 9, that the ratio of the absorbance (A)to the bromine concentration should be the same for the various samples at temperatures at which the dissociation of dimer to monomer is small. However this is not found to be true, suggesting that the activity of the iron(I1) chloride (or, considering the solid as a possible ionic mixture of Fe2+ and Fe3+ and of CI- and Br-, the activities of the various ions) is in general different in the various samples. The same conclusion is found necessary even when other possible modes of vaporization are included. It is noted, Table I, that A,, differs for the various samples; the value also changes slightly with temperature for each sample and changes when part of the sample is removed. Such behavior also indicates that the equilibrium vapors must be mixtures in equilibrium with solid solutions, at the lower temperatures, of variable composition. The behavior of sample 8 strongly indicated formation of solid solution. A relatively large amount, several tenths of a gram, of an approximately equimolar mixture of FeClz and FeBr2 (the latter formed by reaction of bromine with iron wire) was sublimed into the cell. During the preparation of FeBr2,the FeC12-FeBr2 mixture was heated to around 500 "C in a bromine atmosphere. The cell was cooled and a small amount of bromine admitted at room temperature. The value of Co(Br2)indicated by measurement of the bromine absorbance at 100 "C was 2.6 X mol L-l. After the first series of absorbance measurements at ca. 400 OC, no measurable absorbance attributable to bromine could be detected at temperatures below 200 OC. One concludes that the bromine remained in the solid lattice, presumably as bromide ion along with
2FeXdg) = Fe2X&g) 32.25T (ref 6, 11, 12)
AGO4 = -33533
+
2FeX3(s) = Fe2X,(g) AGOS = 32200 - 562' (ref 6, 11, 12)
(4)
(5)
Reactions 4 and 5 , with X = C1 or X = Br have virtually the same free energy expressions and the same equation has been used for both. Equilibrium constants were formulated in terms of ionic activities in the iron(I1) solid phase, e.g., K 1 = (CCiz~Br-2/CBrz~C,-2). Activities were approximated as mole fractions, with Xcl- + XBr-= 1 and XFe2t + 1.5xFe3+= 1, i.e., negative ion sites were assumed to be fully occupied by C1- or Br-, and positive ion sites by Fe2+or Fe3+,with one vacancy for each two Fe3+ions. In the model assuming the presence of mixed halide vapor molecules, each of the five different dimers (and two monomers) was assumed to have thermodynamic properties corresponding to the appropriate weighted average of those of the chloride and bromide parent molecules, with an additional entropy corresponding to relative probability factors based on random distribution of halogen atoms. The sixteen absorbances, from five different samples, measured with all the iron halide in the vapor phase, were found to correlate with the total amounts of iron in the cell and to have no systematic relationship to the bromine concentration (except that the latter must be high enough to bring about complete vaporization). Thirteen of the values, from all samples except 6'r, were observed to have peak maxima in a reasonably narrow range, 280-288 nm. When these were treated as a simple dimer-monomer mixture, with Co(Fe) = CM+ 2CD and K4 CD/C,', and the values of CD and C M used in the equation A(at A,), = EMCM+ EDCD, apparent values of the molar absorptivities of the monomer, E M = 2500, and the dimer, E D = 9500 cm-' mol-l L gave a standard deviation of A(obsd) - A(calcd)/A(obsd) of 4.5% (average deviation 3.5%). By comparison, E D = 10200 (at 250 nm) and E M = 4000 (at 260 nm) for ferric chloride and 9600 (at 305 nm) and 2570 (at 305 nm) for ferric b r ~ m i d e . ~ItJ ~seems most likely, as will be discussed further, that the mixtures studied here contain not only ferric chloride and ferric bromide molecules but all mixed halide species as well. One expects that the molar absorptivities of the mixed halide molecules will be quite similar to those of the parent molecules and that the E M and E D values derived above represent
Interaction of Bromine with Iron(I1) Chloride
The Journal of Physical Chemistry, Vol. 83, No.
weighted averages for the various mixtures. It is possible that while the position of the peak maximum shifts as the composition changes from mixtures largely chloride to largely bromide, the weighted average molar absorptivities may not change appreciably. Sample 6, with an appre, , ,A showed the only significant deviation ciably smaller (ca. 10%). The average molar absorptivities were then used, with K4 values, to derive the apparent total dimer concentration CD and total monomer concentration CM for each of the 50 vapor absorbances measured with a solid phase present. The values derived were not consistent with the vapor composition predicted from thermodynamic data unless it is assumed that the compositions of the solid phases vary and are somewhat different for each sample. The following set of equations was found convenient to derive the mole fractions of the various ions (the relationship of the corresponding equilibrium constants to those of the primary set of equations used as a source of thermodynamic data is shown): 3FeC12(s) + 1.5Rr2(g) = Fe&&(g) + 0.5Fe2Br6(g)(6) & = K1K3(K21/2)
300
6, 1979 691
’
1
I
-. 290 1
.\ I
‘ 270:
T
260 250
1 I 0
01
0 2
03
05
06 Xc, (vapors)
04
0 7
08
09
I 10
Figure 2. Variation of A, with the calculated mole fraction of chlorine atoms (as part of the iron halide molecules) in the vapor phase. See for Fe,Br, (on left) and caption for Figure 1 to identify symbols. A, Fe,CI, (on right) are indicated by the filled squares.
mixed halide molecules; the maximum value of Co/CD was around 0.5 for sample 1, which is also seen to have an observed A, close to the value for the chloride. Co/CD is predicted to be less than 0.05 in the vapors of samples 2 and 8”, which also have observed, , ,A values a t longer 3FeC12(s) + 1.6Br2(g) = 2FeC13(g) + FeBr3(g)(6m) wavelengths. The mole fraction of chlorine atoms in the ?&,= K&-3/2 iron halide vapor (i.e., the total number of chlorine atoms on these molecules/the total number of halogen atoms) 3FeCl,(s) + Br,(g) = FezC&(g) + FeBr2(s) (7) may be shown to be related, by the ideal mixing model, K7 = K1K3 to Xcl-by the equation 1 - l / X a = k ( l - l/Xcr) with k = 2. Figure 2 shows a good correlation between the ob3FeC12(s) Br2(g) = 2FeC13(g)+ FeBr,(s) (7m) served, , A values (estimated uncertainty f 3 nm) and the KIm= K1K4-l chlorine atom vapor mole fraction (XCJ calculated for the 3FeC12(s) + Fe2Br6(g)= 3FeBr2(s)+ Fe&&(g) (8) various samples. K , = K1K3K2-l The concentrations of free chlorine predicted in the various mixtures are smaller by a factor of to lo4 than Apparent mole fractions of Fe3+, shown in Table I, and the concentrations of free bromine. However the bromine Fe2+in the solid phases were first derived from the exexchange reaction with ferrous chloride is enhanced by pression K5 = CDRT/XFe3+2.With the value of XFez+and formation of Fe2C16 and Fe2BrC15. Calculated mole the various equilibrium constants, one may write a polfractions of Fe3+ are also quite small (see Table I); one ynomial in terms of XBi (or Xcl-) with known coefficients, would more reasonably expect a Henry’s law behavior in the solution of which leads to the composition of the solid the dilute solid solution, rather than Raoult’s law, and the and vapor phases. For example, with Co representing significance of these values must be questioned. However Fe2C&and c6 representing FezBr6,c6 = K2CBr,(XBr-4)XF++2 the impact of the presence of ferric ion in the solid on the and co= KsC6(Xcl-/XBr-)6.The bromine concentration calculated vapor phase composition was relatively small; was initially taken as Co(Br2)- CD - 0.5CM and iteration similar results were obtained with XFez+taken as unity. performed to correct for the amount of bromine predicted The dashed lines (Figure 1)represent calculated log A to enter the solid phase (the latter was generally relatively vs. 1/ T values, as derived from Co(Fe),K4,ED = 9500, and very small). E M = 2500, for each of the completely vaporized samples If only Co and c6 (and their respective monomers) are and shows the fit relative to the observed absorbances. assumed present in the vapor, the expression CD = Co The calculated line for sample 6” lies slightly above the c6 leads to a sixth power polynomial in XBr-.The roots observed values, which suggest either that the average of these equations (50) gave XBr-values in the range molar absorptivities should be somewhat smaller for the between 0.03 and 0.11. However the equilibrium vapors chlorine rich vapor mixtures, or that the analytical result are predicted to be virtually all pure Fe2C16 (mole fraction for Co(Fe) is slightly in error. The ionic solution-mixed > 0.998), which is not consistent with the observed behalide vapor model has been used to predict Co(Fe)for the havior of =A., If reaction 6 is considered with Xa- revarious samples from the indicated absorbances a t the maining at unity and Fe2C16 and FezBr6formed in a 2:l intersection points of lines drawn through the data points ratio, predicted absorbances are much lower than the in the temperature regions where a solid phase is present observed values (over half less than 10%). Hence reaction with the calculated vapor lines. Co(calcd)/Co(obsd)ratios 6 must be rejected as the exclusive mode of vaporization. were, respectively, 1.08, 1.03, and 0.997 for samples 2, 3, If mixed halide molecules are formed in the vapor, CD 5; for 6”, the ratio was 0.9; the value for 8” could not and = Co + Cl + Cz + ... C6, where the subscripts of C indicate be determined independently since Co(Br2)could not be the number of bromine atoms on the various dimer determined by experimental observation. molecules and C1 = 6C05/6C61/6, ..., C3 = 20C01/2C61/2, ... . A trial calculation of the heterogeneous sample absor(Similar expressions may be written for the monomers.) bances was made assuming formation of a single mixed The polynomial in XBr- now reduces to a cubic; roots of halide species, Fe2Br2C14(and FeBrCl,), with the estimated these equations gave bromide ion mole fractions (which thermodynamic properties shown for reaction 9. About tend to increase as the temperatures increase) over a wider 2FeC12(s)+ Br,(g) = Fe2Br2C14(g) range for the various samples, 0.02-0.47 (see Table I). (9) AGO9 = 19000 - 21.5T cal Most of the iron is now predicted to be in the form of
+
+
692
The Journal of Physical Chemistry, Vol. 83, No. 6, 1979
one-third of the calculated values were within 15% of those observed (for samples having the lowest absorbances); for those samples having the highest absorbances, calculated values were less than 30% of observed values, when the chloride ion activity was assumed to be unity. A reduced chloride ion activity would lower the predicted absorbances further. Of the possibilities examined the experimental observations seem most consistent with the assumption that the vapor phase generated by the reaction of bromine with iron(I1) chloride in the temperature interval 250-600 "C is a mixture of simple and mixed iron(II1) halides, dimers predominating a t lower temperatures and monomers increasing in importance a t higher temperatures, with the equilibrium iron(I1) phase containing substantial and variable amounts of bromide ion. The results assuming random distribution of halogen atoms among the vapor molecules and ideal ionic mixing in the solid phase do not seem unreasonable; however the absorbance data are not sufficiently discriminatory to determine how closely the system approximates this simple model. The possibility that other species, such as mixed valence (Fe(II)Fe(III)) dimers or trimers, may be present a t significant concentrations cannot be ruled out. The presence of two different halogen atoms appears to lead to a very complex molecular system.
N. W. Gregory
Acknowledgment. Appreciation is expressed to Michael Maroney for performing the atomic absorption analyses. Financial assistance from the National Science Foundation under Grant GP 37033X(CHE 73-08478 A04) is acknowledged with thanks. References and Notes (1) R. 0. MacLaren and N. W. Gregory, J . Am. Chem. SOC.,76, 5874 (1954); N. W. Gregory and T. Wydeven, J . Phys. Chem., 67, 927 (1963). (2) C. G. Maier, U. S. Bur. Mines Tech. Paper 360 (1925); R. Schoonmaker and R. Porter, J . Chem. Phys., 29, 116 (1958); R. J. Sime and N. W. Gregory, J . Phys. Chem., 64, 86 (1959); A. S. Kana'an. J. R. McCreatv. D. E. Peterson. and R. J. Thorn. Hiuh " Temo. Sei., 1, 222 (1969). (3) C. W. DeKock and D. M. Gruen, J . Chem. Phys., 44, 4387 (1966). (4) A. A. Passchier, J. D. Christian, and N. W. Gregory, J . Phys. Chem., 71. 937 (1967). (5) N. W. Gregory; J . Am. Chem. Soc., 73, 5433 (1951). (6) "JANAF Thermochemical Tables", Revised Editions, Thermal Laboratory, Dow Chemical Co., Midland, Mich.; 1965 (iron(II1) chlorides), 1966 (FeBr,), 1970 (FeCI,). (7) M. C. Lenormand, Compt. Rend., 116, 820 (1893). (8) J. D. Christian and N. W. Gregory, J . Phys. Chem., 71, 1579 (1967). (9) D. S. Rustad and N. W. Gregory, Inorg. Chem., 16, 3036 (1977). (10) N. W. Gregory, J . Phys. Chem., 81, 1857 (1977). (11) N. W. Gregory and B. A. Thackrey, J . Am. Chem. Soc., 72, 3136 (1950). (12) N. W. Gregory and R. 0. MacLaren, J . Phys. Chem., 59, 110 (1955). (13) N. Wooster, Z. Krisfaiiogr., 83, 35 (1932); N. W. Gregory, J . Am. Chem. Soc., 73, 472 (1951).
Ultraviolet-Visible Absorption of the Iron-Iodine Vapor Phase N. W. Gregory Department of Chemistry, University of Washington, Seattle, Washington 98 195 (Received September 18, 1978)
Iron iodide vapors are found to absorb light strongly in the UV-visible region of the spectrum; several unresolved peaks contribute to a dominant band with a maximum around 370 nm. Evidence is also seen for additional transitions with absorbances peaking around 305 and 280 nm on the shoulder of a strong band extending into the vacuum ultraviolet region. Vapors in equilibrium with FeI,(s) and Fe(s), unsaturated vapors (at various concentrations) in equilibrium with Fe(s), and saturated and unsaturated vapors in the presence of various concentrations of iodine have been studied. The temperature and concentration dependence of the spectra are considered relative to the concentrations of vapor species predicted from thermodynamic properties derived in earlier work. Apparent values of molar absorptivities at the peak maxima at 370 and 305 nm are derived for the species FeI,(g), Fe214(g),and FeI,(g).
Iron(I1) and iron(II1) chloride and bromide vapor molecules are known to absorb light strongly in the UV-visible region of the s p e ~ t r u m ; l a- ~study of the behavior of iron iodide molecules is now reported.14 The iodides are appreciably less stable than the chlorides and bromides; in earlier work transpiration experiments were used to study equilibrium between the vapor species FeI,, Fe214,and FeI, and the element^.^ Because of the complexity of the system thermodynamic properties derived for the equilibrium 2Fe12 = Fe214were subject to appreciable uncertainty. The present investigation was un-. dertaken to see if absorbance data could provide a test of the validity of the transpiration results as well as to search for evidence concerning the possible presence of other molecular forms. A comparison of the spectra of the 0022-3654/79/2083-0692$01 .OO/O
chloride, bromide, and iodide molecules is also of interest relative to the charge transfer processes believed responsible for these a b ~ o r b a n c e s . ~ Experimental Section Eleven mixtures were prepared by direct combination of the elements (Allied Chemical B&A Reagent Grades; resublimed iodine, 99.8%; iron wire, 99.9%) in Pyrexquartz assemblies attached to a high vacuum system. FeIz forms readily as iodine vapor streams over hot iron (around 500 "C). When vacuum sublimed in the absence of appreciable partial pressures of iodine, FeI, partially decomposes t o the elements (e.g., when a temperature gradient, ca. 600-700 " C , is maintained in evacuated systems containing FeI, and Fe, Fe crystals are observed
0 1979 American Chemical Society
