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Sublimation study of anhydrous ferric chloride
J. Chem. Thermodynamics 38 (2006) 1484–1488 www.elsevier.com/locate/jct
Sublimation study of anhydrous ferric chloride S. Blairs
*
School of Materials Science and Engineering, University of New South Wales, P.O. Box 1, Kensington 2033, Australia Received 20 October 2005; received in revised form 20 December 2005; accepted 22 December 2005 Available online 28 February 2006
Abstract Steady-state sublimation vapour pressures of anhydrous ferric chloride have been measured by the continuous gravimetric Knudseneffusion method from T = 383.9 K to 445.2 K. Based on a correlation of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ, a recommended p(T) equation has been obtained for FeCl3(cr): lg (p/Pa) = 1.895 Æ lg (T/K) 3.047 Æ 103(T/K) 3.672 Æ 107(T/K)2 7.321 Æ 103(K/ T) + 14.017. Condensation coefficients and their temperature dependence have been derived from the effusion measurements. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sublimation; Metal halides; Knudsen-effusion
1. Introduction
2. Experimental and results
The vapourization of anhydrous ferric chloride is complex and an adequate description of the vapour phase must consider: sublimation to the dimeric vapour Fe2Cl6, dissociation of the dimer into the monomer FeCl3, decomposition of the dimeric vapour into solid ferrous chloride and chlorine, and decomposition of solid ferric chloride into solid ferrous chloride and chlorine. At temperatures below 500 K the principal iron-containing vapour species in equilibrium with FeCl3(cr) is the dimer Fe2Cl6 [1]. Dimer dissociation has been studied at high temperature [2,3]. Sublimation vapour pressures of FeCl3(cr) have been made using manometric [4–7], transpiration [6,9,10], Knudseneffusion [11,12], and spectrophotometric [13,14] techniques. Dynamic studies of FeCl3(cr) [6,10,11,15] and Fe2Cl6(g) [6,15,16] decomposition indicate that equilibration is slow. These studies, suggest that a direct study of the sublimation of FeCl3(cr) to the dimeric vapour should be possible using the Knudsen-effusion method without the complication of a concomitant chlorine partial pressure.
Anhydrous FeCl3 used in this study was prepared by direct synthesis in a quartz reactor from iron of 103 total mass fraction of metallic impurities (British Iron and Steel Research Association), and anhydrous chlorine gas containing 105 total mass fraction of gaseous impurities. FeCl3 subliming from the reactor was further purified by repeated sublimations in the anhydrous chlorine flow. An iron effusion cell based on the design by Blairs et al. [17] was filled with FeCl3(cr) inside a nitrogen dry box having a mol fraction of water 62 Æ 105. Five push-fit effusion cell lids carrying knifeedged orifices were used in the continuous gravimetric Knudsen-effusion measurements. The effusion cell was suspended from an annealed and calibrated Pyrex coil with a spring constant of 10.444 ± 0.060 cm Æ g1. Spring contractions during effusion runs were measured by cathetometer to ±0.001 cm. Steady-state effusion rates w/lg Æ s1 at each effusion temperature were derived from linear least-squares plots of spring contraction against time. Effusion cells were heated by a thermostatic silicone oil bath maintained to ±0.5 K. Dynamic vacua 61.33 Æ 105 Pa were maintained during the effusion runs. Effusion cell temperatures were measured by certificated (National Physical Laboratory) mercury-in-glass thermometers to ±0.1 K immersed in the oil bath. Separate experiments confirmed that thermostat temperatures were
*
Present address: 58 Freya Street, Kareela 2232, Australia. Tel.: +61 2 93851000/95286953. E-mail address: [email protected]. 0021-9614/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.12.012
S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488 TABLE 1 Knudsen-effusion orifice quantities: a, l/r, h are the effusion orifice areas, length to radius ratios and cone angles, respectively Cell
a/mm2
l/r
h
Wi
WB
1
1.6701
0.1988 0.2709 0.2528 0.2328
0.9937 0 0.1832 0.8464
0.9953 0.8837 0.9261 0.9924
0.8992
2
0.8440
0.1986 0.3499 0.1324
1.0122 1.0995 0
0.9969 0.9976 0.9375
0.9323
3
0.3942
0.1176 0.6988
1.0908 1.0690
0.9988 0.9996
0.9952
4
0.2105
0.3409 0.5586
1.0428 1.2475
0.9971 0.9964
0.9935
5
0.1386
0.0787 0.6750 0.0899
1.2475 1.0035 0
0.996 0.9944 0.9570
0.8992
Wi and WB are the individual and overall Clausing-factors, respectively.
representative of effusion temperatures. All temperatures are reported in terms of ITS-90. Orifice areas a, lengths l, radii r and angles h were measured using a Vickers metallograph calibrated using an object micrometer. Orifice dimensions were remeasured between effusions runs and were found to be unchanged. At the completion of the effusion measurements, the orifice lids were sectioned for metallographic examination and the previous non-destructively measured orifice dimensions confirmed. a(T) were used in the calculation of sublimation vapour pressures using the appropriate linear thermal expansion coefficient for iron. Effusion orifice quantities are reported in table 1. Orifice Clausing factors WB for convergent/ divergent conical and for cylindrical geometries were taken from Iczcowski et al. [18]. Steady-state Knudsen-effusion sublimation vapour pressures for FeCl3 are reported in table 2 for the range 383.1 K to 445.2 K and are plotted in figure 1 for five different effective orifice areas aWB. Equations of form lg (p/Pa) = (A Æ K/T) + B were derived by linear least-squares treatment of the steady-state sublimation vapour pressures for each orifice size and are summarized in table 3. At temperatures exceeding those reported in this study, p(T) values fall increasingly below the extrapolated lines of lg p(T) against T1. Calculation of the Knudsen-number (k/d), i.e., the ratio of the mean free path of Fe2Cl6(g) to orifice diameter indicates that departure from linearity is attributable to the breakdown of molecular flow conditions within the effusion cell orifice rather than to sublimation-induced surface cooling. k/d varied from ca. 0.9 to 1.8. 3. Discussion Steady-state sublimation vapour pressures p were found to depend on the effective orifice area aWB. A well-known extrapolation function results from the equation: pEq = p(1 + aWB/Aac) where A is the area of the subliming solid,
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TABLE 2 Steady-state sublimation vapour pressure p for FeCl3(cr) determined by the Knudsen-effusion method w/lg Æ s1
p/Pa
386.3 386.3 392.0 396.6 398.0 401.2 406.8
0.096 0.129 0.219 0.416 0.379 0.607 0.837
Orifice 1 0.01586 412.7 0.02159 416.4 0.03693 416.4 0.07053 419.8 0.06426 422.2 0.10346 425.6 0.16572 426.8 431.6
383.9 389.3. 389.7 394.2 394.8 395.1 395.4 399.0 403.0 404.6
0.0678 0.1397 0.1214 0.2021 0.2563 0. 1770 0.2183 0.4167 0.5267 0.7641
Orifice 2 0.021598 408.6 0.044663 409.1 0.038797 409.9 0.065061 413.9 0.091059 414.1 0.057061 419.3 0.070394 419.4 0.134822 424.0 0.171185 424.2 0.249045 428.8
1.0454 1.0006 1.2166 1.7955 1.6149 2.1155 2.4789 3.4752 3.1607 5.2641
0.34250 0.32797 0.38223 0.59208 0.53262 0.70207 0.82286 1.11042 1.05324 1.76678
390.4 394.9 394.9 399.0 403.0 409.3
0.0552 0.1165 0.0727 0.1841 0.2493 0.4666
Orifice 3 0.035329 413.5 0.046796 419.9 0.074927 421.9 0.119057 424.8 0.162119 428.8 0.305841 434.4
0.7281 1.4108 1.4877 2.3191 2.9592 4.6387
0.479692 0.936587 0.989916 1.548535 1.985165 3.132400
389.6 393.8 398.9 403.0 403.3 404.5 408.6
0.0317 0.0703 0.1169 0.1392 0.1481 0.2111 0.3136
Orifice 4 0.038101 413.9 0.085059 418.4 0.142254 423.6 0.170252 426.4 0.181182 430.9 0.258511 434.0 0.385967
0.5723 0.8540 1.3152 1.1843 2.1774 3.0072
0.708740 1.063243 1.647460 1.488407 2.750566 3.811809
392.7 398.6 403.2 407.7 409.8 414.6
0.0330 0.0584 0.1329 0.1695 0.2534 0.3616
Orifice 5 0.062928 419.3 0.112257 424.9 0.257311 429.2 0.329839 434.5 0.494491 439.7 0.709662 445.2
0.6187 0.9729 1.3267 2.0801 3.1785 4.2021
1.330963 1.032502 2.648441 4.176978 6.419454 8.537941
T/K
T/K
w/lg Æ s1
p/Pa
1.895 2.858 2.787 3.638 5.607 8.290 6.843 10.727
0.32744 0.49609 0.48382 0.63395 1.00791 1.45454 1.20230 1.89451
w denotes the steady-state effusion rate (p = 101,325 Pa).
commonly selected as the cell cross-sectional area. Isothermal plots of inverse steady-state sublimation vapour pressure against effective orifice area were linear and were extrapolated to obtain equilibrium sublimation vapour pressures pEq for aWB = 0, table 3. From the slopes of such plots together with pEq, condensation coefficient values ac may be obtained. From these values, apEq(T) equation: lg (pEq/Pa) = (7424.1 ± 9.79)(K/T) + (17.75 ± 0.024) was obtained. To obtain Fe2Cl6 sublimation vapour pressures from manometric total pressure literature values one must consider partial pressure contributions resulting from dimer dissociation and decomposition processes. At
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S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488 TABLE 4 Coefficients of the equation lg (p/Pa) = (A Æ K/T) + B for the sublimation vapour pressures of FeCl3(cr) derived from total pressure and Fe2Cl6(g) and FeCl3(cr) decomposition data; spectrophotometric and effusion data, and Second-law Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ
f
g
FIGURE 1. Steady-state sublimation vapour pressures for FeCl3(cr) measured by the Knudsen-effusion method. d, Orifice 1; , orifice 2; f, orifice 3; , orifice 4; x, orifice 5; ——, equilibrium line; – – –, recommended p(T) this work; and –––, [14].
TABLE 3 Coefficients with standard deviations of the equation lg (p/Pa) = A(K/ T) + B derived from FeCl3, steady-state Knudsen and torsion effusion sublimation vapour pressure measurements determined in the temperature range T1 to T2 Reference
Method
T1/K
T2/K A/K
B
[11] [11] [11] [11] [12] [12] [12] [12] [12] This This This This This This
Knudsen Knudsen Knudsen Knudsena Knudsen Knudsen Knudsen Torsion Torsion Knudsen Knudsen Knudsen Knudsen Knudsen Knudsena
393 393 393 393 369 372 351 403 406 386 384 390 390 393 384
423 423 423 423 474 436 450 442 444 432 432 434 434 445 445
18.395 18.275 16.635 16.645 (16.809 ± 0.042) (16.369 ± 0.061) (15.481 ± 0.026) (11.949 ± 0.097) (13.590 ± 0.076) (17.79 ± 0.370) (16.47 ± 0.438) (17.90 ± 0.508) (17.16 ± 0.460) (17.36 ± 0.568) (17.75 ± 0.024)
a
study study study study study study
7790 7600 6902 6882 (6901 ± 65) (6778 ± 179) (6622 ± 319) (6987 ± 1254) (5867 ± 295) (7537 ± 151.2) (6943 ± 177.9) (7537 ± 208.3) (7187 ± 191.8) (7279 ± 233.3) (7424 ± 9.79)
pEq, extrapolated to aWB = 0.
temperatures below ca. 500 K, the contribution of FeCl3(g) to the total pressure is negligible. By combining chlorine partial pressures resulting from the decomposition of Fe2Cl6(g) with total pressure measurements, sublimation vapour pressures above FeCl3(cr) may be derived. Alterna-
Reference
A/K
B
Dgcr H m ð298:15 KÞ= ðkJ mol1 Þ
Dgcr S m ð298:15 KÞ= ðJ K1 mol1 Þ
[7]a [7]b [7]c [7]d [5]a [5]b [5]c [5]d [4]a [4]b [4]c [4]d [8]a [8]b [8]c [8]d [6]a [6]b [6]c [6]d [16]d [16]b [16]c [10]d [10]b [10]c [6]d [6]b [6]c [11]e [13]f [14]f
6677 7011 6836 6669 6620 6954 6779 6614 7431 7764 7589 7500 7434 7768 7593 7500 7142 7475 7300 7163 7919 7766 7024 7935 7782 7041 7115 6962 6220 6882 6982 7094
16.285 16.845 16.565 16.255 16.145 16.705 16.415 16.115 17.745 18.305 18.015 17.845 17.755 18.315 18.025 17.865 17.235 17.795 17.505 17.265 18.295 17.965 16.775 18.595 18.255 17.065 17.245 16.905 15.715 16.645 17.032 17.300
138.41 145.10 141.76 138.41 137.97 144.25 140.90 137.97 154.20 160.47 157.13 155.45 146.35 152.04 149.28 147.60 147.78 154.06 150.71 148.20 151.47 160.47 146.24 163.74 160.18 146.59 148.83 149.90 132.09 135.75 138.40 143.29
241.64 252.35 246.87 241.05 237.24 247.94 242.51 236.39 268.98 279.73 274.25 270.90 260.03 271.61 266.13 263.03 260.06 270.82 265.34 260.73 279.48 273.21 249.78 285.30 278.61 256.02 260.73 254.03 231.44 233.84 243.92 253.88
a b c d e f
Together with reference [6]. Together with reference [16]. Together with reference [10]. Together with reference [15]. Effusion. Spectrophotometric.
tively, sublimation vapour pressures for FeCl3(cr) may be derived by combining decomposition data for the dimeric vapour with decomposition data for solid ferric chloride. Sublimation vapour pressures obtained in this manner are summarized in table 4 as equations of form: lg (p/ Pa) = (A Æ K/T) + B where the coefficients A and B were obtained by linear least-squares treatment of the literature values. Similarly, the literature sublimation vapour pressures for FeCl3(cr) obtained using the transpiration, effusion and spectrophotometric methods resulted in the coefficients A and B given in table 4. Second-law values of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ also given in table 4 were calculated assuming that the coefficients A and B apply at the mean temperature of the various ranges together with the following C p;m ðT Þ for FeCl3(cr) and Fe2Cl6(g). At temperatures exceeding those reported in this
S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488
study, p(T) values fall increasingly below the extrapolated lines of lg p(T) against T1. For FeCl3(cr), Stuve et al. [19] have observed a large k transition in C p;m ðT Þ at 8.4 K; resulting in S m ð298:15 KÞ ¼ ð147:82 0:29 J K mol1 Þ. This exceeds the value of S m ð298:15 KÞ ¼ ð134:72 1:67Þ J K mol1 reported by Todd and Coughlin [20] which excludes the magnetic spin contribution. S m ð298:15 KÞ and C p;m ðT Þ for FeCl3(cr) as reported by Stuve et al. [19] were adopted in this study. Linear least-squares regression of their C p;m ðT Þ gave the equation: C p;m ðT Þ=J K mol1 ¼ ð66:87 0:80Þ þ ð0:102 0:003ÞðT =KÞ. For Fe2Cl6(g), S m ðT Þ and C p;m ðT Þ given in the JANAF [21] and Barin [22] compilations are based on the molecular structure, bond distances and angles obtained using electron diffraction by Zasorin et al. [23]. The latter values differ significantly from those obtained by Hassel and Viervoll [24]. As an alternative approach, a second-order polynomial was fit to the data tabulated by Frey et al. [25] for the range 298.15 K to 1000 K and resulted in: C p;m =J K mol1 ¼ 149:5 þ 7:315 102 ðT =KÞ 4:22 105 2 ðT =KÞ and S m ð298:15 KÞ ¼ 533:04 J K mol1 for the dimeric vapour. The latter C p;m values result from a farinfra-red matrix spectral study of Fe2Cl6(g), and were calculated for a rigid-rotator harmonic oscillator ideal-gas dimeric molecule and are typically 4 J Æ K Æ mol1 lower than the corresponding JANAF [21] and Barin [22] values. The value Dgcr C p;m =J K mol1 ¼ 15:76 13:045 102 ðT =KÞ 2 4:22 105 ðT =KÞ for the sublimation of FeCl3(cr) to Fe2Cl6(g) was used in the present study. Fe2Cl6 Raman spectral studies by Nalbandian and Papatheodorou [26] interpreted in terms of D2h molecular symmetry, result in nearly identical C p;m to those adopted in this work although their S m ð298:15 KÞ ¼ 506:860 J K mol1 is substantially lower. Using this Dgcr C p;m , results in Dgcr H m ð298:15 KÞ= kJ mol1 ¼ 146:42 and Dgcr S m ð298:15 KÞ=J K mol1 ¼ 255:85 from the present effusion measurements. Application of the same Dgcr C p;m to the two spectrophotometric
g
f
FIGURE 2. Correlation of molar enthalpy and entropy of sublimation at 298.15 K for FeCl3(cr). x, [4]; , [5]; , [6]; f, [7]; g, [8]; d, [10]; , [11]; s, [13]; j, [14]; r, [16]; and n, this work.
1487
studies [13,14], results in Dgcr H m ð298:15 KÞ=kJ mol1 ¼ 138:39 and 143.29, respectively, and Dgcr S m ð298:15 KÞ= J K mol1 ¼ 243:92 and 253.88, respectively. The coefficients A and B from table 4 together with this equation for Dgcr C p;m were used to calculate the values of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ summarized in table 4 and have been plotted in figure 2 as Dgcr H m ð298:15 KÞ against Dgcr S m ð298:15 KÞ. The values in figure 2 were linearly correlated by the least-squares equation: Dgcr H m ð298:15 KÞ=kJ mol1 ¼ ð516:50 27:95ÞDgcr S m ð298:15 KÞ= J K mol1 þ ð14210 7245Þ with a correlation coefficient of 0.92. Previous studies [27,28], have shown that values of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ generated from sets of lg (p/Pa) against T1 are frequently linearly correlated. McCreary and Thorn [27] suggest as an explanation for this type of correlation, that the error or errors inadvertently encountered in vapour pressure determinations, are in the sense of Dgcr H m ðT Þ against Dgcr S m ðT Þ systematic rather than random. Thus, using a value of Dgcr S m ð298:15 KÞ ¼ 237:36 J K mol1 , the corresponding value of Dgcr H m ð298:15 KÞ is 136.86 kJ Æ mol1 as shown in figure 2. This value of Dgcr H m ð298:15 KÞ together with the value selected for Dgcr C p;m has been used to derive a recommended sublimation vapour pressure equation for FeCl3(cr): lg (p/Pa) = 1.895lg (T/K) 3.047 Æ 103(T/K) 3.672 Æ 107(T/K)2 7.321 Æ 103(K/T) + 14.017. The line represented by this recommended equation is shown on figure 1. Rustad and Gregory [14], have used a Second-law Sigma function method to obtain an alternative equation for the sublimation vapour pressure of FeCl3(cr): lg (p/Pa) = 29.69lg (T/K) 2.8107 Æ 102(T/K) + 3.672 Æ 106(T/K)2 5.408 Æ 103(K/T) 54.21. Equilibrium sublimation vapour pressures for aWB = 0 obtained in this study are ca. 1.4 to 1.8 times lower than those determined by Hammer and Gregory [11]. Knudsen effusion p(T) values obtained by Landsberg et al. [12] are of comparable magnitude to the present measurements for the smallest aWB used and the dependence of p(T) on aWB was confirmed. However, Torsion effusion p(T) obtained by Landsberg et al. [12] exhibit lower absolute values and slopes. The enthalpy of sublimation Dgcr H m ð298:15 KÞ is also given by the difference between the enthalpies of formation Df H m ð298:15 KÞ of Fe2Cl6(g) and FeCl3(cr). The former Df H m ð298:15 KÞ may be obtained by Second and Thirdlaw analysis of the relevant equilibria [21]. From JANAF [21], Df H m ð298:15 KÞ for Fe2Cl6(g) is reported as (654.38 ± 8 kJ Æ mol1). Df H m ð298:15 KÞ for FeCl3(cr). has been measured and reviewed by Lavut et al. [29] and report a value of Df H m ð298:15 KÞ ¼ ð396:02 0:14Þ kJ mol1 , (FeCl3(cr), hexagonal, p = 101.325 kPa, 298.15 K). These values result in Dgcr H m ð298:15 KÞ ¼ ð137:66 8:6 kJ mol1 Þ as compared to Dgcr H m ð298:15 KÞ ¼ ð136:83 13:9 kJ mol1 Þ obtained from the correlation of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ. Condensation coefficients ac were obtained from the slopes and intercepts of isothermal linear plots of inverse
1488
S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488
TABLE 5 g # Condensation coefficients ac, temperature coefficients d(lg ac)/d(1/T), activation sublimation enthalpy Dgcr H # m and entropy Dcr S m , activation condensation cr # # enthalpy Dcr H and entropy D S for FeCl (cr) at 408 K 3 g g m m ac
d(lg ac)/d(1/T)
1 Dgcr H # m =ðkJ mol Þ
Dgcr S # m =R
1 # Dcr g H m =ðkJ mol Þ
# Dcr g S m =R
Dgcr H m =ðkJ mol1 Þ
Dgcr S m =R
0.007 [29] (0.0296 ± 0.006)a
1381 0
162.7 142.2
210.9 167.6
26.35 0
23.4 29.2
136.4 142.2
234.3 244.1
a
This study.
steady-state sublimation vapour pressures and aWB and are summarized in table 5 with those obtained by Hammer and Gregory [11]. ac = (0.0296 ± 0.005) was not observed to change significantly with temperature within the precision of the present effusion measurements. In contrast, ac(T) was observed to increase with temperature (dac/ dT > 0) by Hammer and Gregory [11]. A necessary criticism of the latter values is that ac(T) resulted from an inordinate extrapolation of three data points in a procedure which is inherently lever-sensitive. From the slopes and intercepts of lg ac against T1, activation sublimation g # enthalpies Dgcr H # m and entropies Dcr S m and corresponding cr # cr # values Dg H m and Dg S m (relative to the solid) for the condensation process may be obtained. Values obtained by Gregory [30] and in this study are summarised in table 5 for 408 K. For sublimation at 408 K, the activated state g for Fe2Cl6(g) indicates that Dgcr H # m ¼ Dcr H m , the customary g standard sublimation enthalpy, while Dgcr S # m ¼ Dcr S m 1 76:5 J K mol . For condensation, the activation # condensation enthalpy Dcr g H m ¼ 0 and the activation concr # densation entropy Dg S m ¼ Dgcr S m 76:5 J K mol1 . The corresponding quantities suggested by Gregory [30] are: g 1 g Dgcr H # and Dgcr S # m ¼ Dcr S m 23:4 m ¼ Dcr H m þ 26:3 kJ mol 1 cr # g J K mol for sublimation; Dg H m ¼ Dcr H m þ 26:3 1 g 1 # kJ mol and Dcr for cong S m ¼ Dcr S m 23:4 J K mol cr # densation. Gregory suggests that Dg S m > 0 and # Dcr g H m > 0 for condensation suggest an activated state comprising FeCl3(g) molecules or pairs of FeCl3(g) molecules i.e., incipient Fe2Cl6 molecules. Factors resulting in low ac are poorly understood. A general correlation exists between the structure of gas phase species and their existence or otherwise in the solid state. Thus, low ac are often observed in systems where there is difficulty in incorporating into a solid surface structural units which are absent in the solid state. FeCl3(cr) forms a layer-type lattice in which sheets of Fe3+ in an hexagonal array are sandwiched between sheets of Cl also in an hexagonal array such that each Fe3+ is almost perfectly octahedrally coordinated by Cl with an Fe–Cl bond distance of 0.249 nm. Electron diffraction and infra-red spectra of Fe2Cl6 are consistent with an out of plane bridged structure of the Al2Cl6 type in which iron is tetrahedrally coordinated with an Fe–Cl bond distance of 0.211 nm to 0.217 nm. For an Fe2Cl6 molecule to condense from the vapour there is a radical change of iron coordination and an increase in Fe–Cl bond length. Appraisal of the lattice unit cell indicates the existence of distorted Fe2Cl6 mole-
cules with tetrahedral coordination of iron. It is suggested that the above factors result in the observed low ac observed in this system and that the activation enthalpies and entropies are consistent with an alternative activated state comprising distorted Fe2Cl6(g) molecules. References [1] J. Wilmhurst, J. Mol. Spectrosc. 5 (1960) 343. [2] K. Kangro, H.F. Bernstorff, Z. Anorg. Allgem. Chem. 263 (1949) 316. [3] H.Z. Schafer, Anorg. Allg. Chem. 53 (1949) 259. [4] E. Stirnemann, Neues Jahrb. Mineral. Geol. Palaeontol. Bellageband, Abt. A 52 (1925) 368. [5] K. Sano, J. Chem. Soc. Jpn. 1073 (1938) 59. [6] L.N. Wilson, N.W. Gregory, J. Phys. Chem. 52 (1958) 433. [7] C.G. Maier, U.S. Bur. Mines Tech. Paper 1925, No. 360. [8] H.F. Johstone, H.C. Weingartner, W.E. Winsche, J. Am. Chem. Soc. 64 (1942) 241. [9] K. Jellineck, A. Rudat, Z. Phys. Chem. Abt. A 143 (1929) 55. [10] O.E. Ringwald, Doctoral Dissertation, Princeton University, New Jersy, 1949. [11] R.R. Hammer, N.W. Gregory, J. Phys. Chem. 66 (1962) 1705. [12] A. Landsberg, A. Adams, S.D. Hill, U.S. Bur. Mines R.I. (1977) 8207. [13] C.F. Shieh, N.W. Gregory, J. Phys. Chem. 79 (1975) 828. [14] D.S. Rustad, N.W. Gregory, J. Chem. Eng. Data 28 (1983) 151. [15] H. Shafer, E. Oeler, Z. Anorg. Allg. Chem. 271 (1953) 206. [16] W. Kangro, E. Petersen, Z. Anorg. Allg. Chem. 216 (1950) 157. [17] S. Blairs, R.A.J. Shelton, R. Unsworth, J. Sci. Instrum. 38 (1961) 469. [18] R.P. Iczcowski, J.L. Margrave, S.M. Robinson, J. Phys. Chem. 67 (1963) 229. [19] J.M. Stuve, M.J. Ferrante, D.W. Richardson, R.R. Brown, U.S. Bur. Mines R.I. (1980) 8420. [20] S.S. Todd, J.P. Coughlin, J. Am. Chem. Soc. 73 (1951) 4184. [21] M.W. Chase, C.A. Davies, J.R. Downey, D.J. Frurip, R.A. McDonald, A.N. Syverud, JANAF Thermochemical Tables, third ed., J. Phys. Chem. Ref. Data (1985) 14. [22] I. Barin, Thermochemical Data of Pure Substances, VCH, Weinheim, 1989. [23] E.Z. Zazorin, N.G. Rambidi, P.A. Akishin, Zh. Strukt. Chim. 4 (1963) 910. [24] O. Hassel, H. Viervoll, Tidskr. Kjem. Berg. Metall. (1943). [25] R.A. Frey, R.D. Werder, Hs.H. Gu¨nthard, J. Mol. Spectrosc. 35 (1970) 260. [26] L. Nalbandian, G.N. Papatheodorou, High Temp. Sci. 28 (1990) 49. [27] J.R. McCreary, R.J. Thorn, J. Chem. Phys. 48 (1967) 3290. [28] R.F. Brebrick, High Temp. Sci. 8 (1976) 11. [29] E.G. Lavut, B.I. Timofeyev, V.M. Yuldashva, J. Chem. Thermodyn. 16 (1984) 101. [30] N.W. Gregory, J. Phys. Chem. 67 (1963) 618.
JCT 05-259
Sublimation study of anhydrous ferric chloride S. Blairs
*
School of Materials Science and Engineering, University of New South Wales, P.O. Box 1, Kensington 2033, Australia Received 20 October 2005; received in revised form 20 December 2005; accepted 22 December 2005 Available online 28 February 2006
Abstract Steady-state sublimation vapour pressures of anhydrous ferric chloride have been measured by the continuous gravimetric Knudseneffusion method from T = 383.9 K to 445.2 K. Based on a correlation of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ, a recommended p(T) equation has been obtained for FeCl3(cr): lg (p/Pa) = 1.895 Æ lg (T/K) 3.047 Æ 103(T/K) 3.672 Æ 107(T/K)2 7.321 Æ 103(K/ T) + 14.017. Condensation coefficients and their temperature dependence have been derived from the effusion measurements. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Sublimation; Metal halides; Knudsen-effusion
1. Introduction
2. Experimental and results
The vapourization of anhydrous ferric chloride is complex and an adequate description of the vapour phase must consider: sublimation to the dimeric vapour Fe2Cl6, dissociation of the dimer into the monomer FeCl3, decomposition of the dimeric vapour into solid ferrous chloride and chlorine, and decomposition of solid ferric chloride into solid ferrous chloride and chlorine. At temperatures below 500 K the principal iron-containing vapour species in equilibrium with FeCl3(cr) is the dimer Fe2Cl6 [1]. Dimer dissociation has been studied at high temperature [2,3]. Sublimation vapour pressures of FeCl3(cr) have been made using manometric [4–7], transpiration [6,9,10], Knudseneffusion [11,12], and spectrophotometric [13,14] techniques. Dynamic studies of FeCl3(cr) [6,10,11,15] and Fe2Cl6(g) [6,15,16] decomposition indicate that equilibration is slow. These studies, suggest that a direct study of the sublimation of FeCl3(cr) to the dimeric vapour should be possible using the Knudsen-effusion method without the complication of a concomitant chlorine partial pressure.
Anhydrous FeCl3 used in this study was prepared by direct synthesis in a quartz reactor from iron of 103 total mass fraction of metallic impurities (British Iron and Steel Research Association), and anhydrous chlorine gas containing 105 total mass fraction of gaseous impurities. FeCl3 subliming from the reactor was further purified by repeated sublimations in the anhydrous chlorine flow. An iron effusion cell based on the design by Blairs et al. [17] was filled with FeCl3(cr) inside a nitrogen dry box having a mol fraction of water 62 Æ 105. Five push-fit effusion cell lids carrying knifeedged orifices were used in the continuous gravimetric Knudsen-effusion measurements. The effusion cell was suspended from an annealed and calibrated Pyrex coil with a spring constant of 10.444 ± 0.060 cm Æ g1. Spring contractions during effusion runs were measured by cathetometer to ±0.001 cm. Steady-state effusion rates w/lg Æ s1 at each effusion temperature were derived from linear least-squares plots of spring contraction against time. Effusion cells were heated by a thermostatic silicone oil bath maintained to ±0.5 K. Dynamic vacua 61.33 Æ 105 Pa were maintained during the effusion runs. Effusion cell temperatures were measured by certificated (National Physical Laboratory) mercury-in-glass thermometers to ±0.1 K immersed in the oil bath. Separate experiments confirmed that thermostat temperatures were
*
Present address: 58 Freya Street, Kareela 2232, Australia. Tel.: +61 2 93851000/95286953. E-mail address: [email protected]. 0021-9614/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2005.12.012
S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488 TABLE 1 Knudsen-effusion orifice quantities: a, l/r, h are the effusion orifice areas, length to radius ratios and cone angles, respectively Cell
a/mm2
l/r
h
Wi
WB
1
1.6701
0.1988 0.2709 0.2528 0.2328
0.9937 0 0.1832 0.8464
0.9953 0.8837 0.9261 0.9924
0.8992
2
0.8440
0.1986 0.3499 0.1324
1.0122 1.0995 0
0.9969 0.9976 0.9375
0.9323
3
0.3942
0.1176 0.6988
1.0908 1.0690
0.9988 0.9996
0.9952
4
0.2105
0.3409 0.5586
1.0428 1.2475
0.9971 0.9964
0.9935
5
0.1386
0.0787 0.6750 0.0899
1.2475 1.0035 0
0.996 0.9944 0.9570
0.8992
Wi and WB are the individual and overall Clausing-factors, respectively.
representative of effusion temperatures. All temperatures are reported in terms of ITS-90. Orifice areas a, lengths l, radii r and angles h were measured using a Vickers metallograph calibrated using an object micrometer. Orifice dimensions were remeasured between effusions runs and were found to be unchanged. At the completion of the effusion measurements, the orifice lids were sectioned for metallographic examination and the previous non-destructively measured orifice dimensions confirmed. a(T) were used in the calculation of sublimation vapour pressures using the appropriate linear thermal expansion coefficient for iron. Effusion orifice quantities are reported in table 1. Orifice Clausing factors WB for convergent/ divergent conical and for cylindrical geometries were taken from Iczcowski et al. [18]. Steady-state Knudsen-effusion sublimation vapour pressures for FeCl3 are reported in table 2 for the range 383.1 K to 445.2 K and are plotted in figure 1 for five different effective orifice areas aWB. Equations of form lg (p/Pa) = (A Æ K/T) + B were derived by linear least-squares treatment of the steady-state sublimation vapour pressures for each orifice size and are summarized in table 3. At temperatures exceeding those reported in this study, p(T) values fall increasingly below the extrapolated lines of lg p(T) against T1. Calculation of the Knudsen-number (k/d), i.e., the ratio of the mean free path of Fe2Cl6(g) to orifice diameter indicates that departure from linearity is attributable to the breakdown of molecular flow conditions within the effusion cell orifice rather than to sublimation-induced surface cooling. k/d varied from ca. 0.9 to 1.8. 3. Discussion Steady-state sublimation vapour pressures p were found to depend on the effective orifice area aWB. A well-known extrapolation function results from the equation: pEq = p(1 + aWB/Aac) where A is the area of the subliming solid,
1485
TABLE 2 Steady-state sublimation vapour pressure p for FeCl3(cr) determined by the Knudsen-effusion method w/lg Æ s1
p/Pa
386.3 386.3 392.0 396.6 398.0 401.2 406.8
0.096 0.129 0.219 0.416 0.379 0.607 0.837
Orifice 1 0.01586 412.7 0.02159 416.4 0.03693 416.4 0.07053 419.8 0.06426 422.2 0.10346 425.6 0.16572 426.8 431.6
383.9 389.3. 389.7 394.2 394.8 395.1 395.4 399.0 403.0 404.6
0.0678 0.1397 0.1214 0.2021 0.2563 0. 1770 0.2183 0.4167 0.5267 0.7641
Orifice 2 0.021598 408.6 0.044663 409.1 0.038797 409.9 0.065061 413.9 0.091059 414.1 0.057061 419.3 0.070394 419.4 0.134822 424.0 0.171185 424.2 0.249045 428.8
1.0454 1.0006 1.2166 1.7955 1.6149 2.1155 2.4789 3.4752 3.1607 5.2641
0.34250 0.32797 0.38223 0.59208 0.53262 0.70207 0.82286 1.11042 1.05324 1.76678
390.4 394.9 394.9 399.0 403.0 409.3
0.0552 0.1165 0.0727 0.1841 0.2493 0.4666
Orifice 3 0.035329 413.5 0.046796 419.9 0.074927 421.9 0.119057 424.8 0.162119 428.8 0.305841 434.4
0.7281 1.4108 1.4877 2.3191 2.9592 4.6387
0.479692 0.936587 0.989916 1.548535 1.985165 3.132400
389.6 393.8 398.9 403.0 403.3 404.5 408.6
0.0317 0.0703 0.1169 0.1392 0.1481 0.2111 0.3136
Orifice 4 0.038101 413.9 0.085059 418.4 0.142254 423.6 0.170252 426.4 0.181182 430.9 0.258511 434.0 0.385967
0.5723 0.8540 1.3152 1.1843 2.1774 3.0072
0.708740 1.063243 1.647460 1.488407 2.750566 3.811809
392.7 398.6 403.2 407.7 409.8 414.6
0.0330 0.0584 0.1329 0.1695 0.2534 0.3616
Orifice 5 0.062928 419.3 0.112257 424.9 0.257311 429.2 0.329839 434.5 0.494491 439.7 0.709662 445.2
0.6187 0.9729 1.3267 2.0801 3.1785 4.2021
1.330963 1.032502 2.648441 4.176978 6.419454 8.537941
T/K
T/K
w/lg Æ s1
p/Pa
1.895 2.858 2.787 3.638 5.607 8.290 6.843 10.727
0.32744 0.49609 0.48382 0.63395 1.00791 1.45454 1.20230 1.89451
w denotes the steady-state effusion rate (p = 101,325 Pa).
commonly selected as the cell cross-sectional area. Isothermal plots of inverse steady-state sublimation vapour pressure against effective orifice area were linear and were extrapolated to obtain equilibrium sublimation vapour pressures pEq for aWB = 0, table 3. From the slopes of such plots together with pEq, condensation coefficient values ac may be obtained. From these values, apEq(T) equation: lg (pEq/Pa) = (7424.1 ± 9.79)(K/T) + (17.75 ± 0.024) was obtained. To obtain Fe2Cl6 sublimation vapour pressures from manometric total pressure literature values one must consider partial pressure contributions resulting from dimer dissociation and decomposition processes. At
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S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488 TABLE 4 Coefficients of the equation lg (p/Pa) = (A Æ K/T) + B for the sublimation vapour pressures of FeCl3(cr) derived from total pressure and Fe2Cl6(g) and FeCl3(cr) decomposition data; spectrophotometric and effusion data, and Second-law Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ
f
g
FIGURE 1. Steady-state sublimation vapour pressures for FeCl3(cr) measured by the Knudsen-effusion method. d, Orifice 1; , orifice 2; f, orifice 3; , orifice 4; x, orifice 5; ——, equilibrium line; – – –, recommended p(T) this work; and –––, [14].
TABLE 3 Coefficients with standard deviations of the equation lg (p/Pa) = A(K/ T) + B derived from FeCl3, steady-state Knudsen and torsion effusion sublimation vapour pressure measurements determined in the temperature range T1 to T2 Reference
Method
T1/K
T2/K A/K
B
[11] [11] [11] [11] [12] [12] [12] [12] [12] This This This This This This
Knudsen Knudsen Knudsen Knudsena Knudsen Knudsen Knudsen Torsion Torsion Knudsen Knudsen Knudsen Knudsen Knudsen Knudsena
393 393 393 393 369 372 351 403 406 386 384 390 390 393 384
423 423 423 423 474 436 450 442 444 432 432 434 434 445 445
18.395 18.275 16.635 16.645 (16.809 ± 0.042) (16.369 ± 0.061) (15.481 ± 0.026) (11.949 ± 0.097) (13.590 ± 0.076) (17.79 ± 0.370) (16.47 ± 0.438) (17.90 ± 0.508) (17.16 ± 0.460) (17.36 ± 0.568) (17.75 ± 0.024)
a
study study study study study study
7790 7600 6902 6882 (6901 ± 65) (6778 ± 179) (6622 ± 319) (6987 ± 1254) (5867 ± 295) (7537 ± 151.2) (6943 ± 177.9) (7537 ± 208.3) (7187 ± 191.8) (7279 ± 233.3) (7424 ± 9.79)
pEq, extrapolated to aWB = 0.
temperatures below ca. 500 K, the contribution of FeCl3(g) to the total pressure is negligible. By combining chlorine partial pressures resulting from the decomposition of Fe2Cl6(g) with total pressure measurements, sublimation vapour pressures above FeCl3(cr) may be derived. Alterna-
Reference
A/K
B
Dgcr H m ð298:15 KÞ= ðkJ mol1 Þ
Dgcr S m ð298:15 KÞ= ðJ K1 mol1 Þ
[7]a [7]b [7]c [7]d [5]a [5]b [5]c [5]d [4]a [4]b [4]c [4]d [8]a [8]b [8]c [8]d [6]a [6]b [6]c [6]d [16]d [16]b [16]c [10]d [10]b [10]c [6]d [6]b [6]c [11]e [13]f [14]f
6677 7011 6836 6669 6620 6954 6779 6614 7431 7764 7589 7500 7434 7768 7593 7500 7142 7475 7300 7163 7919 7766 7024 7935 7782 7041 7115 6962 6220 6882 6982 7094
16.285 16.845 16.565 16.255 16.145 16.705 16.415 16.115 17.745 18.305 18.015 17.845 17.755 18.315 18.025 17.865 17.235 17.795 17.505 17.265 18.295 17.965 16.775 18.595 18.255 17.065 17.245 16.905 15.715 16.645 17.032 17.300
138.41 145.10 141.76 138.41 137.97 144.25 140.90 137.97 154.20 160.47 157.13 155.45 146.35 152.04 149.28 147.60 147.78 154.06 150.71 148.20 151.47 160.47 146.24 163.74 160.18 146.59 148.83 149.90 132.09 135.75 138.40 143.29
241.64 252.35 246.87 241.05 237.24 247.94 242.51 236.39 268.98 279.73 274.25 270.90 260.03 271.61 266.13 263.03 260.06 270.82 265.34 260.73 279.48 273.21 249.78 285.30 278.61 256.02 260.73 254.03 231.44 233.84 243.92 253.88
a b c d e f
Together with reference [6]. Together with reference [16]. Together with reference [10]. Together with reference [15]. Effusion. Spectrophotometric.
tively, sublimation vapour pressures for FeCl3(cr) may be derived by combining decomposition data for the dimeric vapour with decomposition data for solid ferric chloride. Sublimation vapour pressures obtained in this manner are summarized in table 4 as equations of form: lg (p/ Pa) = (A Æ K/T) + B where the coefficients A and B were obtained by linear least-squares treatment of the literature values. Similarly, the literature sublimation vapour pressures for FeCl3(cr) obtained using the transpiration, effusion and spectrophotometric methods resulted in the coefficients A and B given in table 4. Second-law values of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ also given in table 4 were calculated assuming that the coefficients A and B apply at the mean temperature of the various ranges together with the following C p;m ðT Þ for FeCl3(cr) and Fe2Cl6(g). At temperatures exceeding those reported in this
S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488
study, p(T) values fall increasingly below the extrapolated lines of lg p(T) against T1. For FeCl3(cr), Stuve et al. [19] have observed a large k transition in C p;m ðT Þ at 8.4 K; resulting in S m ð298:15 KÞ ¼ ð147:82 0:29 J K mol1 Þ. This exceeds the value of S m ð298:15 KÞ ¼ ð134:72 1:67Þ J K mol1 reported by Todd and Coughlin [20] which excludes the magnetic spin contribution. S m ð298:15 KÞ and C p;m ðT Þ for FeCl3(cr) as reported by Stuve et al. [19] were adopted in this study. Linear least-squares regression of their C p;m ðT Þ gave the equation: C p;m ðT Þ=J K mol1 ¼ ð66:87 0:80Þ þ ð0:102 0:003ÞðT =KÞ. For Fe2Cl6(g), S m ðT Þ and C p;m ðT Þ given in the JANAF [21] and Barin [22] compilations are based on the molecular structure, bond distances and angles obtained using electron diffraction by Zasorin et al. [23]. The latter values differ significantly from those obtained by Hassel and Viervoll [24]. As an alternative approach, a second-order polynomial was fit to the data tabulated by Frey et al. [25] for the range 298.15 K to 1000 K and resulted in: C p;m =J K mol1 ¼ 149:5 þ 7:315 102 ðT =KÞ 4:22 105 2 ðT =KÞ and S m ð298:15 KÞ ¼ 533:04 J K mol1 for the dimeric vapour. The latter C p;m values result from a farinfra-red matrix spectral study of Fe2Cl6(g), and were calculated for a rigid-rotator harmonic oscillator ideal-gas dimeric molecule and are typically 4 J Æ K Æ mol1 lower than the corresponding JANAF [21] and Barin [22] values. The value Dgcr C p;m =J K mol1 ¼ 15:76 13:045 102 ðT =KÞ 2 4:22 105 ðT =KÞ for the sublimation of FeCl3(cr) to Fe2Cl6(g) was used in the present study. Fe2Cl6 Raman spectral studies by Nalbandian and Papatheodorou [26] interpreted in terms of D2h molecular symmetry, result in nearly identical C p;m to those adopted in this work although their S m ð298:15 KÞ ¼ 506:860 J K mol1 is substantially lower. Using this Dgcr C p;m , results in Dgcr H m ð298:15 KÞ= kJ mol1 ¼ 146:42 and Dgcr S m ð298:15 KÞ=J K mol1 ¼ 255:85 from the present effusion measurements. Application of the same Dgcr C p;m to the two spectrophotometric
g
f
FIGURE 2. Correlation of molar enthalpy and entropy of sublimation at 298.15 K for FeCl3(cr). x, [4]; , [5]; , [6]; f, [7]; g, [8]; d, [10]; , [11]; s, [13]; j, [14]; r, [16]; and n, this work.
1487
studies [13,14], results in Dgcr H m ð298:15 KÞ=kJ mol1 ¼ 138:39 and 143.29, respectively, and Dgcr S m ð298:15 KÞ= J K mol1 ¼ 243:92 and 253.88, respectively. The coefficients A and B from table 4 together with this equation for Dgcr C p;m were used to calculate the values of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ summarized in table 4 and have been plotted in figure 2 as Dgcr H m ð298:15 KÞ against Dgcr S m ð298:15 KÞ. The values in figure 2 were linearly correlated by the least-squares equation: Dgcr H m ð298:15 KÞ=kJ mol1 ¼ ð516:50 27:95ÞDgcr S m ð298:15 KÞ= J K mol1 þ ð14210 7245Þ with a correlation coefficient of 0.92. Previous studies [27,28], have shown that values of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ generated from sets of lg (p/Pa) against T1 are frequently linearly correlated. McCreary and Thorn [27] suggest as an explanation for this type of correlation, that the error or errors inadvertently encountered in vapour pressure determinations, are in the sense of Dgcr H m ðT Þ against Dgcr S m ðT Þ systematic rather than random. Thus, using a value of Dgcr S m ð298:15 KÞ ¼ 237:36 J K mol1 , the corresponding value of Dgcr H m ð298:15 KÞ is 136.86 kJ Æ mol1 as shown in figure 2. This value of Dgcr H m ð298:15 KÞ together with the value selected for Dgcr C p;m has been used to derive a recommended sublimation vapour pressure equation for FeCl3(cr): lg (p/Pa) = 1.895lg (T/K) 3.047 Æ 103(T/K) 3.672 Æ 107(T/K)2 7.321 Æ 103(K/T) + 14.017. The line represented by this recommended equation is shown on figure 1. Rustad and Gregory [14], have used a Second-law Sigma function method to obtain an alternative equation for the sublimation vapour pressure of FeCl3(cr): lg (p/Pa) = 29.69lg (T/K) 2.8107 Æ 102(T/K) + 3.672 Æ 106(T/K)2 5.408 Æ 103(K/T) 54.21. Equilibrium sublimation vapour pressures for aWB = 0 obtained in this study are ca. 1.4 to 1.8 times lower than those determined by Hammer and Gregory [11]. Knudsen effusion p(T) values obtained by Landsberg et al. [12] are of comparable magnitude to the present measurements for the smallest aWB used and the dependence of p(T) on aWB was confirmed. However, Torsion effusion p(T) obtained by Landsberg et al. [12] exhibit lower absolute values and slopes. The enthalpy of sublimation Dgcr H m ð298:15 KÞ is also given by the difference between the enthalpies of formation Df H m ð298:15 KÞ of Fe2Cl6(g) and FeCl3(cr). The former Df H m ð298:15 KÞ may be obtained by Second and Thirdlaw analysis of the relevant equilibria [21]. From JANAF [21], Df H m ð298:15 KÞ for Fe2Cl6(g) is reported as (654.38 ± 8 kJ Æ mol1). Df H m ð298:15 KÞ for FeCl3(cr). has been measured and reviewed by Lavut et al. [29] and report a value of Df H m ð298:15 KÞ ¼ ð396:02 0:14Þ kJ mol1 , (FeCl3(cr), hexagonal, p = 101.325 kPa, 298.15 K). These values result in Dgcr H m ð298:15 KÞ ¼ ð137:66 8:6 kJ mol1 Þ as compared to Dgcr H m ð298:15 KÞ ¼ ð136:83 13:9 kJ mol1 Þ obtained from the correlation of Dgcr H m ð298:15 KÞ and Dgcr S m ð298:15 KÞ. Condensation coefficients ac were obtained from the slopes and intercepts of isothermal linear plots of inverse
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S. Blairs / J. Chem. Thermodynamics 38 (2006) 1484–1488
TABLE 5 g # Condensation coefficients ac, temperature coefficients d(lg ac)/d(1/T), activation sublimation enthalpy Dgcr H # m and entropy Dcr S m , activation condensation cr # # enthalpy Dcr H and entropy D S for FeCl (cr) at 408 K 3 g g m m ac
d(lg ac)/d(1/T)
1 Dgcr H # m =ðkJ mol Þ
Dgcr S # m =R
1 # Dcr g H m =ðkJ mol Þ
# Dcr g S m =R
Dgcr H m =ðkJ mol1 Þ
Dgcr S m =R
0.007 [29] (0.0296 ± 0.006)a
1381 0
162.7 142.2
210.9 167.6
26.35 0
23.4 29.2
136.4 142.2
234.3 244.1
a
This study.
steady-state sublimation vapour pressures and aWB and are summarized in table 5 with those obtained by Hammer and Gregory [11]. ac = (0.0296 ± 0.005) was not observed to change significantly with temperature within the precision of the present effusion measurements. In contrast, ac(T) was observed to increase with temperature (dac/ dT > 0) by Hammer and Gregory [11]. A necessary criticism of the latter values is that ac(T) resulted from an inordinate extrapolation of three data points in a procedure which is inherently lever-sensitive. From the slopes and intercepts of lg ac against T1, activation sublimation g # enthalpies Dgcr H # m and entropies Dcr S m and corresponding cr # cr # values Dg H m and Dg S m (relative to the solid) for the condensation process may be obtained. Values obtained by Gregory [30] and in this study are summarised in table 5 for 408 K. For sublimation at 408 K, the activated state g for Fe2Cl6(g) indicates that Dgcr H # m ¼ Dcr H m , the customary g standard sublimation enthalpy, while Dgcr S # m ¼ Dcr S m 1 76:5 J K mol . For condensation, the activation # condensation enthalpy Dcr g H m ¼ 0 and the activation concr # densation entropy Dg S m ¼ Dgcr S m 76:5 J K mol1 . The corresponding quantities suggested by Gregory [30] are: g 1 g Dgcr H # and Dgcr S # m ¼ Dcr S m 23:4 m ¼ Dcr H m þ 26:3 kJ mol 1 cr # g J K mol for sublimation; Dg H m ¼ Dcr H m þ 26:3 1 g 1 # kJ mol and Dcr for cong S m ¼ Dcr S m 23:4 J K mol cr # densation. Gregory suggests that Dg S m > 0 and # Dcr g H m > 0 for condensation suggest an activated state comprising FeCl3(g) molecules or pairs of FeCl3(g) molecules i.e., incipient Fe2Cl6 molecules. Factors resulting in low ac are poorly understood. A general correlation exists between the structure of gas phase species and their existence or otherwise in the solid state. Thus, low ac are often observed in systems where there is difficulty in incorporating into a solid surface structural units which are absent in the solid state. FeCl3(cr) forms a layer-type lattice in which sheets of Fe3+ in an hexagonal array are sandwiched between sheets of Cl also in an hexagonal array such that each Fe3+ is almost perfectly octahedrally coordinated by Cl with an Fe–Cl bond distance of 0.249 nm. Electron diffraction and infra-red spectra of Fe2Cl6 are consistent with an out of plane bridged structure of the Al2Cl6 type in which iron is tetrahedrally coordinated with an Fe–Cl bond distance of 0.211 nm to 0.217 nm. For an Fe2Cl6 molecule to condense from the vapour there is a radical change of iron coordination and an increase in Fe–Cl bond length. Appraisal of the lattice unit cell indicates the existence of distorted Fe2Cl6 mole-
cules with tetrahedral coordination of iron. It is suggested that the above factors result in the observed low ac observed in this system and that the activation enthalpies and entropies are consistent with an alternative activated state comprising distorted Fe2Cl6(g) molecules. References [1] J. Wilmhurst, J. Mol. Spectrosc. 5 (1960) 343. [2] K. Kangro, H.F. Bernstorff, Z. Anorg. Allgem. Chem. 263 (1949) 316. [3] H.Z. Schafer, Anorg. Allg. Chem. 53 (1949) 259. [4] E. Stirnemann, Neues Jahrb. Mineral. Geol. Palaeontol. Bellageband, Abt. A 52 (1925) 368. [5] K. Sano, J. Chem. Soc. Jpn. 1073 (1938) 59. [6] L.N. Wilson, N.W. Gregory, J. Phys. Chem. 52 (1958) 433. [7] C.G. Maier, U.S. Bur. Mines Tech. Paper 1925, No. 360. [8] H.F. Johstone, H.C. Weingartner, W.E. Winsche, J. Am. Chem. Soc. 64 (1942) 241. [9] K. Jellineck, A. Rudat, Z. Phys. Chem. Abt. A 143 (1929) 55. [10] O.E. Ringwald, Doctoral Dissertation, Princeton University, New Jersy, 1949. [11] R.R. Hammer, N.W. Gregory, J. Phys. Chem. 66 (1962) 1705. [12] A. Landsberg, A. Adams, S.D. Hill, U.S. Bur. Mines R.I. (1977) 8207. [13] C.F. Shieh, N.W. Gregory, J. Phys. Chem. 79 (1975) 828. [14] D.S. Rustad, N.W. Gregory, J. Chem. Eng. Data 28 (1983) 151. [15] H. Shafer, E. Oeler, Z. Anorg. Allg. Chem. 271 (1953) 206. [16] W. Kangro, E. Petersen, Z. Anorg. Allg. Chem. 216 (1950) 157. [17] S. Blairs, R.A.J. Shelton, R. Unsworth, J. Sci. Instrum. 38 (1961) 469. [18] R.P. Iczcowski, J.L. Margrave, S.M. Robinson, J. Phys. Chem. 67 (1963) 229. [19] J.M. Stuve, M.J. Ferrante, D.W. Richardson, R.R. Brown, U.S. Bur. Mines R.I. (1980) 8420. [20] S.S. Todd, J.P. Coughlin, J. Am. Chem. Soc. 73 (1951) 4184. [21] M.W. Chase, C.A. Davies, J.R. Downey, D.J. Frurip, R.A. McDonald, A.N. Syverud, JANAF Thermochemical Tables, third ed., J. Phys. Chem. Ref. Data (1985) 14. [22] I. Barin, Thermochemical Data of Pure Substances, VCH, Weinheim, 1989. [23] E.Z. Zazorin, N.G. Rambidi, P.A. Akishin, Zh. Strukt. Chim. 4 (1963) 910. [24] O. Hassel, H. Viervoll, Tidskr. Kjem. Berg. Metall. (1943). [25] R.A. Frey, R.D. Werder, Hs.H. Gu¨nthard, J. Mol. Spectrosc. 35 (1970) 260. [26] L. Nalbandian, G.N. Papatheodorou, High Temp. Sci. 28 (1990) 49. [27] J.R. McCreary, R.J. Thorn, J. Chem. Phys. 48 (1967) 3290. [28] R.F. Brebrick, High Temp. Sci. 8 (1976) 11. [29] E.G. Lavut, B.I. Timofeyev, V.M. Yuldashva, J. Chem. Thermodyn. 16 (1984) 101. [30] N.W. Gregory, J. Phys. Chem. 67 (1963) 618.
JCT 05-259