All the investigated systems show negative enthalpies of mixing with a minimum value of approximately -1.25, - 8 .3 , -17.0, - 20 . 0 and -22.5 kJ mol- 1, for M = Li, ...
Mixing Enthalpies of TbBr3-MBr Liquid Mixtures (M = Li, Na, K, Rb, Cs) L. Rycerz and M. Gaune-Escard 3 Institute of Inorganic Chem istry and M etallurgy o f Rare Elem ents, Wroclaw University o f Technology, W ybrzeze W yspianskiego 27, 50-370 W roclaw, Poland 3 IUSTI - CNRS UM R 6595, Technopole de Chateau Gom bert, 5 rue Enrico Fermi, 13453 M arseille Cedex 13, France Reprint requests to M. G.-E.; Fax: +33 (0)4 91 11 74 39; E-m ail: m ge@ iusti.univ-m rs.fr Z. Naturforsch. 56 a, 859-864 (2001); received N ovem ber 14, 2001 The molar enthalpies o f mixing, A mixHm in the binary liquid systems TbBr3-M Br (M = Li, Na, K, Rb, Cs) have been m easured with a C alvet-type high-tem perature microcalorim eter over the entire composition range with an accuracy of about 6 %. M ixing of the two liquid components was achieved by using the “break-off am poule” technique. All the investigated systems show negative enthalpies of mixing with a minim um value of approxim ately -1 .2 5 , - 8 .3 , -1 7 .0 , - 2 0 . 0 and -22.5 kJ m ol- 1, for M = Li, Na, K, Rb and Cs, respectively. The mixing enthalpy in the TbBr3LiBr system is positive in the T bB r3-rich region. For all the system s, the enthalpy minim um occurs at mole fraction xXbBr ss 0.3 - 0.4. The molar enthalpies o f form ation A formHm (3MBr, TbBr 3 , 1) for M = Li, Na, K, Rb and Cs at 1113 K (arising from the reaction 3 M B r(I) +TbB r 3 (1) = (3MBr, TbBr3) (1)) are found to be -4 .8 , —31.3, -6 3 .3 , —70.3 and -8 1 .2 kJ m ol- 1 , respectively. The leastsquares coefficients A, B, C, D and E in the equation A (kJ m ol- 1) = A + B x + C x 2 + D x 3 + E x 4, where A is an interaction param eter and x = x-rbßr,, are also reported.
Key words: Terbium Bromide; A lkali Bromides; M ixing Enthalpy; Formation Enthalpy.
Introduction
Experimental
Rare earths are extracted and processed into metals, magnet alloys, oxides and other forms. Lanthanide’s extraction and processing is largely based on m olten salt technologies. Many processes are still under de velopment; particularly those dealing with reprocess ing o f spent nuclear fuel or nuclear waste processing [1, 2]. Data on lanthanide com pounds, however, are scarce and not easily accessible in literature. A s a consequence, intensive efforts are being made at an international level both on the research and develop ment aspects and also on data bank developm ent. The present work is part o f a research performed on LnX 3-M X system s (where Ln is lanthanide, M is alkali metal and X is halide) [ 3 - 1 2 ] . It reports the experimental enthalpies o f m ixing o f the TbBr3MBr liquid system s, which were measured by direct calorimetry. A comparison with the analogous TbC l3MC1 liquid mixtures [13] is also presented. The cur rent data are discussed in terms o f “relative ionic po tentials” [14].
Chemicals TbBr 3 was prepared by sintering bromination o f Tb 20 3 w ith N H 4Br (POCh G liw ice, Poland). The Tb 20 3 -N H 4Br mixture (molar ratio = 1:14) was heated slow ly up to 570 K. After 3 hours reaction at 570 K, the temperature was increased up to 650 K and N H 4Br was sublimated. Finally the salt was m elted at 1150 K and an am poule with TbBr 3 was transferred to the g lo v e box. Further purification o f crude terbium bromide was obtained by distillation under reduced pressure ( ~ 0.1 Pa) at 1170 K. MBr, alkali metal bromides were Merck Suprapur reagents (min. 99.9% ). Before their use, they were treated by progressive heating up to fusion under gaseous HBr atmosphere. Excess HBr was removed from the m elt by argon. A ll the chem icals were handled in an argon glove box with a measured volum e fraction o f water o f about 2 x l 0 ~ 6 and continuous gas purification by forced recirculation through external molecular sieves.
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L. Rycerz and M. Gaune-Escard • M ixing Enthalpies o f TbBr3-M Br Liquid Mixtures
860
Table 1. Molar enthalpies of mixing Amix H m and interaction param eter A of the T bB r3-LiBr liquid system at 1113 K. A/ kJ mol- 1
^TbBr3
kJ mol- 1 -0.542 -0.439 -1.482 -1.284 -1.125 -1.045
-6.624 -3.574 -9.116 - 6. 101 -4.908 -4.312
0.5458 0.5958 0.6972 0.8077 0.8972 0.8984
ZTbBr3 0.0899 0.1434 0.2043 0.3011 0.3559 0.4124
AmixHm/
A/ kJ mol- 1
^TbBr3
kJ mol- 1 -0.140 -0.542 0.547 0.441 0.432 0.551
-0.565 -2.251 2.591 2.839 4.684 6.036
0.0462 0.0996 0.1521 0.1993 0.2218 0.2273 0.2449 0.2694 0.3008 0.3483 0.4059 0.4448 0.5027 0.5369
Table 2. M olar enthalpies of mixing Z W Hmand interaction param eter A of the T bB r3-NaBr liquid system at 1113 K. •ETbBr3 0.0512 0. 1011
0.1465 0.2008 0.2257 0.2960 0.3465
AmixHm/
A/ kJ mol- 1
^TbBrj
kJ mol- 1 -1.770 ^4 . 122 -5.306 -7.229 -8.009 -8.180 -8.300
-36.436 ^5 .357 -42.435 ^5 .0 4 6 ^5.829 -39.254 -36.655
0.3928 0.5056 0.5921 0.6526 0.6955 0.7989 0.8960
Table 3. Molar enthalpies o f mixing zAmix H mand interaction param eter A of the T bB r3-K B r liquid system at 1113 K.
kJ mol- 1
A/ kJ mol- 1
-7.768 -6.891 -5.349 -3.989 -3.017 -1.872 -0.787
-32.569 -27.567 -22.147 -17.595 -14.246 -11.652 -8.446
AmixHm/
Am\xHm/
A/ kJ mol- 1
ZTbBr3
kJ mol- 1 -2.995 -6.862 -10.655 -13.610 -14.255 -16.413 -15.921 -17.127 -17.245 -17.156 -15.927 -16.233 -13.586 -13.943
-67.967 -76.517 -82.619 -85.287 -82.587 -93.450 -86.095 -87.017 -81.994 -75.581 -66.047 -65.733 -54.345 -56.077
0.5565 0.5770 0.6025 0.6072 0.6369 0.6579 0.6923 0.7460 0.781 0.7986 0.8499 0.9004 0.9384 0.9585
Ana\Hml kJ mol- 1 -12.715 -13.526 -10.829 -10.897 -10.204 -8.430 -8.600 -7.345 -7.447 -5.906 ^1.586 -3.001 -2.178 -1.291
A/ kJ mol- 1 -51.518 -55.418 ^15.216 -45.688 -44.124 -37.455
-A0312 -38.763 -43.540 -36.720 -35.949 -33.463 -37.678 -32.455
Procedure The m ixing experiments were all o f the sim ple liquid-liquid type, performed under argon at atm o spheric pressure. The calorimetric apparatus, Calvettype high-temperature microcalorimeter, m ixing de vices used and experimental methods have been de scribed in [15]. For every system under investigation, the terbium bromide was w eighed in the glove box within ± 1 0 - 5 g and placed in the “b reak -off’ quartz am poule. The alkali metal bromide, w eighed in the same way, was placed in a quartz crucible. The breako ff am poule was evacuated under controlled argon pressure in order to obtain the proper pressure at the temperature o f the experiment. It was then w elded to a quartz tube, which could be m oved up and down through a special gas-tight ring. After attainment o f thermal equilibrium the “break-off am poule” was crushed and the thermal effect resulting from m ix ing was recorded. Calibration o f the calorimeter was performed with NIST a-alum ina; known amounts o f alumina were dropped directly into the melt. The area o f the thermograms was obtained automatically by a computer. The calibration o f the calorimeter had an accuracy o f about 3%. Therefore the molar enthalpies o f m ixing were obtained with an accuracy o f about 6%, because they were measured during separate e x perimental runs.
x , TbBr3
Fig. 1. Experim ental mixing enthalpies A mixHm in TbBr3M Br liquid system s at 1113 K. Open circles: TbBr 3 -LiBr, squares: T bB r 3 -NaBr, triangles: T bB r 3 -KBr, diamonds: T bB r 3 -RbBr, full circles: T bB r 3 -CsBr.
Results Calorimetric experim ents were conducted at 1113 K for all the TbBr3-M Br liquid system s (M = Li, Na, K, Rb, Cs). At this temperature, liquid mixtures are formed in the w hole com position range. The experim ental enthalpies o f the investigated system s are presented in Tables 1 - 5 and plotted vs. com position in Figure 1. The interaction parameter X = -^mix-^m/C^’MBr^'TbBrj)} w hich represents the energetic asymmetry in a melt, has been calculated for all the system s. The lim it ing interaction parameter A(0), i.e . A at infinite di-
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L. Rycerz and M. Gaune-Escard • M ixing Enthalpies o f TbBr3-M Br Liquid Mixtures
Table 4. M olar enthalpies of m ixing zimix H m and interaction parameter A of the TbBr3-RbBr liquid system at 1113 K. A/ kJ mol- 1
^TbBr3
kJ mol- 1 -3.333 -6.421 -7.856 -11.988 -13.732 -17.231 -17.596 -18.856 -19.009 -19.428 -19.783 -18.435 -18.236
-75.482 -96.099 -85.545 -94.753 -95.028 -93.803 -93.920 -100.139 -90.972 -89.254 -86.235 -76.883 -75.927
0.4482 0.5087 0.5462 0.6094
^TbBr3 0.0463 0.0720 0.1023 0.1486 0.1752 0.2425 0.2497 0.2516 0.2974 0.3202 0.3565 0.3989 0.4009
0.6668 0.6778 0.7513 0.7645 0.8098 0.8473 0.8974 0.9237 0.9552
kJ mol- 1
A/ kJ mol- 1
-18.935 -17.296 -16.414 -14.277 -11.382 -11.817 -8.591 -10.382 -6.193 -5.126 -3.319 -1.968 -1.692
-76.562 -69.205 -66.221 -59.979 -51.229 -54.110 -*5.978 -57.665 -40.208 -39.619 -36.047 -27.923 -39.539
861
Table 6 . Least-squares coefficients for the equation of A for the liquid alkali bromide - terbium bromide mixtures: A = A + B x + C x 2 + D x 3 + E x 4, in kJ m ol - 1 (x = mole fraction of TbBr3).
>1
System
B
C
D
E
kJ mol- 1 kJ mol- 1 kJ mol- 1 kJ mol- 1 kJ mol- 1 LiBr-TbBr3 NaBr-TbBr3 KBr-TbBr3 RbBr-TbBr3 CsBr-TbBr3
_ -3.9727 -27.476 82.102 -44.75 -40.525 -58.80 268.56 -217.27 39.50 -59.320 -328.27 1277.96 -1391.78 517.51 -72.540 -258.21 889.70 -853.64 255.16 -80.344 -334.48 1153.79 -1138.92 352.46
Table 5. M olar enthalpies of mixing
