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Experimental Enthalpy of Fusion and Heat Capacity. Estimation of Thermodynamic Functions up to 1300 K

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Experimental Enthalpy of Fusion and Heat Capacity. Estimation of Thermodynamic Functions up to 1300 K Thermodynamics of SmCl3 and TmCl3: Experimental Enthalpy of Fusion and Heat Capacity. Estimation of Thermodynamic Functions up to 1300 K L. Rycerza;b and M. Gaune-Escardb a Institute of Inorganic Chemistry and Metallurgy of Rare Elements, University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland b Universite´ de Provence, IUSTI, CNRS UMR 6595, Technopoleˆ de Chateauˆ Gombert, 5 rue Enrico Fermi, 13453 Marseille Cedex 13, France Reprint requests to M. G.-E.; Fax: +33 (0)4 91 11 74 39; E-mail: [email protected] Z. Naturforsch. 57 a, 79–84 (2002); received December 19, 2001 Heat capacities of solid SmCl3 and TmCl3 were measured by differential scanning calorimetry in the temperature range from 300 K up to the respective melting temperatures. The heat capacity of liquid SmCl3 was also investigated. These results were compared with literature data and fitted by a polynomial temperature dependence. The temperature coefficients were given. Additionally, the enthalpy of fusion of SmCl3 was measured. Furthermore, by combination of these results with the 0 literature data on the entropy at 298.15 K, Sm(LnCl3, s, 298.15 K) and the standard molar enthalpy 0 H of formation of form m(LnCl3, s, 298.15 K), the thermodynamic functions were calculated up to T = 1300 K. Key words: Samarium Chloride; Thulium Chloride; Heat Capacity; Enthalpy; Entropy; Formation; Fusion; Differential Scanning Calorimetry. Introduction preparation of a mixture in the molar ratio Tm O 2 3 :NH Cl=1:9, The enthalpies of phase transitions of lanthanide 4 sintering and chlorinating under vacuum at 600 K chlorides as well as the heat capacities of LaCl , 3 during 3 hours, CeCl , PrCl , NdCl , GdCl , DyCl , and EuCl3 have 3 3 3 3 3 sublimation of unreacted ammonium chloride at been measured and reported previously [1 - 3]. This 640 K under vacuum, work continues an investigation program on lan- thanide halides. It presents results of enthalpies of melting of crude thulium chloride, phase transitions and specific heat capacity measure- purification of crude chloride by distillation under ments of the pure lanthanide chlorides SmCl and reduced pressure (0.1 Pa). 3 TmCl performed with a SETARAM DSC 121 dif- As the product after sintering chlorinating and 3 ferential scanning calorimeter. The results are com- melting was contamined by oxychloride, TmOCl, the pared with original literature data and with literature next step of preparation was a double distillation of estimations. the crude TmCl from the less volatile residue (mainly 3 TmOCl). Details of the distillation procedure have Experimental been described in [4]. However, this method should not be used for Sample Preparation the synthesis of SmCl3 because of the decompo- sition tendency of this compound. Thus samarium Thulium chloride (TmCl3) was prepared from the trichloride was prepared by chlorinating the oxide oxide of 99.9% purity, supplied by Merck, by chlo- (Merck, 99.9%) with a current of high purity argon rinating with ammonium chloride (POCh Gliwice, (water and oxygen content less than 2 and 0.5 ppmV, Poland – pure for analysis). The synthesis included respectively) saturated with SOCl vapour in a quartz 2 the following steps: reactor, at 793 - 813 K for 24 hours. This procedure, 0932–0784 / 02 / 0100–0079 $ 06.00 c Verlag der Zeitschrift fur¨ Naturforschung, Tubingen¨ www.znaturforsch.com 80 L. Rycerz and M. Gaune-Escard · Thermodynamics of SmCl3 and TmCl3 ∆ Table 1. Chemical analysis of the lanthanide trichlorides. T i. Therefore the heat capacity of the sample is Compound Cl Cl Ln Ln 0 Q M = ∆T m ; observed theoretical observed theoretical C =( ) ( ) p;m i s i s mass % mass % mass % mass % M where ms is the mass of the sample and s the molar SmCl3 41.44 41.43 58.56 58.57 TmCl3 38.62 38.63 61.38 61.37 mass of the sample. The same operating conditions (e. g. initial and fi- nal temperatures, temperature increment, isothermal which revealed satisfactory, was developed empiri- delay and heating rate) are required for the two ex- cally, and no attempt was made to reduce the duration perimental runs. The original SETARAM program of the chlorinating cycle, although this may be pos- performs all necessary calculations. sible. The heat capacity measurements were performed The chemical analysis of the synthesised lan- by the “step method”. Each heating step of 5 K was thanide chlorides was performed by titration methods followed by 400 s isothermal delay. The heating rate for the chloride (mercurimetric) and lanthanide (com- was 1.5 K min1. All experiments were started at plexometric). The results are presented in Table 1. 300 K and were performed up to 1100 K. The mass All handling of lanthanide chlorides was per- difference of the quartz cells in a particular experi- formed in an argon glove box (water content less than ment did not exceed 1 mg (mass of the cells: 400 - 2 ppmV). Continuous argon purification was achieved 500 mg). The mass of the samples was 200 - 500 mg. by forced recirculation through external molecular sieves. Results and Discussion Differential Scanning Calorimeter (DSC) Enthalpy of Phase Transition The enthalpies of phase transitions and heat ca- The enthalpy of fusion was determined for SmCl3. pacities were measured with a SETARAM DSC 121 As supercooling was observed in DSC cooling curves differential scanning calorimeter. The apparatus and (about 19 K), the temperature and fusion enthalpy the measurements procedure were described in details were determined from heating thermograms. in [1 - 3]. The temperature and fusion enthalpy of TmCl3 have been obtained earlier [1] by Calvet high-tempe- Measurements rature microcalorimetry, since the high melting tem- perature was beyond the experimental range of the Quartz cells (7 mm diameter and 15 mm long) DSC 121 apparatus. were filled with the lanthanide chlorides in a glove- box, sealed under vacuum and then placed into the SmCl3 DSC 121 calorimeter. SmCl was found to melt at 950 K with a corre- Enthalpy of transition measurements were carried 3 0 H sponding enthalpy and entropy of fusion = out with heating and cooling rates between 1 and fus m 1 0 1 1 S 47.6 k J mol and = 50.3 J mol K , 5 K min 1. fus m respectively. Our measured melting temperature of The so-called “step method”, used for C measure- p SmCl agrees well with that measured by Poly- ments, was already described in [1, 2]. In this method, 3 achenok and Novikov [5] and that selected by small heating steps are followed by isothermal equi- Pankratz [6], although by about 5 K lower than other librations. Two correlated experiments should be car- and older literature data [7]. ried out to determine the heat capacity of the sample. The first one, with two empty containers of identical TmCl mass, and the second one with one of these loaded 3 with the sample. The heat flux is recorded as a func- TmCl melts at 1092 K with a corresponding en- 3 0 1 H tion of time and temperature in both runs. The differ- thalpy and entropy of fusion fus m = 35.6 k J mol 0 1 1 S ence of heat flux in both runs is proportional to the and fus m = 32.6 J mol K , respectively [1]. The amount of heat (Qi) necessary to increase the temper- melting temperature and fusion enthalpy agree well ature of the sample by a small temperature increment with the reference data of Thoma [8], but this melting 82 L. Rycerz and M. Gaune-Escard · Thermodynamics of SmCl3 and TmCl3 0 0 0 3 6 2 T H T G T T –(Gm( )– m(298.15))/ = form m = 651.101 10 + 14.978 10 3 2 1 9 3 T T T 91.27 ln T + 14.534 10 – 4.939 10 – 1.743 10 5 2 1 3 T T T T – 0.17510 + 28621 – 470.02. – 62.366 10 ln – 1015.0. SmCl3 liquid, 950 K < T < 1300 K: SmCl3 liquid, 1190K<T<1300 K: 0 0 3 6 2 H T T C = 145.26, = 42.966 10 – 0.188 10 p;m form m 2 1 0 0 3 T T H T H – 4.268 10 – 1010.7, m( )– m(298.15) = 145.26 10 – 19.16, 0 0 3 6 2 G T T T T S = 524.911 10 + 0.188 10 m( ) = 145.26 ln – 671.02, form m 0 0 2 1 3 T T T T H T T G – 2.134 10 – 42.966 10 ln – 1009.6. –( m( )– m(298.15))/ = 145.26 ln 1 + 19160 T – 816.46. The results obtained for selected temperatures are pre- sented in Table 2. Having the thermodynamic functions for SmCl3, one can calculate the thermodynamic functions of its formation as a function of temperature. TmCl3 The formation of SmCl from the elements can be 3 The only existing literature data on the TmCl heat described by the reaction 3 capacity are the experimental data at low tempera- Sm(s) + 1.5 Cl2(g) = SmCl3(s,l), (2) tures (10 - 300 K) obtained by Tolmach et al. [15] by adiabatic calorimetry (100.10 J K1mol 1 at 300 K). and the related thermodynamic functions of SmCl3 formation depend on the thermodynamic functions Barin and Knacke [12] as well as Knacke et al. [11] made heat capacity estimations. Our experimental of metallic Sm and chlorine gas Cl2. The latter 0 heat capacity values on TmCl3, are presented in Fig- were calculated using literature data for C and p;m 0 ure 2. They agree very well with these estimations Sm(298.15 K) [9].
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