JOURNAL OF CHEMICAL EDUCATION
m
PRASEODYMIUM TETRAFLUORIDE THEODORE P. PERROS, THOMAS R. MUNSON,' and CHARLES R. NAESER George Washington University, Washington, D. C.
AMONG the elements of the rareearth group, only cer-
ium is known to form quadrivalent salts. These are the familiar ceric salts. It might be expected that praseodymium, the element of next higher atomic number, would also form comparable compounds. However, the dioxide Proz and one or two of its derivatives such as PsOn and NazPrOs are the only reported compounds which contain praseodymium in the quadrivalent state. A report (1) claiming that praseodymium (IV) has been prepared as a complex with 8-quinolinol-5-sulfonic acid has not been substantiated. From a consideration of the electronic configuration of praseodymium, one would even predict the existence of the pentavalent oxidation state. Evidence for this has been presented by Prandtl and Rieder (2) who concluded from their investigations that the oxide, Pr20r,was a component of the mixed oxide Pr6011. However, Marsh (9) in repeating their work found no evidence for PrzOs but only for Pros. 1
Present address: National Bureau of Standards, Washington,
D. C.
Since Proz contains praseodymium in the quadrivalent state, a number of investigators have used it as a starting material in an effort to prepare quadrivalent salts of praseodymium (4, 5, 6, 7 , 8 , 9 ) . In no instance was evidence found for the formation of quadrivalent praseodymium. If oxygen is capable of producing the quadrivalent oxidation state, it would be expected that fluorine could also produce that state, i. e., as praseodymium tetrafluoride. The cerium analogue is easily prepared and quite stable (10). Klemm and Henkel (11) treated praseodymium trichloride with fluorine and obtained a mixture of the trichloride and trifluoride but no evidence of the tetrafluoride. Recent attempts to fluorinate the oxides (18) and the trifluoride (IS) of praseodymium produced no evidence for quadrivalent praseodymium. I n spite of the experimental failures to prepare praseodymium tetrafluoride, there is strong evidence for its possible formation. This is to be found by calculatinq the equilibrium constants for some of the reactions by which praseodymium tetrafluoride may be prepared.
AUGUST, 1853
403
*
The standard heats of formation, AH,",for the rare taken for praseodymium trifluoride and 32 3 e.u. for earth fluorides are not known. Therefore, estimates of praseodymium tetrafluoride. these values are made which leave little doubt as to the Using these values the increment in the Gibhs free validity of the conclusions derived from the calculations energy may be calculated for reaction (1): using these values. The heats of formation of praseoPr (c) 2Fp(g) = PrF, (c) (1) dymium trifluoride and praseodymium tetrafluoride are AH", = -478 i 20 kg.-cal./mal estimated first. The heats of formation used in makinn AS" = -79 zt 4 e.u. A F o = AH" - TASo these estimates are from the recent compilation by t h i AFo = -454 i 21 kg.-cal./mol National Bureau of Standards (14). The table lists The equilibrium constant for reaction (1) may now be AH," at 298.1G°K. for some of the trihalides of rare earth elements together with those for uranium and ohtainedfrom the familiar expression A Fo = -RT in K. neptunium. The third column contains the difference between the heat of formation of the trichloride and the triiodide. From this it can be seen that the equilibrium in reaction (1) is far in favor of the formation of praseody--AH/" (kg.-cal./mol)mium tetrafluoride. By making rough estimates of the Element XClr XIx Diffwenee necessary quantities it can be shown that even a t temCerium 260 164 96 peratures up to about 700°C. the equilibrium should be Praseodymium 258 162 96 substantially in favor of the praseodymium tetrafluoride. Neodvmium 254 159 9.5 The melting point of praseodymium tetrafluoride is estimated to be 1000 50°K. It may be concluded that praseodymium tetrafluoride is quite stable with respect to the elements. In the same manner, the equilibrium constant for From an examination of the data in column three of reaction (2) may be determined: the table, it seems reasonable to assume that the values PrF, (c) ' / 8 2 (g) = PrFd ( c ) AH,' of praseodymium trifluoride and praseodymium (2) AHo = -478 - (-402) = -76 i 30 kg.-cal./mol tetrafluoride should bear the same relationship to that A S o = - 15 i 32 o.u. for praseodymium trichloride that the corresponding AF* = AN- - r n s D uranium and neptunium tri- and tetrafluorides do to uranium trichloride and neptunium trichloride. The heats of formation of uraninm trifluoride and neptunium The value here also indicates that praseodymium trifluorideare -357 kg.-cal./mol and -360 kq.-cal./mol, respectively. The differencebetween the values for the tetrafluoride, once prepared, would be stable. On the trichloride and the trifluorideis in each case 144kg.-cd./ basis of the above calculations, which are felt to be valid mol. Hence, the heat of formation of praseodymium within the limits indicated, it would appear that further experimental work in developing a method for the preptrifluoride is taken as aration of praseodymium tetrafluoride is warranted.
+
*
+
The heats of formation of uranium tetrafluoride and neptunium tetrafluoride are -443 kg.-cal./mol and -428 kg.-cal./mol, respectively. The differencebetween these figures and those for the corresponding trichlorides are 230 and 212 kg.-cal./mol for the uranium and neptunium salts, respectively. The value estimated for the difference between the heats of formation of the praseodymium trichloride and the tetrafluoride is 220 kg.-cal./mol. The heat of formation of praseodymium tetrafluoride is then taken as: AH/' = (-258)
- (220)
=
-478 i 20 kg.-eal./mol
The entropies of praseodymium, praseodymium trifluoride, praseodymium tetrafluoride, and fluorine are still required. The entropy of praseodymium is taken as 13.7 e x . from therecent work of Latimer (15), that of fluorine as 48.6 e.u. from the NBS compilation (14). By recourse to the work of Latimer and by noting trends in the periodic table the value 23 i 3 e.u. is 'The assigned uncertainties are the authors' estimates of the maximum uncertaintr involved in the stated values.
LITERATURE CITED (1) NAKATSWKA, Y., AND T. CHANG, Acla Chimica Teiwanica, 1, 37 (1949). W., AND G. RIEDER, 2. anorg. Chem., 238, 225 (2) PRANDTL, (1938). (3) MARSH,J. K., J. Chem. Soe., 1946, 15. B., PTOC. Chem. Soc., 17, 66 (1901). (4) BRAUNER, B., ibid.. 14,70 (1898). (5) BRAUNER, J., AND I. KLIMENKG, Chem. Zmtr., 1,172 (1902). (6) MELIKOW, (i) MATIGNON, C., Ann. Chim. Phys., 8, 364 (1SO6). W., AND K. HUTTNER,Z. anorg. Chem., 149, 235 (8) PRANDTL, (1925). A,, AND M. BABOR, Bull. sect. sci. a d . roumaine, (9) ATANASIU, 20, No. 1-3, 27 (1938). G , Z. anorg. allgem. Chernie, 244, 337 (10) VONW A R T E N B ~H., (1940). ibid., 220, 180 (1934). (11) KLEMM,'w., AND J . HENKEL, J. Am. Chem. Soc., 74,1357 (12) Popov, A. I., AND G. GLOCKLER,
- - -- .
(1 , 959),
(13) PERROS, T.,AND C. R. NAESER,ibid., 74, 3694 (1952). WAGMAN, EVANS, LEYINE,AND J A F "Selected ~ ~ (14) ROSSINI, Values of Chemical Thermodynamic Properties," NBS Circular 500. Government Printine Office. 1952. W. M., "Oxidation ~ o t e n t h , "2nd ed., Prentice(15) LATIMER, Hall, Inc., New York. 1952.
