HEATSOF FORMATION OF RHENIUM HEPTOXIDE AND 'TRIOXIDE
Dec. 5, 1953 [CONTRIBUTION FROM
THE
OAK RIDGENATIONAL LABORATORY, AND TENNESSEE]
THE
CHEbfISTRY DEPARTMENT, UNIVERSITY
5783 OF
Thermodynamic Properties of Technetium and Rhenium Compounds. III. Heats of Formation of Rhenium Heptoxide and Trioxide, and a Revised Potential Diagram for Rhenium1 BY G. E. BOYD,J. W. COBBLE^
WM.T. SMITH,
AND
JR.
RECEIVEDJULY 20, 1953 Elemental rhenium and rhenium Crioxide have been burned in a semimicro bomb calorimeter t o redetermine the heats of formation of RelOT(c) and ReOa(c). These were -296.9 f 2.0 and -146.0 f 3.0 kcal. mole-', respectively. The latter value is in disagreement with that previously reported, but is now consistent with the observed stability of the compound in the presence of water. Using experimental and estimated entropy values, and the recently published potential for the ReOgReo,- couple, revised thermodynamic functions are given for rhenium and for some of its compounds. The oxidationreduction scheme shown in Fig. 1 for the element in acid solution is proposed.
Heats of combustion of elemental rhenium and of rhenium trioxide were determined incidental to other measurements of the thermochemical properties of technetium compounds.8 Previously, heat values for both of these rhenium substances have been reported by Roth and B e ~ k e r . ~However, more accurate data for rhenium trioxide seemed desirable to establish the relative stability of rhenium(VI), which was in doubt, as well as to afford a possible guide for an estimation of the stability of the corresponding technetium compound. Using the new data obtained in this work, a revised potential diagram for rhenium was formulated which appears to be in better agreement with the known chemistry of this element. Experimental Materials and Apparatus.-A semi-micro combustion calorimeter already described8 was employed in the heat measurements. Rhenium metal, prepared by hydrogen reduction of ammonium perrhenate a t 500°, was obtained from the University of Tennessee rhenium project. This metal appeared t o bum much more completely than that used in the experiments of Roth and Becker which had been formed by reduction of potassium perrhenate; hence, the interpretation of the heats observed by us was not complicated by the necessity to correct for incomplete combustion. To achieve >99% combustion, however, an oil accelerator was required. The heat equivalent of the latter was determined quite accurately in several independent calorimeter experiments. Very small amounts of metallic rhenium which occasionally remained in the semi-micro bomb were weighed t o obtain a corrected combustion weight. Rhenium trioxide, also obtained from the University of Tennessee, had been collected as a by-product from numerous previous burnings of rhenium t o give rhenium heptoxide. The trioxide did not burn completely even when an oil accelerator was used, so that corrections for the weights of unreacted compound left in the bomb were necessary. In the calculations one calorie was taken as equal to 4.1840 absolute joules, while 186.31 was employed as the atomic weight of rhenium.
%
Experimental Results Calorimetric data for the combustion of rhenium and rhenium trioxide are given in Table I. For rhenium the process measured was 2Re(c)
+ 7/202(g, 30 atm.) 4- HtO(1) = 2HReO,(B)
(1)
(1) This work was performed for the Atomic Energy Commission. (2) From the Thesis of J. W. C. submitted to the Graduate SchooI of the University of Tennessee in partial fulfillment of the requirements for the Ph.D. degree in June, 1952. (3) J. W. Cobble, W. T. Smith, Jr., and G . E. Boyd, THISJOURNAL, 76, 5777 (1953); paper I1 of this sequence. (4) W. A. Roth and G. Becker, Z. Qhysik. Chcm., A160, 27 (1932); data have been recalculated and summarized in Bureau of Standards "Selected Values of Chemical Thermodynamic Properties," 1948.
and for rhenium trioxide 2ReO,(c)
+ 1/202(g, 30 atm.) + HsO(1) = 2HReO,(@) (2)
where i@ represents the average concentration of the perrhenic acid solution formed with the water in the combustion bomb. Actually, only about one-half of the rhenium heptoxide formed dissolved to give a 0.1 M perrhenic acid solution while the rest remained in the ignition cup. The largest errors, accordingly, arose from the uncertainty about the concentration distribution of perrhenic acid inside the bomb after the combustion. TABLE I HEATDATAFOR R ~ N I U AND M RHENIUMTRIOXIDE ATcor.,'
deg.
ATnet,
AT&
deg.
deg.
Sample wt., Temp. rise, mg. deg. g.-1
Rhenium-metal 0.5775 0.0739 129.09 0.573 ,0569 .3895 98.77 .576 .0635 .5532 111.00 .572 Average* 0.574 f 0.001 AE = -298.6 f 0.4 kcal. mole-' of Re207
0.6514 .4464 .6167
Rhenium trioxide 0.9196 0.0024 253.1 0.0095 1.1309 .0082 253.7 .0323 0.6190 .0008 78.3 .0103 Averageb 0.0174 f 0.008 AE = -11.4 f 6.0 kcal. mole-' of Res07 a Corrected for fuse and ignition 'energy and nitric acid Standard deviations calculated using the reformation. lation S = Z ( A X ) * / n ( n 1). 0.9220 1.1391 0.6198
-
Correcting to unit fugacity of oxygen, and the observed energies to enthalpies, we obtain 2Re(c)
+ 7/202(g) + HgO(1) = 2 H R e 0 4 ( a )
(3)
AH = -300.9 f 0.4 kcal.
and, using the data of Roth and Becker4to estimate the heat of solution of rhenium heptoxide to give 0.1 Mperrhenic acid solutions PRe(c)
+ 7/20&)AH == Re~Odc) -295.9 f 2.0 kcal.
(4)
Because of the modest level of accuracy in the rhenium and rhenium trioxide combustions the Washburn6 corrections were not applied. The value of -295.9 f 2.0 kcal. mole-' for the heat of (6) E. W. Washburn, J . Research Natl. Bur. Standards, 10, 525 (1933).
5784
6. E. BOYD,J. W. COBBLE AND Wx T. SMITH, JR.
Vol. 75
formation of rhenium heptoxide may be compared with the value, -297.5 =k 2.0 kcal. mole-', of Roth and B e ~ k e r . ~The adoption of the average, -296,700 =k SO0 cal. mole-', is recommended.
This figure is based on the assumption that the heat of neutralization of HRe04 aq, by KOH aq, is that of any strong acid-strong base reaction (-13,310 cal. mole-' HzO formed). From the experimental values of the entropy of K R e 0 4 ( ~ ) ~ Calculations of 40.12 cal. deg.-' mole-' and the entropy of RePerrhenate Ion (ReOd-, aq).-Using - 13.2 ( c ) ~of 8.89 cal. deg.-' mole-', A X becomes -82.0 kcal. mole-' for the heat of solution of rhenium cal. deg.-l mole-', and AFP equals -238.3 kcal. h e p t ~ x i d e ,the ~ heat of formation of HRe04, aq, mole-'. Rhenium Trioxiae (Re03(c)).-Correcting the becomes -189.2 =k 0.8 kcal. mole-'. Since perrhenic acid is a strong acid, this is also the heat heats given in Table I for Re03(c) to unit oxygen of formation of Reo4-, aq. Using - 74.2 cal. mole-1 fugacity gives deg.-' for the entropy of formation,6 AFP becomes %ReOa(c) 1/202(g) = RezO7(c) -167.1 =k 0.S kcal. mole-'. The calculated poAff = -6 & 6 kcal. X I I O ~ C - - 'of Re03 tential for the Re-ReOh- couple then becomes E o = (7) - 0.367 V. The errors are large in this case due to the unRhenium Heptoxide (RezO,(c)) .-The entropy certainty of corrections for the incomplete comof RezO,(c) can be estimated in three dzerent bustion of the oxide. The calculated heat of ways: (a) by the additivity principle:? 48.6 cal. formation of the trioxide then is Af?? = - 146.0 i mole-' deg.-l; (b) by comparison to T c ~ O , ( C ) ~3.0 kcal. mole-'. Roth and Becker4 reported correcting for mass differences -83.0 kcal. mole-'. The poor agreement is probably due to the fact that their calculation was based upon the heats observed when rhenium was burned to form a mixture of the heptoxide and which gives 44.9 cal. mole-I deg.-l when the value varying amounts (4-870) of the trioxide. The 43.6 cal. mole-' deg.-l is used for Tca07(c); (c) direct determination reported here should be more from the entropy of KReO4(c) and the entropy of accurate. dehydration of HRe04(c). The entropy6 of KReThe estimated value of AS? is -61.5 e.u. for O ~ ( Cis ) 40.12 cal. deg.-l mole-' and since 9.2 is ReOs(c) based on an estimated' entropy of 19.8 the contribution of K+(c),' Re04-(c) becomes 30.9 e.u. and the experimental values of 8.89 e.u. for the cal. deg.-' mole-'. The entropy of HReOl(c) is entropy of rhenium. Thus AFF = -127.3 kcal. estimated to be 30.9 e.u. Smith, et a,!.,* have meas- inole-'. ured the entropy of dehydration of perrhenic acid Rhenium Dioxide (ReOz(c)).-Latimer7 reports HReOa(c) = 1/2HzO(1) 1/2RezO7(c) that Hugus has obtained -0.510 v. for the ReOzReOd- couple. Thus the free energy of formation A S = -3.2 cal. mole-' deg.-1 (6) for ReOo(c) based on this voltage and the free 'Thus the entropy calculated by this method for energy formation given above for Reo4-, aq, Rep07(c) is 38.6 e.u. Using the average of the becomesof -S9.2 kcal. mole-'. Estimating the three values, 44.0 =k 3.6 cal. mole-' deg.-', AFf" entropy of ReOa(c) as 17.4 cal. mole-' deg.-', is calculated to be -258.7 kcal. mole-'. AS? = -40.5 cal. mole-' deg.-' and AHf = Potassium Perrhenate (KRe04(c)).-With the - 101.3 kcal. mole-'. value for the heat of formation of HReOr aq, of -189.2 kcal. mole-', and the heat of solution3 Oxidation-Reduction Potentials of KReO4(c) of 13.8 kcal. mole-', AH? (KRe04 From the above free energies of formation, the (c)) is calculated to be -263.0 kcal. mole-'. potential diagram (Fig. 1) for rhenium in acid solution has been calculated. The reliability of the I -0.510 V. voltage values is, in general, indicated by the number -0.260 v. 1 -0.40 v.-ReO3--0.73 V. Re Reoy Reo*- of significant figures given. The voltages of the -0.367 v. ! Re03-Re04- and Reo2-Reos couples are now in agreement with the observed stability of ReOa(c) Fig. 1.-Oxidation-reduction scheme for rhenium in acid toward disproportionation in acid media. In this iolutioii respect, ReOs(c) differs from Tc03(c), which ap(61 5 . W. Cobble, G. D. Oliver and A'm. T . Smith, J r . , THIS pears to disproportionate.
+
+
I
76, 5786 (10531, paper V of this sequence. (7) A'. M. Latimer, "Oxidation Potentials," 2nd ed., Prentice-Hall, Inc., New York, N. Y . , 1932. (8) W. T. Smith, J r . , L. E. Line and W. A. Bell, THISJOURN.4L, 74, JOURNAL.
4964 (1932).
OAKRIDGE,TENN. KSOXVILLE,TENX. (9) W. T. Smith, Jr., G. D. Oliver and J. W. Cobble, ibid.. 78, 5785 (1953). paper I V of this sequence.
