May, 196CI
HEATSOF SOLUT~ON A N D OF FORMATION OF Mo, \V
ANU
NB FLVOEIDJCS
6'31
a modified Berthelot equation (takiiig 1', T B = 1.55 and b = 80 cc.), and Ruff and Ascher's2 vapor pressure data. The result of 80.6 cal./deg. --S, oal./mole deg.MoFs NbFs mole is to be compared with 80.05 cal./deg. mole 0-50°K. (cxt,rap.) 8.15 4.27 calculated by Gaunt from infrared and Raman 50-263.6' f C, d In t 36.50 spectra. The spectroscopic value is t o be preferred Transition 7.72 for the gas, but not necessarily for the liquid, prin263.6-290. tiOK. cipally because of the uncei tainty in the entropy 3.56 f C, d l r t of vaporization. Fusion 3.65 Table TI1 also lists the entropy for solid EbF6 at 290.6-298. 15'K. 1.04 (50-298.16) 298.15, 33.3 cal. mole deg. For the extrapola34.02 f C, d 111 t tion from 50 to 0OK. there are iio data to guide the SZY8.12K. 60.6 (liquid) 38.3 (solid) choice of Einstein functions; a Debye function of Vaporization 6000/298.15 20.1 6 degrees of freedom (6, = 170'K.) and eight EinGas imperfcction 0.2 stein functions (Y = 247 crn.-l) were found to Compression R 111 (645/ reproduce the smoothed experimental curve to 760) -0.3 i 0.27; between 50 and 100°Ii. 80.6 (ideal gas) 5 1 2 3 8 . 1 ~ 1~ exp.) . Acknowledgments.-The help of 111.. C. 1. (calcd.) 80.05 Glassbrook in the fundamental design of apparatus was obtained by use of Kelley's estimate2 for the and of Mr. Glassbrook sild Mr. L. A. Caranagh heat of vaporization, a crude (but relatively un- in construction is gratefully acknowledged. importani.) correction for gas imperfection using (18) J. G a u n t , Trans. F a r a d a y Sac., 49, 1122 (195i). TABLE I11 ;\IUI,AL K N ~ R U I 'AT Y 25' OF MoF6 A N D NbF,
~
THEltMODYNAIVIIC PROPERTIES OF HIGHER FLUORIDES. 11. THE HEA4TSOF SOLUTION ASD OF FORMATIOK OF JIOLYBDESUM HEXAFI UORIDE, TUXGSTES HEXAFLUORIDE AND KIOBIUM PENTAFLUORIDE'" 'BY 0. E. MYERSAND A. P. B R B U Y ' ~ C'ontribiLtzon from Stanford Research Znstztute, Menlo I'urk, Cal. Recezued October 81. 1969
From solution calorimetry, AHzss"for hloFe(1) was found to be -388.6 kcal./mole and that for WFe(g) -416 kcal./mole. No clear cut thermodynamic cycle could be found for N b F , but the average for three approaches giving entirely different end products, and fairly concordant results, was - 432 kcal./mole. These data, together with previously obtained entropies, permit calciilation of the free energies of formation of the three compounds.
Introduction Estiniat,es of the thermodynamic functions for the higher fluorides of molybdenum, tungsten and niob i ~ m ~have - ~ been largely intelligent guesswork. Entropies of the ideal gas have been calculated for MoF6and WF6from Raman and infrared spectra5-' but other data have been completely lacking. In a preceding paper* from these laboratories, experimental third law entropies were given a t 298.15 for liquid MoF6 and solid NbF5 and ST - Szas up to approxim:itely the normal boiling point for NbFB. (1) (a) This work was supported in p a r t b y t h e Materials Laboratory, Wright Air Development Center; (h) t o whom inquiries about this article should be sent,. (2) 0. Kuliachewski and E. Evans, "hletallurgical Thermochcinistry," Academic Prees, New York, h'. Y., 1951. (:i) A. Glassner, "The Thermochemical Properties of the Oxidw, Fluorides. and Chlorides for 250OoK.," 1857. (1) L. Brewer, "National Nuclear Series," Vol. I\', 19B, Ed. by I,. I,. Quill, McGraw-Hill Book Co.. New York, N. Y., 1951. ( 5 ) K . N.'fanner and 9. B. F. Duncan, J . Am. Chem. Soc., 73, 1164 (1951). (6) T. G. Burke, D. F. Smith and H. A. Nielsen, J . Chem. Phys., 20, 447 (1951). (7) J. Garnet, Tpans. Faraday SOC.,34, 368 (1963). (8) A . P. Hrady, 0. E. Myers a n d J. K. Clauss, THISJ O U R N A L64, , 383 (1960).
ANL-5750,
The present paper records the results of heats of solution experiments which, together with the data now extant, permit calculation of the free energy functions of the three fluorides. Experimental Materials.-The molybdenum hexafluoride, purified by trap to trap distillation in glass over sodium fluoride, was the same material used for the heat capacity determinations discussed in the previous paper.8 Tungsten hexafluoride, also obtained from the General Chemical Division, Allied Chemical and Dye Corporation, was purified in a similar manner. It is worthy of note that the use of Monel valves with Teflon packing for isolating the traps was found to be much more convenient than Fluorolube-lubricated glass stopcocks, which had a bad tendency to stick, previously used in the NoFe purification train. The preparation of the N ~ F s , from. metallic Nb and gaseous HF, has been described in the previous paper. It contained 0.9% Nb metal as an impurity, which has been corrected for in the calorimetric. Ivork. The 1100sused was a hand-picked lot assaying 1O0.Oc;, supplied by the S. W. Shattuck Co. It was dried a t 250" for six hours prior to use. All other reagents were C.P. grade. Temperature Measurement -The temperature of the calorimeter was measured by a Westinghouse 14B thermistor in a bridge circuit. The voltage supply (a 1.3-v. mercury rell) and ratio arms were immersed in a thermostat. The variable arm (a decade box) was in a constant temperature
$
platinum foil caps a t both ends, which werr attached to the vacuum system for filling by means of 3/32-in. platinum tubes hard-soldered to copper Honsekerper seals to Pyrex. The ampoules were removed from the vacuum line by pinching off the small platinum tube. For emptying the ampoules in the calorimeter, a plunger was constructed so that the blade forced both foils downward, but during the withdrawal proress the upper flap is pulled out. The Teflon pipet, used for MoFe, had the advantage of positive filling to a prerhosen amount, but if the nickel rod became corroded enough not to have a high luster, the subsequent emptying operation was a difficulty. Thc metal ampoules gave positive emptying, but filling to a convenient level was difficult because they could be wcighcd only after removal from the vacuum line.
G R O U N D GL. A S S AOlNT
BLADE NlCKIrL ROD _.--0
0005 I N PT
TEFLON -1/4 I N N I C K E L
PIPE FHARD
--
Results The results of the heat of solution runs are listed in Table I, in terms of the reacting condensed phase, the solution in the calorimeter (before the ampoule is broken), and the heat of reaction per mole of condensed phase. The subscripts used below refer to the reaction numbers in Table I. TABLE I
SOLDER
0 0005 IN PT
liE.YrS OF SOLUTION
AN, hcn1.l mole of ReacconTemp., densed tion r ReactnntsOC. phase Solution no. Condeneed phase 1 0 . 1 7 5 9 g. MoFs (1) 400 ml. 0.1075 f NaOII 25 0 -154 7 2 4 . 9 - 17 6 Same 2 ,2485 g. MoOa Same plm 0 . 2 6 7 2 g. 3 ,2434 g. N a P 009 25.0 MOO, 750 ml. 0 , 1 1 3 3 f NaOH 1 5 . 0 -149 4 4 2 . 2 7 8 0 g. WFe (1) 750 ml. 0 . 1 0 8 4 I NnOIi 2 5 . 0 -150 8 5 2 , 5 1 3 8 g. WFB( 1 ) 97.0 760 ml. 0.1123 f N a O H 24 8 6 0 . 7 7 5 0 g. NbFs 96.8 24.9 Same 0 . 6 7 1 6 g. NbFs 750 ml. 0,100 f NII4OH 2 5 . 1 -100 3 7 1 . 1 0 5 0 g. NbFi 5.61 24 4 750 ml. 2 . 9 8 f &SO4 8 0.5604 g. NbFs
.
I INCH
A
0
Fig. 1.--Mixing devices: A, Teflon-nickel pipe used for 1loFe; B, platinum-nickel ampule and plunger used for \FrFe and KbFs room. A Brown recorder modified to read 500 v. full scale was used as a null and drift indicator. The thermistor was calibrated electrically before and after each run; changes corresponding to about 0.1 cal. (ca. lo-* ")could be detected easily. Calorimeters.--The solution calorimeters w ( w of conventional type, consisting of a dewar vcsscl eqnipped wit,h an insulai,ed closure through which was led a shaft for a st'irrer, leads for a manganin heater of known rcuistance rind the thermistor, and a shaft for a capsule-emptying device. The whole was immersed in a constant temperature bath. The MoFe experiments were run in an all-glass calorimeter; because of a change in the design of the method for introducing the sample (see below) the WF6 and NbF, experiments were run in a dewar with a glass body but with metal shields for the heater and a metal stirrer. Metallic parts leading out of the calorimeter were of thin-walled Kovar to minimize heat losses. For ordinary materids (such as LIoO;j), it frangible glass bulb, containing the sealed-off samplc and attarhed to the bottom of the stirring rod by Apieson W, served as a convenient method of mixing for starting the solution process. For the higher fluorides, however, this method is objectionable for two reasom: the moisture released during the sealing-off proress worild start :L oyclical reactjion with the glass, and mommtary high concentrations of fluoride in the presence of an appreciable glass surface area a t the instant of breaking might lead to a not inconsequential side reartion. In order to overcome this difficulty two methods for sample introduction were devised. The first, shown in Fig. lrl in the filling position, was :L pipet with a Teflon body and a nickel plunger. A~prosimately0.075 ml. could be trapped in the annular space between nickel : m l Teflon by withdrawing the plunger t,o thc intermediate position. Thc: pipet then w:ts with(iraivr1from the filling assembly, cleaned cwernally and plac~vl in thc solution calorimeter. The liquid was discharged through the iiiachined slot in the Teflon by further withdrawal of the plunger after drift rates had been established. The actual amount of MOFGdischarged from the pipet was determined a t the end of the experiment by direct analysis of the solution by the Jones reduction methods. The second method for sample introduction is illustrated in Fig. lI3 The capsllles werc sections of nickel pipc, with
+ -
Molybdenum Hexafluoride.-The solut,ion reaction 1 corresponds to the skeleton react.ion MoF6(l)
+ iKaOH = ih;a:MoO4 + GXaF + 4IT:O
The heat of format'ion of MoF6, A N R I ~ then F ~ is given by A H > ~=~ ~F H~ ~ a z ~(ino soln.) o a - AH,
The chosen reaction conditions were such that the concentration of the NaOH is in a range where its heat of formation is nearly constant, -112.12 kcal./mole a t the initial concentration, - 112.14 kcal. at the finaL9 The heat of formation of liquid mater should then suffice, and was taken a's AHII~O = -68.32 kcal./mole.8 Reaction 2 was used in combination with reaction 3 to estimate f i l ~ N a l m , o l (in solution) via A H N ~ ~(in~ soln.) ~ ~ o =, AH, + I ) - ~ H N ~( ionIwln.'l s 4A N > f 0 n 3- AI€R,O
The heat of formation of RloOs was t,akeii as - 178.0 kcal./mole. lo Reaction 3 was to est'imate AHs~F (in soln.) via
+ Aff?;,~
L I H N(in ~ F soln.) = AHA
The heat of formation of NaF was taken as - 136.0 kcal./mole.8 The results obtained were AHN~~M (solution) ~o, = -357.6 AHN~F (solution) = - 1 3 5 . 9 = -388.6 i.4 keal./mole AHmoF, (9) National Bureau of Standards, Circular 500, 1951. (10) B. A . StaskiPwica, J. P. Tucker and P. E. Snyder, J. A m . Chem. Soc., 77, 2987 (1955).
I-lexrs OF S o ~ u ~ r oAND iu
Nay, lYGO
OF
FORMATION OF hlo, W AND NB FLUORIDES
The value for A H N ~ F(solution) is very close t o (as.), - 135.84 kcal./mole, that reportedg for AHN~F so there appears to be little ionic interaction in solution. Neglecting the heat of dilution, the data permit an estimate of -237.0 kcal./mole for the heat of formation of aqueous Mooa- (taking A",+ = -57.3). This is over 17 kcal. less negative than the value given in the Bureau of Standards compilation, but is in reasonable agreement with the more recent value of -238.2 kcal./mole, obtained by Graham' and Hepler" a t a somewhat higher concentration. Tungsten Hexafluoride.-Solution reaction7 4 and 5 in Table I are analogous to reaction 1. Becaiiie the second type of ampoule was uqed here. a sinall correction for \IT6 in the vapor phase was necessary (0.2 and 0.3 kcal./mole for reactions 4 and 5, respectively); the data of Ruff and Ascher12 mere used. The reaction analogous to reaction 2 could not be carried out in a reasonable length of time within the temperatures acceFsible to our calorimeter. However, the molybdenum data indicate a lack of strong interactions in solution; indeed, use of the best values for the heat of formation of the aqueous ions together with AHl would have led to an error of less than 0.5 kcal. in the estimation of AHxora. Consequently, the use of such a procedure with tungsten should yield a value reliable to about 2 kcal. Taking the aqueous heats of formation of OH-, F- and W04= as -54.96, -78.66, and -2.66.6 k ~ a l . / m o l etogether ,~ with the average of reactions 4 and 5 of - 150.1 kcal./mole (correcting for the t,emperature difference is not justified), the estimat,e for AHwF~ (liquid) is -422 f 4 kcal./ mole. The estimate of the error includes both that from the heat of solution measurements and that from the use of heats of formation at infinite dilution. Niobium Pentafluoride.--.It thc time of thcw experiment,
