Mass spectrometric studies at high temperatures—XV

J. inorg, nucl. Chem., 1967, Vol. 29, pp. 673 to 680. Pergamon Press Ltd. Printed in Northern Ireland

MASS SPECTROMETRIC STUDIES AT HIGH TEMPERATURES~XV SUBLIMATION PRESSURES OF CHROMIUM, MANGANESE IRON TRIFLUORIDES AND THE HEAT OF D I S S O C I A T I O N O F Fe2F6(g)

AND

K. F. Z~mov* and J. L. MARGRAVE Department of Chemistry, Rice University, Houston, Texas (Received 15 September 1966)

Abstract---Vapours effusing from Knudsen cells containing CrF3, MnF3 and FeF3 have been analysed mass spectrometrically. Only monomers of CrFs and MnF8 were observed in the vapour phase, while the mass spectrum of the vapour over FeF8 showed the presence of both monomers and dimers. The sublimation pressures and heats of sublimation of CrF3, MnF3 and FeF8 were measured and the dissociation energy of FesF6(g') was determined. INTRODUCTION

N o VAPOUR pressure data have been available for CrFs, M n F a and FeFa, but the heats o f vapourization o f these c o m p o u n d s have been estimated, tl) The heat o f sublimation o f F e F a was calculated t2> as AHB.~g8.15= 52-8 kcal mole -1 based on an estimated entropy o f sublimation ASs ° = 40 e.u. and the observation ta> that FeFa(c ) sublimed without fusion at temperatures near 1000°C. In the present experiments, a mass spectrometer equipped with a K n u d s e n effusion cell has been employed to study the sublimation of C r F a, M n F a and FeFa. EXPERIMENTAL The general characteristics of the experimental technique have been described previously, c4~ In the present work a tantalum cell was used in conjunction with a platinum liner. A platinum lid was used which had an effusion hole I mm i.d. The cell was heated by resistance heating from a doubleloop of tungsten wire. The temperature was measured by a Pt-Pt(10~.Rh) thermocouple, which passed through the hole in the bottom of the tantalum cell and was fastened in the base of the Pt-liner. The samples were obtained from A, D. Mackay, New York, and were used without further purification. RESULTS

CrF3 A typical mass spectrum of the v a p o u r above C r F 3 is given in Table 1. Appearance potentials were measured by the vanishing-current method, using b a c k g r o u n d mercury as a standard. As one can see f r o m the Table 1, the v a p o u r of C r F a consists only of m o n o m e r CrFa molecules. * On leave from the Boris Kidrich Institute of Nuclear Sciences, Belgrade, Yugoslavia. {1>C. E. WicKs and F. E. BLOCK,Bur. Mines Bull., Wash. 605 (1963). ~2~JANAF Thermochemical Tables (Edited by D. R. STOLL). Clearinghouse for Federal Scientific and Technical Information, Springfield, Va., No. PB-168-370 (1965). c3) POOLSNC,Ann. chim. Phys. 2, 1 (1894). ~4>G. BLot, J. W. GREEN,R. G. BAtrrtSTAand J. L. MAROrtAV~,J. phys. Chem. 67, 877 (1963). 673 6

674

K . F . ZMaov and J. L. M~ORAVE

TABLEI . ~ M A s s svFxrrRtrMoF CrFs

VAPOUR

AT I073°K

Ion

Relative ion intensity (50-eV electrons)

Appearance potential (eV)

Cr+ CrF + CrFi + CrFs +

16.0 15.2 100.0 19.3

20.1 :t: 0.3 14.0 -4- 0.3 13"5 4- 0"3 12.2 + 0.3

The sublimation rate of CrFa was measured by following the temperature dependence of the CrF3+-ion intensity in the temperature range 906--1058°K. By plotting log I+T vs lIT, one obtains the enthalpy change for the reaction CrFa(s) = CrFa(g)

(1)

from the slope of this plot. This treatment yielded the heat of sublimation of CrFa at the mean temperature of the experiment, AH~A0o o x = 57.9 4- 0.6 kcal mole -1, where the uncertainty quoted represents the standard deviation of the least-squares treatment of the data. With an estimated value ACp(s -- g) ---- 3.5 cal mole -1 °K -1, one obtains AH°zgs[CrFs(s)] = 60-3 -4- 3 kcal mole -1, with the uncertainty reflecting possible errors in ,temperature measurements and the estimation of the thermodynamic functions. The pressure calibration was obtained by vapourizing a weighed a m o u n t of a previously outgassed CrFs sample at a constant temperature, With known evaporation time, Knudsen ceU orifice, temperature and weight loss, the absolute vapour pressure was obtained .by employing the Knudsen-Hertz equation, tS~ This quantity was then used to evaluate the sensitivity constant k in the current-pressure relationship, (e~ P ---kI+T. The value of k, combined with the (I+T) data yielded the least-square vapour pressure equation: log Patm(CrFa) = --(1"264 ~: 0"012) × 104/T + 7"20 -4-0"12

(2)

The entropy of sublimation obtained from the second-law plot is AS~,loot = 32.9 + 0.6 e.u. T h e sublimation pressure data for CrF3(s) are plotted in Fig, 1. MnFz Reagent-grade MnFa was placed in a tantalum cell with a platinum liner and its vapourization studied in the temperature range 999°-1133°K. The mass spectrum (TabIe 2) and the appearance potentials indicated that MnFs evaporates principally as monomeric MnFa molecules. The appearance potentials of the various ions resembled those of the CrFs-system, while the relative intensities showed clear differences, the most pronounced of which is the relatively small abundance of the parent MnF3+-ion compared with that of the CrFz + ion. ts~j. L. MARORAW,in Physieo-ChemicalMeasurementsat High Temperatures(Edited by J'. BOCKRIS, J. Wm'r~ and J. MAClCENZm). Butterworths, London (1959). c.~ W. A. Ctrer..A and M. G. INGrm~, J. phys. Chem. 59, 100 (1955).

Mass spectrometric studies at high temperatures--XV

675

-4.0,

I

w ~-5.C o

a. - 6 . 0 o

MEASUREMENT o MASS SPECTROMETERDATA, RELATIVE PRESSURES

-7.0 9.0

o

912

914

~6

918

,;.o

,d2

IO 4 / T

(*K'll

,0.4

,o=6

,o'.8

~'~

i!

.'o

.'.2 j

Fie. 1.--Sublimation pressure data for CrF3,

TABLE2.--MAss SPECTRUMOF VAPOUROVERMnFa(s) AT 1070°K

Ion Mn + MnF + MnFa + MnF3+

Relative ion intensity (50-V electrons)

Appearance potential (eV)

20.2 100.0 35"3 3'2

19.5 :~ 0-5 14.2 :L 0.3 13.7 + 0,3 12 i 0.8

Two independent measurements of the heat of sublimation were made by monitoring the temperature dependence of M n F + and MnF2+-ion currents, The corresponding heats of sublimation obtained were AH1,,970 -- 66.1 + 0.7 kcal mole -1 and AH2,.9, = 65.2 :~ 0.5 kcal mole -x, respectively, in good agreement and indicative that both ions originate from the same precursor. When corrected to 298°K the heat of sublimation is 68.0 ± 3 kcal mole -I. A weight-loss measurement at 1069°K with an orifice of 1 m m dia. gave Am ~1.20 × 10-Sg in 80 min and yielded the absolute pressure P = 2.23 × 10-6 atm. This quantity was used to evaluate the sensitivity constant k. The entropy of sublimation is A ~ s 2 = 35.1 ~: 0,5 e.u. and the derived vapour pressure equation log Patm(MnF3) = --(1"425 -4- 0"011) × The vapour pressure data are shown in Fig. 2.

104/T+ 7"68 ± 0"10

(3)

676

ZmBov and J. L. MARGRAVE

K.F.

,Q'

i

J

i

,

~

i

-SD

-5.2

~ -,,54 -5,6 w -5.S

~ -6.0 ~ -62

MEASUREMENT o MASS SPECTROMETERDATA, RELATIVE PRESSURES

-6.4

o~ Xo, ~ o

-6.6 I

8.6

8~

,'o

,i~

;, I0 4 / T

;~

,'~

I

i

,oo

10.2

(°K'!)

FIG. 2 . - - S u b l i m a t i o n pressures o f MnFs.

FeF a The mass spectrum for FeF S vapour is presented in Table 3. The close values of the appearance potentials of Fe +, FeF +, FeF2+ and FeFa+ ions to those of corresponding CrFn+ ions indicates that they are products of dissociative ionization of monomeric FeFs molecules. The only polymeric ions of appreciable intensities are FezFs+ and F%F4+. The arguments presented earlier (6) which excluded the possibility of formation TABLE 3.--MASs

SPECTRUM OF

FeFa

VAFOtm

AT

926°K

Relative ion

Ion Fe + FeF + FeFi + FeFa + Fe~F4 + FosF~ +

intensity (50-V electrons) 15.6 34"2 100.0 16"4 0.2 0.7

Appearancepotentials (eV) 20-5 16.0 13.7 12"5

4444-

0.3 0.3 0.3 0"3

-12.1 4- 0-5

of polymeric ions by ion-molecule mechanisms under the given experimental conditions can be applied to the present experiments. The Fe2F6+ and Fe2F4+ ions were thus assumed to be formed primarily by dissociative ionization of the Fe~F6 molecule. The heat of sublimation of FeF 3 was determined by employing the method described for CrFa and MnF a. The second-law plot of log (I~el%++× T) vs. lIT yielded the heat of sublimation at the average experimental temperature, AHs.~l -~ 52.7 4- 0.6 kcal mole -1. Again, an absolute pressure calibration yielded the sensitivity

Mass spectrometric studies at high temperatures--XV

677

constant k, which, combined with the least-squares data for log (I+T) vs. 1/T, makes possible the calculation of the vapour pressure equation: logP --~ --(1.152 ~ 0.012) × 104/T+ 8.18 i 0.14

(4)

0 and the entropy change, ASs,as 1 = 37.4 ~ 2 e.u. The vapour pressure data are presented in Fig. 3.

-35

-4C

uJ -4~=

~

O V_SC u~ ~-55

-6£

MEASUREMENT e

-6.5

,o.,

,oi~

,o'.8

~ .

MASS SPECTROMETER DATA, RELATIVE PRESSURES

,!o

,,i~

I

,,.,

,'.~

,,~.8

I04/T

(°K'l)

12.0 ~

12.2 '

121.4

12.6

12.8

FIG. 3.--Sublimationpressure of FeF3. When corrected to 298°K with the heat-capacity data from the JANAF Tables, (~) one obtains AH°29s ----- 55.6 ± 3 kcal mole-z. The values of the free energy functions from the JANAF Tables (z) were therefore combined with the experimentally determined vapour pressures to obtain the third-law values of the heat of sublimation. The results are presented in the Table 4. The error of 0.07 kcal mole-1 attached to the average value is the standard deviation from sixteen measurements, while the true uncertainty, which includes possible errors in the estimation of the thermodynamic functions, must be higher. The third-law heat is therefore, taken to be 60.8 ± 2.0 kcal mole-1. The difference of 5 kcal mole-x between the second- and third-law values suggests possible errors in the second-law determinations, due probably to non-uniform temperature distribution in the effusion cell, and/or molecular parameters which are significantly different from those assumed in the JANAF-compilation.

Heat of dimerization of FeF3(g ) A separate experiment was performed in order to determine the enthalpy of the dimerization reaction: 2FeFa(g) = Fe2F~(g)

(5)

678

K. F. Zr~mov and J. L. MAROI~,W TABLE4.--MAss

SPECTROMETRIC DATA FOR

FeF8 strma~aaou

T (°K)

(arbitrary units)

-- Log P (atm)

(cal. deg-x rnole-0

AHO0s (kcal mole-t)

788 814 833 832 833 847 865 880 896 898 915 932 935 936 950 926

300 750 1600 1620 1650 2800 5500 9200 15,000 15,400 28,200 44,000 43,000 44,500 71,000 41,000

6"323 6.003 5'663 5"666 5.652 5.414 5"128 4"876 4"661 4.648 4"378 4'177 4"185 4.169 3"959 4.210

47"180 46"678 46"835 46.840 46"835 46.886 46.704 46"643 46.577 46"571 46.504 46'439 46"427 46"423 46.370 46"461

59"98 60"55 60.60 60.59 60.63 60"66 60-71 60"78 60.92 61"01 60"97 61"17 61"40 61"40 61.36 60"91 Average 60'85 4- 0-07

* See Ref. 2. The procedure in this determination was similar to that described previously. {7~ The partial pressure ratio of dimer to m o n o m e r molecules is represented by the equation:

L%

J

(6)

where or,,, and cr~ are the relative ionization cross sections and S~ and Sa multiplier efiiciencies for the m o n o m e r and dimer respectively. The ratio cr,,/cr~ was estimated to be 0"5, and the S,JSa ratio was calculated to be approximately 1"5. By using then the previously determined sensitivity constant of the mass spectrometer and vapour pressure data for FeFa (Fig. 3), one calculates the partial pressure of Fe2F6. The semi-log plot of P1%1% vs. 1/T is shown in Fig. 4. A least square calculation of the slope yields AH°1001[Fe2Fe(g)] = 86.8 -4- 2.9 kcal mole -1. In Fig. 4 is also shown the temperature variation of the equilibrium constant of the reaction: Fe2F6(g ) = 2FeFa(g )

(7)

F r o m the slope of the dimerization equilibrium plot, one obtains A//~00x -----29"8 + 3"8 kcal mole -1 for the heat of dissociation of Fe2F e. DISCUSSION The data obtained on the C r F a, MnF3 and FeF 3 can be utilized to evaluate the atomization energies of these compounds. The available data on the heats of formation of the fluorides, A H / ( M F a ) , eaJheats of sublimation ofcorrespondingmetals, AH~(M), tg) tT~R. F. PORTERand R. C. SCrlOONMAK~R,d. chem. Phys. 29, 1070 (1958). c8~R. C. FEnER, U.S. Atomic Energy Report LA-3164 (1965). ~0~R. HULaX3~N,R. L. ORR, P. D. ANDERSONand K. K. KELLEY,Selected Valuesof Thermodynamic Properties of Metals and Alloys. J. Wiley, New York (1963).

Mass spectrometric studies at high temperatures--XV

679

F~F6(g) = 2 FeF3(tj)

I

FeF~Cs)= FeF~(g)

I

10-5

10-7I 97

98

99

I0.0

104 / T (*K-')

I01

102

103

FIG. 4.--Equilibrium plots for the FeF3-system.

and the dissociation energy of fluorine, D(Fz), (~) were used together with the heats of sublimation in the thermochemical cycle AHa(MFa) ---- --AHy°(MF3) -- AHs°(MFa) + AHs°(M) + 3/2[D(Fz)]

(8)

The results of the calculations are presented in Table 5.

TABLE5.--HEATS OF ATOMIZATIONAND AVERAGEBONDENERGIESFOR CrUz, MnF3 AND FeFs Compound

AH~(MF3) (kcal mole-0

AHs°ub(MF3) AHs°~b(M) (kcal (kcal mole-0 mole -1)

CrF3 MnF3 FeF3

--265.2 --238 ! 5 --235 4- 13

60.3 4- 3.0 68"0 4- 3.0 60.8 4- 3.0

94.85 + 0.5 67.06 4- 0-4 99.55 4- 0.2

3/2[D(F,)] AH°tm(MUs) I/3AH°t(MFa) (kcal (kcal (kcal mole-0 mole-0 mole -i ) 56.5 + 1.5 56.5 4- 1-5 56.5 4- 1-5

356-3 4- 10 293.6 4- 10 339.3 4- 17

118.8 4- 4 97.9 4- 4 110.1 4- 6

Table 6 lists the bond dissociation energies of the chromium, manganese and iron fluorides, calculated on the basis of the present results and of earlier experimental data. (1°-12) The general trends are as predicted but the breaking up of the half-filled 3d-shell in MnFz to form MnFz seems to cost 25-40 kcal mole -1 in binding energy. (lo) R. A. KENT, T. C. EI-ILERTand J. L. MARGRAVE,J. Am. chem. Soc. 86, 5060 (1964). ~11) R. A. KENT and J. L. MARGRAVE,J. Am. chem. Soc. 87, 3582 (1965). (lz~ R. A. KENT and J. L. MARGRAVE,J. Am. chem. Soc. 87, 4754 (1965).

680

K . F . ZMaOV and J. L. MAnt~KAW TABLE6.--BOND DIS~CIATION

ENERGIES FOR

CHROMIUM, MANGANESE AND IRON FLUORIDES

Element Cr Mn Fe

Acknowledgements--Thiswork

Bond energy (kcal mole-1) D(M-F) D(FM-F) D(FsM-F) 106 101 (108)

121 119 112

129 74 100

was supported by the United States Atomic Energy Commission. The authors would also like to thank Mr. ROBSRTB. C~Tm~LD for programming the IBM computations.