NOTES
3174
Knudsen and Langmuir Measurements of the Sublimation Pressure of
Thermodynamic calculations show that such reaction is unlikely under the conditions of these experiments. The Clausing factor used in the calculations was To=
Cadmium(I1) Fluoride
by G. Besenbruch, A. S. Kana'an, and J. L. Margrave Department of Chemistry, Rice University, Houston, Texas (Received March 8,1966)
Sublimation pressures for CdFz have been measured over wide ranges of temperature by both the Knudsen and Langmuir techniques utilizing a vacuum semimicro balance. Vaporization data for CdFz were reported by Ruff and LeBoucher,' who investigated the vapor pressure of CdF2 over the temperature range 1689-2023OK. by a boiling point technique. Brewer, et a1.,2in their review of thermodynamic properties of gaseous metal dihalides, derived the heat of sublimation, A H o Z ~= s 76 kcal. mole-l.
Experimental
*
The vapor pressure studies were carried out with an Ainsworth RVA-AU-2 semimicro balance which recorded the weight loss of the sample as a function of time. The sample (either a single crystal or a powder contained in a Knudsen cell) was suspended by a tungsten wire and enclosed in a quartz or Vycor tube. A Kanthal wire-wound tube furnace with a BarberColeman controller, Model 622, was used for maintaining the sample at the desired temperature ( f1") as measured by a calibrated Pt-Pt-lO% Rh thermocouple suspended inside the Vycor tube, with the junction and sample at the same level. The Knudsen cell was machined from a spectroscopic grade graphite rod and the orifice in the lid was 2.46 mm. in diameter and 0.56 mm. deep. The CdF2 single crystal (surface area = 1.43 f 0.2/cm.9 was obtained from SemiElements Inc., Saxonburg, Pa. After the Langmuir studies were completed, the crystal was powdered for the Knudsen studies. For increased accuracy, two or more measurements of the rate of weight loss a t each temperature were made and the vapor pressure was calculated from the appropriate equations3 on the assumption that CdFz(g) is the vapor species. Mass spectrometric studies of related fluorides by Klemperer4 and Ehlert, et U E . , ~ showed that only the metal difluoride vapors result from heating the solids at temperatures in the range of these studies. Also, there was no evidence for a reaction between the graphite crucible and CdFz(g). The Journal of Physical Chemistry
Table I: Sublimation Pressures of CdFz by the Knudsen Method -A(F", Press. obsd., atm.
T , OK.
2.87 x 4.07 X 6.70 X 8.36 X 1.02 x 1.33 x 1.67 X 1.68 x 2.32 x 2.92 x 3.60 X 3.78 x 4.79 x 6.13 x 7.43 x 9.71 X 1.19 x 1.51 x
1092 1098 1118 1128 1138 1148 1158 1160 1172 1183 1191 1193 1203 1214 1223 1235 1245 1255
~
10101010-6 10-3
10-3 10" 10-6 1010-5 10-6 10-6
10-6 10-
10-4 10-4
-
H o a ~ r i ) / Toal. ,
AHo29u.~s,
deg.-' mole-1
keal. mole-'
41.8 41.8 41.7 41.7 41.7 41.6 41.6 41.6 41.6 41.5 41.5 41.5 41.5 41.4 41.4 41.4 41.3 41.3
73.18 73.17 73.27 73.32 73.38 73.42 73.45 73.47 73.53 73.58 73.61 73.63 73.65 73.70 73.70 73.75 73.78 73.82 Av. 73.52
~~
Table II : Sublimation Pressures of CdF, by the Langmuir Method -A(F'T
-
H O W ) / T , cal.
T , OK.
921 941 951 961.5 971 982 992 1002 1012 1022 1041
Press. obsd., atm.
9.76 X 2.08 X 3.31 X 4.25 X 6.88 X 8.52 X 1.35 x 1.70 x 2.50 X 3.36 x 6.20 x
10-6 10% 1010" 10" 10" 10-7 10-7 109 10-7 10-7
deg.-l mole-1
42.3 42.3 42.2 42.2 42.2 42.2 42.1 42.1 42.1 42.0 42.0
AH"sw.ir, kcal. mole-1
72.72 72.81 72.87 72.91 72.97 73.03 73.08 73.11 73.15 73.20 73.27 Av. 73.01
(1) 0.Ruff and L. LeBoucher, 2. anorg. allgem. Chem., 219, 376 (1934). (2) L. Brewer, G. R. Somayajulu, and E. Brackett, Chem. Rev., 63, 111 (1963). (3) J. L. MarErave in "Physioo-Chemical Measurements at High Temperatures, J. O'M. Bockris, J. L. White, and J. D. MacKenzie, Ed., Butterworth and Co., Ltd., London, 1969, Chapter 10. (4) W. Klemperer, J . Chem. Phys., 40, 3471 (1964). (5) T.C.Ehlert, R. A. Kent, and J. L. Margrave, J. Am. Chem. SOC., 86, 6093 (1964).
NOTES
3175
0.492 as obtained from the tabulations of Iczkowski, et aZ.,6 relating the length/radius of the orifice to Wo. The surface area was assumed to decrease linearly with the weight loss and corrections for this changing area were made in the calculation of the rate of weight loss per unit area. This change was less than 35% of the original area.
I
'
I
I
0
-4.0
KNUDSSEN LANGMUIR
Results and Discussion The observed sublimation pressures, PK and PL, are given in Tables I and 11. The experimental data were fitted to a log P vs. 1/T equation by the least-squares method using an IBM 1620 computer. The Knudsen and Langmuir data are fitted by eq. 1 and 2, respectively log PK= 7.391
f 0.015
-
(14.089
f 0.080)
I
X loa
'
1
1
I
0.8
0.9
I .o
1.1
T
q
PI0
Figure 1. Sublimation pressure of cadmium fluoride.
log PL = 7.563
f
0.021
-
(14.341
=k
0.113) X loa
T (2)
and are plotted in Figure 1. The second-law heats of sublimation obtained from Clausius-Clapeyron plots are
AHolls5= 64.47
f 0.36 kcal.
mole-1 (Knudsen experiments)
and
AHoggo = 65.6% f 0.52 kcal. mole-' (Langmuir experiments) When corrected to 298'K. assuming ACp = -9.0 cal. deg.-l mole-l for the sublimation process, these heats are identical within experimental error, AHozgs= 72.0 f 3.0 kcal. mole-l. The small uncertainties are standard deviations of the experimental points from the straight line and do not include the uncertainties in the temperature, surface area, orifice cross section, and ACp values; a more realistic appraisal of the results suggests =k3 kcal. mole-' as the possible error for the second-law heat of sublima,tion. A combination of the vapor pressure data with the free energy functions from Brewer, et aZ.,2produces an average third-law heat of sublimation at 298'K. of 73.3 f 3.0 kcal. mole-l. Thus, AHozas= 72.6 f 3.0 kcal. mole-* is the selected sublimation energy for CdF2. PL is always slightly lower than PKand from one concludes that LYL,the Langmuir the ratio, PLIPR, coefficient, is in the range 0.9-1.0.
The second-law entropies of sublimation obtained from Clausius-Clapeyron plots are A S O I I S S = 33.82
f
0.07 e.u.
(Knudsen experiments)
and ASo9g0
= 34.61 f 0.10
e.u. (Langmuir experiments)
These values are in excellent agreement with the corresponding values obtained from the data given by Brewer, et aL2 When corrected to 298'K., the entropy of sublimation of CdF2, AS'Zg8 = 45.8 f 0.5 e.u. The heats and entropies of sublimation obtained by both methods are in satisfactory agreement when one considers the difficulty of maintaining isothermal conditions and of finding a nonreactive cell, along with the uncertainties in the thermodynamic functions. The results are also in reasonable agreement with the early work of Ruff and LeBoucher.' The atomization energy of CdFz(g) may be calculated from the heat of sublimation and other available thermochemical One finds AH.' = 158.6 f 3 kcal. mole-1 as compared with 153.2 given by Brewer, et aZ.,2 and thus an average Cd-F bond energy (6) R. P. Iczkowski, J. L. Margrave, and S.M. Robinson, J. Phys. C h a . , 67, 229 (1963). (7) G. N. Lewis and M. Randall, "Thermodynamics," revised by K. S. Pitzer and L. Brewer, 2nd Ed., McGraw-Hill Book Co., Inc., New York, N. Y., 1961, p. 672. (8) (a) R. P. Iczkowski and J. L. Margrave, J. Chem. Phys., 30, 403 (1959); (b) J. G. Stamper and R. F. Barrow, Trans. Furadau SOC., 54, 1592 (1958). (9) E. Rudsitis, H. M. Feder, and W. N. Hubbard, J. Phys. Chem., 67, 2388 (1963).
Volunta 69,Number 9 September 1966
NOTES
3176
of 79.3 f ]I kcal. mole-’. HerzberglO and Gaydonll found the data on CdF(g) inadequate for establishing the dissociation energy. By comparison with the ratios D(Mn-F)/AHao (MnF2) = 0.46,12 D(Cr-F)/ AH,O(CrFZ) = 0.47,13and the general conclusions from studies of group I1 and group IV mono- and dihalides,l4> l5 one estimat,esD(Cd-F)/AH,’(CdFz) = 0.46 f 0.02 and D(Cd-F) = 73 f 5 kcal. mole-’. Aclcnowileclgments* The authors wish to acknowledge the financial. support of this work by the United States Atomic Energy Commission, by the Advanced Research Projects Agency through funds administered by the Army Research Office, Durham, and by the Robert AFoundation* We wish to thank Mr- David Bonnell for carrying out the calculations on the IBA4 computer.
w*
(10) G.Herzberg, “Molecular SEectra and Molecular Structure. I. Spectra of Diatomic Molecules, D. Van Nostrand Co., Inc., New York, N. Y., 1’350. (11) A. G. Gaydon, “Dissociation Energies,” Chapman and Hall, Ltd., London, 1953. (12) R. A.Kent, T. C. Ehlert, and J. L. Margrave, J. Am. Chem. Sac., 86, 5090 (1964). (13) R.A. Kent and J. L. Margrave, to be published. (14) G. D. Blue, J. W. Green, T. C. Ehlert, and J. L. Margrave, Nature, 199,804 (1963). (15) T. C. Ehlert and J. L. Margrave, J . Chem. Phys., 41, 1066 (1964’1.
ably close to that of Br-. NCO- appears to reach its peak in the vacuum ultraviolet ~ p e c t r u m . ~The spectrum of SCN- was first reported by Rabinowitch,5 who assigned the inflection in the 220-mp region to a c.t.t.s. band with Xmax 224 mp. No evidence was given to this assignment and it was recently stated that the peak actually lies in the vacuum ultraviolet spectrum.6 The electronic spectra of SeCN- and TeCN- have not been reported before. We wish to report here some concerning the spectraof SCN-, SeCT\ii, and TeCN- md to show the relation between the electrochemical and spectroscopic series.
Experimental KNCS (C.P.) was twice recrystallized from ethanol and dried Over CaClz in a vacuum desiccator. The solvents used were of Spectrograde quality and all other materials of Analar grade. Aqueous solutions of NaNCSe and NaNCTe were prepared by shaking Se and Te, respectively, in lo-’ M NaCN solutions. Accurately weighed quantities of Se were dissolved (ca. 1 mg. in 50 ml.) whereas Te dissolved only slightly after long shaking and warming, so the concentration of TeCN- could not be determined. All of the spectra were taken with a Hilger Uvispek spectrophotomer in a thermostated cell compartment ; 5-mm. silica cells were used. The sample differed from the reference solution only by containing -2 X mole/l. of the solute investigated. “
I
The Relation between Electrochemical and
Results and Discussion
SpectroscopicProperties of the Halide and
Figure 1 shows some of the results obtained. There is a close resemblance between the spectra of the three anions, showing a gradual shift to long wave lengths as the electronegativity of the group - VI atom decreases. I n water theyall display a long wave length “shoulder” with a relatively high intensity, which is presumably due to the overlap of two electronic transitions. The properties of the long wave length band (A), as studied for SCN-, lead to its assignment to a c.t.t.s. origin. Thus, it appears to be sensitive to environmental effects and its response is of the same type as that of a c.t.t.s. band’: relative to HzO it is blue shifted by alcohol and red shifted by CH3CN. A large red shift
Pseudohalide Ions in Solution
by E. Gusarsky and A. Treinin Department of Physical Chemistry, Hebrew University, Jerusalem, Iwael (Received March 10, 1966)
The reducing power of the halide and pseudohalide ions, as measured electrochemically, increases in the following order’: F-, NCO-, C1-, N3-, Br-, SCN-, I-, SeCN-, TeCN-. This sequence also should be reflected in .the electronic spectra of the ions, especially in their charge-transfer-to-solvent (c.t.O.s.) bands which involve a kind of an oxidation-reduction process (the halide ions display only this type of excitation). Thus we expect a gradual shift of the c.t.t.s. band to longer wave lengths when proceeding along the series from F- to TeCN-. This is actually the case for the halide ions.2 The c.t.t.s. band of N3- has been studied3 and though its Amax could not be exactly located it is probThe Journal of Physical ChEmi3tTy
(1) (a) L. Birckenbach and K. Kellermann, Ber., 58, 786 (1925); (b) G . R. Levi and A. Perotti, Gazz. chirn. i t d . , 88, 640 (1958). (2) For recent data see J. L. Weeks, G. M. A. C. Meaburn, and S. Gordon, Radiation Res., 19, 559 (1963). (3) I. Burak and A. Treinin, J . Chem. Phys., 39, 189 (1963). (4) I. Burak and A. Treinii, unpublished results. (5) E. Rabinowitch, Rev. Mod. Phys., 14, 112 (1942). (6) J. C. Barnes and P. Day, J. Chem.SOC.,3889 (1964). (7) I. Burak and A. Treinin, Trans. Faraday Soc., 59, 1490 (1963).
