NOT^
1760
substances give long-lived negative ions.8.8 The molar conductivity of 0%is 65 ohm-' cm2 mol-';' a plausible value of 30 ohm-' cm2 mol-' was used for CsHaNOz-. Nitrobenzene also acts as scavenger X, for OH and H according to eq 3 and 5a.1° I n the c a s of 02,isopropyl alcohol was added as OH scavenger. The peroxy radical (CH&C(OH) (02.) that is formed in isopropyl alcohol-OZ solutions does not ionize in slightly acid solutions. It finally gives conducting tetroxides in a comparatively slow reaction which can he neglected on the time scale of our present experiments." The electron pulse always lasted 0.3 psec; the dose was 4&2200 rads per pulse. The H + concentration as measured by conductometry before the pulse amounted to more than 4 X lo-' M in most of the experiments (range, 2 X lo-' to 8 X lo-' M ) . Since OH- is formed in concentrations of only 0.2-1.5 X lo-' M , the neutralization after the pulse leads to a pseudo-first-order decrease in conductivity. Knowing the total H + concentration after the pulse, the rate constant for the reaction of H + with OH- could he calculated from the measured half-life of the decrease in K. A value of 1.1 0.2 X 10'' M-I sec-' mas obtained in sufficient agreement with Eigen's measurements which shows that the initial drop in conductivity is really due to the neutralization H + OHH20. For several microseconds after the pulse, the conductivity signal is distorted. By extrapolating the first-order decay of the conductivity back to time zero, the increase in conductivity ~i immediately after the pulse was obtained. K , was obtained from the conductivity change of the solution after 100 psec. Figure 1 shows a typical oscillogram and a semilogarithmic plot of (K - K f ) / K f us. time. The average of the ratio G(0H-)/G(e.,-) obtained 0.02 in solufrom 40 experiments amounted to 0.28 tions containing nitrobenzene at 1 X lo-' M and de0.02 with increasing nitrobenzene creased to 0.23 concentrations up to 1 X M . It would indeed he expected that nitrobenzene scavenges electrons in the spurs at higher concentrations which would increase G(eaq-). An average value for G(0H-)/G(e.,-) of 0.26 0.02 was obtained for the isopropyl alcoholsaturated 0%system (isopropyl alcohol, 1 X to 1.4 X lo-* M ) . Taking G(e.,-) = 2.7, we obtain G(0H-) = 0.73 0.1 from the average value of the measured ratios G(0H-)/G(e.,-) in agreement with Barker, et aL6 A few remarks should be made ahout some minor corrections to the measurements. Since the initial H + concentration of the solution was rather loa, a slight decrease in pH resulted from the production of H + XI-. Some of the OH- ions initially present in the solution were therefore neutralized. The correction due to this effect generally was leas than 12%. The conductivity of the water used amounted to 40-S070 ahove the theoretical value at 25". The worst im-
*
+
-
*
*
*
*
+
The J o u d of Phys*al Chmistru, Vel. 76,No. 11.1971
pulse + LIOOps-cl
0
LO
20
tIpslFigure 1. Upper part, oscillogram for the change in = 2.9 X IO' M before conductivity m a function of time; ["+I the pulse. Additional [H+l produced by the pulse w'ss 0.95 X 10-7 M ; nitrobenzene Concentration, 1.7 X 10-8 M . Ordinate scale, 2 mV per major division. Lower part, semilogarithmic plot of (K - x r ) / s . The extrapolation to 1 = 0 yields (6
- xr)/xt
= 0.41.
purity that can influence the measurements is hicarbonate. Even if one assumes that bicarbonate is responsible for all the excess conductivity the results are not changed. (8) K.-D. Asmus. A. Wigger, and A. Hengllein, Be?. B=nsengm. Phyr. Chem., 70,862 (1966). (9) (a) J. Rabani and 8 . 0 . Nielsen. J . Phya. Chem., 73,3736 (1969): (b) D. Behar, G . Czapski. J. Rabani, L. M. Dorfman, and H. A. Schwarz. i6L1... 74.. 3209 (1970). . . (10)K.-D. Asmu. B. Cercek. M. Ebert. A. Henglein, and A. Wigger. Tlons. Faraday Soe., 63,2435 (1967). (11) K. Stockhausen, A. Foitik. and A. Henglein. Be,. Bunaewes. Phus. Chem.. 74, 34 (1870).
Mass Spectrometric Determination of the
H e a t s of F o r m a t i o n of AlOCl(g) and AlOF(g)' by R. D. Srivastava and M. Farher* Space Sciences. Inc.. Monrooia, CdiforniO (Receiwd Nooember 843 1070)
91016
P u b l d i o n msts oaswtd bu Spoce Sciences. Inc.
The possible existence of the AlOCl molecule was first reported by Fischer and Gewehr.? Schafer, (1) This work was sponsored by the Air Force Rocket Propulsion Laboratory, Air Force Systems Command. United States Air Fore,
Edwards. Calif. (2) W. Fisoher and R. Gewehr, Z. Anorg. All. Chem., 209.17 (932).
NOTES
1761
et aZ.,3,4 obtained a value of -190 kcal/mol for the AHr of crystalline AlOCl based on calorimetric measurements of the heat of solution of AlOCl(c) in aqueous HCI. Employing the molecular flow effusion method, Farber, et aZ.,6 studied the heat of formation of AlOCl(g) at 2400°K by means of a study of the reaction AlC13(g)
+ A1203(1) = 3AlOCl(g)
(1)
and reported a value of -83.2 f 5 kcal/mol for AHf298 for AlOCl(g). The only reported values for the AlOF molecule are the results of Iparber and Petersena using the molecular flow effusion method. For the reaction
+
A1203(~) A11?3(g) = 3A10F(g)
(2)
third law studies at 2200°K yielded a value of - 140.2 f 2.6 kcal/mol for AHf298 of AlOF(g). Since none of the previous work directly identified AlOCl(g) and AlOF(g), the present work involved a mass spectrometric study to identify the AlOCl(g) and AlOF(g) species in the temperature ranges 1473 to 1600°K and 1483 to 1923°K) respectively. The results are compared with the published data.
Cl(g)
Results and Discussion AlOCZ Molecule. A mass spectrometric determination was made of the reaction of A1208 with gaseous chlorine in the temperature range 1473 to 1600°K. Mass spectrometer intensities were obtained for the species involved in the isomolecular reaction
=
+ Al(g)
AlOCl(g)
(3)
The equilibrium constants in terms of ion intensities for reaction 3 are given by
K, =
IAIOC JA1 ____ IAi201c1
(4)
The appearance potential for AlOCl(g) was found to be 12 f 1 eV. The intensities and thermodynamic data for the third-law calculations based on JANAF thermochemical datag for the species AlzO(g), Cl(g), and Al(g) are presented in Table I. The second-law AHr at an average temperature of 1540°K is -6.4 f 1 kcal. This yields a AHf2g8 of -86.4 i= 2 kcal/mol. The average third-law value for AHf298 of AlOCl(g) is -82.5 f 1 kcal/mol, which is nearly identical with the value of -83.2 kcal/mol obtained from the effusion experiment s.j Table I : Ion Intensities and Thermodynamic Data for the Reaction Cl(g) AlzO(g) = AlOCl(g) Al(g)
+
+
TAST, T, OK
Experimental Section The mass spectrometer, vacuum systems, and furnace apparatus employed for these studies have been described previously.’ The effusion cell was fabricated of alumina (inside diameter 6.8 mm, length 25.4 mm) with an orifice diameter and thickness of 0.986 and 6.6 mm, respectively. An alumina connecting tube with an inside diameter of 1 mm was used both as a reservoir for the AlF3and as a flow tube for the C1,. The gaseous chlorine was contained in a lowpressure vessel and was metered directly into the effusion cell for the studies involving AlOCl(g). For the AlOF(g) investigation the alumina tube acted as a reservoir and contained AlF3 crystals. The AIF3reservoir was heated with a tungsten heater to a temperature which produced the desired vapor pressure of A1F3in the reaction cell. The ion intensities were identified by their masses, isotopic distribution, and appearance potentials. The shutterable, or chopped, portion only of the ion intensities was directly recorded. I n order to minimize contributions due to fragmentation, ionizing electron energies of 3 eV above the appearance potentials were used to ensure that the species were the parent ions. The previously reported ion appearance potentials8 used in these studies were for AI, 6 eV, for AlF, 9 f 1eV, for ALO, 7.7 f 0.5 eV, and for C1,13 f 1 eV.
+ Al2O(g)
Relative intensities A1 AlOC1 A120 C1
36 1473 120 4 18 40 1528 320 6 62 1543 9000 40 500 1000 1553 2000 18 120 340 1600 500 50 60 450
log K I
kcal
kcal/ mol
-0,120 -0.110 -0,056 -0.053 -0.034
-0.808 -0.770 -0.722 -0.426 -0.293
-2.044 -2.076 -2.085 -2.089 -2.112
AF ,
AZOF Molecule. A mass spectrometric study of the reaction between Al2O3(c) and gaseous A1F3 has been ~~
Table 11: Ion Intensities and Thermodynamic Data for the Reaction AlF(g) AllO(g) = 2Al(g) AlOF(g) T, OK
1540 1560 1640 1700 1723 1773 1800 1923
+ + _-____ Relative intensities------41 AlOF -4lF 140 180 1000 2300 2000 3000 3000 3800
11 12 8 5 10 6 11 26
180 180 260 200 120 360 160 12
A120
log Kz
100 100 300 500 620 1720 260 280
4.26 4.53 5.23 5.65 5.96 6.22 6.64 7.26
(3) H . Schafer, G. Goser, and L. Bayer, 2.Anorg. Allg. Chem., 263, 87 (1950).
(4) H. Schafer, F. E. Wittig, and W. Wilborn, ibid., 297, 48 (1958). (5) M . A. Greenbaum, J. A. Blauer, M . R . Arshadi, and M . Farber, Trans. Faraday SOC., 60, 1592 (1964). (6) M.Farber and H. L. Petersen, ibid., 5 9 , 836 (1963). (7) M. Farber, M . A. Frisch, and H. C. KO, ibid., 65, 3202 (1969). (8) “Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions,” National Bureau of Standards Publication NSRDS-NBS 26, Washington, D. C . , June 1969. (9) “JANAF Thermochemical Tables,” The D o a Chemical Company, Midland, Mich., 1961, 1965. The Journal of Physical Chemistry, Vol. 76, N o . 11, 1071
1762 performed in the temperature range 1483 to 1923°K to identify the AlOF species. In this temperature range an appearance potential of 10.5 f 1 eV was obtained for the AlOF(g) species. The mass spectrometer intensities for the reaction
are presented in Table 11. A second-law least-squares plot of log K 2 us. 1/T, where
The Journal of Physical Chemistry, Vol. 7 6 , No. 1 2 , 1971
NOTES
yielded a value of 109 f 1.8 kcal for AHr at an average temperature of 1700°K. This yields a value of -148 f 1 kcal/mol for AHt of AlOF(g). Reduction of these data to 298°K using the heat content data in the current JANAF Thermochemical Tables gives - 143.8 f 1 kcal/mol for AHf298, which is in close agreement with the previously published third-law effusion studies at the single temperature of 22OO0K,which yielded a value of - 140.2 f 2.6 kcal/mol for AHme of A10F(g).6
