3028
NOTES
Table I : Dissociation Field Effect Relaxation Data for a Coupled Aqueous Hydrazoic Acid-Bromocresol Green System a t 25" kl8,f
Ca," 10-0 M
50 50 50 10 10 25 25 25 100
Ci,b
10-0 M
50 50 50 50 50 25 25 25 50
ail:
1010 M-1
pHC
108 sec-1
100 880-1
sec-1
4.25 4.60 4.05 4.32 4.75 4.79 4.32 4.54 4.54
7.75 3.82 7.50 4.98 3.24 3.03 4.19 3.49 4.46
4.18 2.91 5.78 3.79 3.02 2.14 1.91 2.62 3.04
8.64 4.37 6.34 7.06 4.86 5.36 8.04 5.62 4.04
Total molar concentration of hydrazoic acid. Total molar concentration of bromocresol green. Average of p H measured in the sample cell before and after D F E experiments. Faster of two experimental reciprocal relaxation times, average of four experiments. E Calculated reciprocal relaxation time ([H+] [Inz-] Ki)kll of bromocresol green under the same conditions using Ki = 1.26 X 10-6 M and klz = 5.4 x 10'0 M-' sec-'.
+
+
h a ' Specific rate of reaction H + + Ns- + NHa calculated from
these data.
hydrazoic acid made in this laboratory with a conductometric dissociation field effect apparatus. The somewhat lower value of k13 for N3- than for the spherically symmetric F- ion could indicate that not all directions of approach of the H+ ion to the linear N3- ion are equally favorable for reaction.
Acknowledgment. The authors wish to acknowledge preliminary conductometric experiments by Dr. David L. Cole and to thank Professor K. Kustin for having suggested this problem. This work has been sponsored by AFOSR (SRC)-OAR, USAF, Grant No. 69-1717-F.
Molecular Complexes in the Vapor of Sodium Bromide and Zinc Bromide Mixtures by Douglas W. Schaaf and N. W. Gregory* Department of Chemistry, University of Washington, Seattle, Washington 98106 (Received February 16, 1971) Publication costs assisted b y the National Science Foundation
The equilibrium vapors above a number of alkali halide-metal halide systems have been shown to contain molecules in which both metal and alkali metal atoms are present. Of particular interest in relation to the present work is evidence for formation of molecules of the form (NaC1),(ZnC12)y,found in transpiration studies by Rice and Gregory1 and in mass spectrometric studies by Bloom, O'Grady, Anthony, and ReinsborThe Journal of Physical Chemistry, Vol. 76, No. 19, 1971
oughn2 The latter work indicates that N&ZnzClais an important constituent, whereas the former suggests, from the observed dependence of the apparent pressure of the complex, derived from the sodium content of the vapor, on the pressure of ZnClz in equilibrium with solid sodium chloride, that molecules containing more than one zinc atom are not present in significant amounts. The experimental conditions of the two studies were somewhat different : the transpiration work was done over the range 495-550" with an argon carrier gas a t cu. 1000 Torr containing various partial pressures of zinc chloride equilibrating with solid sodium chloride; the mass spectrometric results were obtained from vapor issuing from a cell in which a condensed mixture of ZnCl2 and NaCl (42-51 mol % of ZnClJ was dispersed on alumina powder and heated in the range 250-450". I n the mass spectrometric work the partial pressures of ZnCl2 are expected to be relatively higher (in relation to sodium chloride) than in the transpiration studies which may enhance the contribution of Na2Zn2Cla; however, the apparent heats of formation derived for NaZnC13(g) from the two studies are widely divergent (- 18 as compared with -41 kcal/mol for formation from NaCl(g) and ZnClz(g)). We wish to report results of a related transpiration study of the NaBr-ZnBr2 system. Experimental Section The experimental method was virtually identical with that described earlier in the tvork on the chloride system.lJ Very similar results were obtained. Data are presented in Table I.4 The partial pressures derived for the various components of the equilibrium vapor were independent of flow rates between 15 and 50 cm3 min-'. The argon was made to flow either first over a sample of pure ZnBrz, in a compartment adjacent to the main reactor and heated by a separate furnace to introduce the desired partial pressure of ZnBr2, and then over a sample of NaBr(s) (method l),or directly over heated NaBr-ZnBr2 mixtures (method 2). Condensed mixtures used in method 2 had a mole fraction of NaBr ca. 0.9; i t was assumed that NaBr(s) remained a t unit activity in both types of experiment, analogous to the behavior indicated by the phase diagram for the chloride systems6 I n method 2 a liquid complex phase, assumed in equilibrium with NaBr(s), was present. The phase diagram for the bromide system does not appear to have been reported. (1) D. W. Rice and N. W. Gregory, J . Phys. Chem., 72, 4524 (1968). (2) H. Bloom, B. V. O'Grady, R. G. Anthony, and V. C. Reinsborough, Aust. J . Chem., 23, 843 (1970). (3) D. W. Rice, Doctoral Dissertation, University of Washington, Seattle, Wash., 1968.
(4) Table I will appear immediately following this article in the microfilm edition of this volume of the journal. Single copies may be obtained from the Reprint Department, ACS Publications, 1155 Sixteenth Street, N . W., Washington, D. C. 20036. Remit 13.00 for photocopy or $2.00 for microfiche. (5) N . Nikonowa, S. P. Pawlenko, and A. G. Bergman, Bull. Acad. Sci. U R S S , CI. Sei. Chim., 391 (1941).
NOTES Starting samples of ZnBrz (Alfa Inorganics, reagent grade) and NaBr (Baker and Adamson, reagent grade) were vacuum dried; the ZnBr2 was vacuum sublimed prior to use. Apparent ideal gas partial pressures in the equilibrium vapors were calculated from the relative numbers of moles of Zn, Na, and Ar found in the condensed vapor. The total zinc transported was determined by EDTA complexometric analysis;6 a Beckman DU flame photometer was employed to determine the quantity of sodium. The pressure of argon was determined manometrically and the number of moles of argon
3029
-0.0-
Y
0
-
-0.6-
I
I
1.10
I
1.20
l
I
1.30
l
1
1.40
1
1
1
1.10
I
la00
1000 / T PK)
Figure 2. Transpiration results for the assumed equilibrium NaBr(s) ZnBrl(g) = NaZnBr&): apparent equilibrium constant, K = P N R ~ n B r l / P ~ n ~ r 2 ; 0,method 1; 0, method 2.
+
Results and Discussion Apparent equilibrium constants were calculated for the reaction LI
E E
zNaBr(s)
+ zZnBrZ(g) -+ (NaZnBrd,
(1)
Y
n
-
0 X
m
m c
N 0
nz
Figure 1. Pressure of NaZriBrl(g) us. the pressure of ZnBr%(g). Pressures of ZnHrz lower than those necessary to form a NaBr-ZriBrr condensed phase: 0 , 784 i 1 5 ° K ; 0,817 f 1.5”K. Pressures of ZnBrl above NaBr-ZnBrt condensed phase in equilibrium with NaBr(s): *, 784 f l..i°K; @,817 f 1 5 ° K .
flowing through the reactor during a given experiment was determined by measuring the pressure of the quantity collected after expansion into a calibrated volume. The contribution to the total number of moles of sodium expected from the vapor pressure of sodium bromide was predicted from the data of Cogin and Kimball’ with allowance for the presence of dimer from the data of Guion, Hengstenberg, and Blander.8 This contribution ranged from a negligible amount a t the lowest temperatures to about 20% a t the highest temperatures; see Table I.
The value of z was assumed to be unity when it was found, Figure 1, that a plot of the apparent pressure of NaZnBr3 us. the pressure of ZnBrz was linear. The absence of any noticeable curvature in these lines suggests that species with more than one zinc atom are not of major importance in the equilibrium vapor. This behavior is similar to that found earlier in the chloride system.’ The transpiration results do not provide evidence concerning the relative importance of species such as Na2ZnBrr, Na3ZnBr6,etc., since the activity of NaBr was not varied; however, these “higher polymers” are not expected to be present at significant concentrations, particularly in view of the low partial pressure of the sodium halide. The equilibrium constants listed in Table I are the values derived from the data when NaZnBra(g) was assumed the only complex species. A log K us. 1/T plot is shown in Figure 2; the associated least-squares linear line shown corresponds to the equation log K = (-2786
300)T-’
+ 1.503
f
0.425
Values of the equilibrium constants at 784 and 817”K, respectively, were taken as the slopes of the respective lines in Figure 1. The value at 810°K was taken as the numerical average of the values in Table I because the small variation in ZnBr2 pressures precluded a meaningful plot of data at this temperature. These averaged constants were weighted in the least-squares treatment to reflect the number of runs represented. The closed (6) A. I. Vogel, “A Textbook of Quantitative Inorganic Analysis,” Wiley, New York, N. Y . , 1963, p 433. (7) G. E. Cogin and G. E. Kimball, J . Chem. Phys,, 16, 1035 (1948). (8)J. Guion, D. Hengstenberg, and M. Blander, J . Phys. Chem., 7 2 , 4620 (1968).
The Journal o/ Physical Chemistry, Vol. 76, N o . 19, 1971
COMMUNICATIONS TO THE EDITOR
3030 circles, Figure 2, represent results when argon was passed over the preformed mixtures of NaBr and ZnBrz (method 2). As a set they appear more consistent than the open circles (method 1). It may be reasonable to anticipate that the approach to equilibrium by decomposition of the condensed mixture is more rapid than by interaction of vapor with the solid phase, although no evidence for a systematic dependence of the results of method 1 on flow rate was apparent. The apparent enthalpy and entropy changes for (l), derived by a least-squares treatment (using all points), a t the mean temperature of 740°K are 12.7 kcal mol-l and 6.9 cal mol-’ deg-l, respectively; these values are quite similar to those found in the chloride system. The properties derived from the transpiration data for the “mixed metal dimer” molecules NaZnX,(g) (X = Br, Cl), when compared with those of (NaX)Z
and ( X n X , ) z , are reasonably close to what might be expected for a random exchange without serious modification in bond energie~.~-’l This is not true of the mass spectrometric value for the relative enthalpy of the chloride. It is to be emphasized that the transpiration data provide no direct verification of the molecular form of the complex, however.
Acknowledgment. We are pleased to acknowledge financial support from the National Science Foundation, Grant NSF G P 6608x. (9) “JANAF Thermochemical Tables,” revised ed, The Dow Chemical Co. Midland, Mich., 1963. (IO) S. Data, W. T. Smith, and E. H. Taylor, J . Chem. Phys., 34, 558 (1961). (11) F. J. Keneshea and D. Cubiccotti, ibid., 40, 19 (1964).
C O M M U N I C A T I O N S T O THE E D I T O R
Formation of Ozonide Ions in y-Irradiated Aqueous Solutions of Alkali Hydroxides Publication costs borne completely by The Journal of Physical Chemistry
We have used a wide range of solutions and bleaching and annealing procedures but have completely failed to detect any features that could possibly be associated with hyperfine coupling to alkali metal
Sir: I n a recent communication Nazhat and Weiss claimed to have observed a phenomenon of considerable significance to the theory of trapped and solvated electrons.’ They exposed 1 M aqueous solutions of the alkali metal hydroxides to e°Co y radiation at 77”K, and, after various annealing and bleaching procedures, they detected esr spectra comprising hyperfine features characteristic of the various alkali metal cations used. The data reported’ are included in Table I. They postulated that the unpaired electrons are trapped in expanded orbitals on the hydrated alkali metal cations. These results, if correctly interpreted, constitute a major piece of evidence in the topic of solvated electrons, since such centers have long been one of the major models used to interpret the properties of metal although no firm evidence for their formation in water or ammonia has been forthcoming prior to this work.‘ Before using these results for theoretical purposes, we felt it wise to repeat the work, particularly since no esr spectra were actually presented, and various phrases1 such as “to some extent characteristic of the alkali metal nuclei” and “the number of hyperfine lines corresponds roughly to the nuclear spin” made us somewhat cautious. The Journal of Physical Chemicrtry, Vo1. 76, No. 19, 1971
Table I : Esr Data Obtained from 7-Irradiated Aqueous Solutions of Alkali Metal Hydroxides after Annealing, Together with Data for OaMatrix
Pav
NaOH-H*O KOH-Hz0 CsOH-Hz0
2.0056 2,0060 2,0048
Mstrix
LiOH, NaOH, KOH in 020 or He0 NaOa NaBr03 KClOs KrBrOs
Ull
a,
G
Ref
14.8 10.0 14.1 022
1 1 1 QSS
Ref
2.004
2.018
2.011
u
2.0025 2.006 2.0026 2.003
2.0174 2.022 2.018 2.0165
2.0104 2.022 2.016 2.011
b c d e
a Present work, 10.001. * P. W. Atkins, J. A. Brivati, N. Keen, M. C. R. Symons, and P. A. Trevalion, J. Chem. SOC., 4785 (1962). T. Anderson, J. R. Bzberg, and K. J. Olsen, J. Phys. Chem., 71, 4129 (1967). R. S. Eachus and M. C. R. Symons, J. C h a . SOC.A, 2433 (1968). a A. Begum, S. Subramanian, and M. C. R. Symons, ibid., A , 918 (1970).
B.Naahat and J. J. Weiss, J . Phys. Chem., 74,4298 (1970). M. C. R. Symons, Quart. Rev., Chem. SOC.,99 (1959); P. W. Atkins and IM. C. R. Symons, “The Structure of Inorganic Radicals,” Elsevier, Amsterdam, 1967. (1) N. (2)
