Thermal Decomposition of Sodium Azide We do not wish to imply that the spectrophotometric measurements can in any way substitute for more direct ways of characterizing the products. However they can make a significant contribution to the problem by indicating the spectral characteristics of the products to be sought. Once the products have been isolated and their spectra determined, a comparison can be made between the values of exX obtained, respectively, from eq 4 and 6 and from eq 2; this comparison would provide a check on the completeness of the products inventory.
Acknowledgments. The work described above was aided by a Biomedical Sciences Support Grant from the National Institutes of Health to the Research Foundation, Oklahoma State University, and by NSF Grant No. MPS 71-03499 and 75-18967 to L.M.R. References and Notes (1) All Gvalues are in the units molecules (I00eV)-l. (2) D. J. Kanazir, Prog. Nucl. AcidRes. Mol. Biol., 9, 117 (1969). (3) A. P. Casarett, “Radiation Biology”, Prentice-Hall, Englewood Cliffs, N.J., 1968. (4) “End Results in Cancer, Report No. 4”, L. M. Axtell, S. J. Cutler, and M. H. Myers, Ed., Department of Health, Educatlon and Welfare, National Cancer Institute, 1972 (DHEW Publication NIH 72-272). (5) G. A. Infante, E. J. Fendler, and J. H. Fendler, Radiat. Res. Rev., 4, 301 (1973). (6) M. Polverelli and R. Teoule, Z.Naturforsch. C, 29, 12, 16 (1974). (7) C. A. Ponnamperuma, R. M. Lemmon, and M. Calvin, Science, 137, 605 (1962). (8) J. F. Ward and I. Kuo, lnt. J. Radiat. 8io/., 15, 293 (1969). (9) G. Scholes, J. F. Ward, and J. Weiss. J. Mol. Biol., 2, 379 (1960). (10) D. Barszcz and D. Shugar, Acta Chim. Polon., 8, 455 (1961).
119 (11) (12) (13) (14) (15)
8. Ekert and R. Monier, Nature (London), 188, 309 (1960). M. N. Khattak and J. H. Green, lnt. J. Radiat. Biob, 11, 131 (1966). J. Holian and W. M. Garrison, Nature (London), 212, 394 (1966). J. Holian and W. M. Garrison, Nature (London), 221, 57 (1969). M. Polverelli and R. Teoule, C. R. Acad. Sci. Paris, Ser. C, 277, 747 (1973). (16) Consistent units must of course be used: if 6 is expressed, as usual, in mmol-’ cm2 (= M-’ cm-‘), G is in molecules (100 eV)-‘, and 1-cm cells are used for the absorbance measurements, one must multiply by the factor (10-23/6.02) mol molecule-’ and express E, In (100 eV) dm-3. If the density is 1 kg dm-3, 1 rad = 6.242 X I O l 4 (100 eV) dm-3. (17) “Radiation Dosimetry: X-Rays and Gamma Rays with Maximum Photon Energies Between 0.6 and 60 MeV” (ICRU Report No. 14), International Commission on Radiation Units and Measurements, Washington, D.C., 1969. (18) C. A. Mannan, M. S. Thesis, Oklahoma State University, 1972. (19) D. Shugar and J. J. Fox, Biochim. Biophys. Acta, 9, 199 (1952). (20) H. Stephen and T. Stephen, ”Solubilities of lnorganic and Organic Compounds”, Vol. 1, Pergamon Press, Oxford, 1963, part 2, pp 87 and 88: W. F. Linke, Ed., “Solubilities”, Vol. 11, American Chemical Society, Washington, D.C., 1965, p 1229. (21) The values of Y are based on @OH.) = G(H20.)- = 2.7, @Ha) = 0.5, a(H202) = 0.7, @H2) = 0.45. There is not complete agreement on the exact values, but the discrepancies are of no consequence for the present purpose (cf. ref 22a,b). We prefer to use the symbol (H20.)- instead of eaq- to represent the hydrated electron, because with the former one can write formally balanced equations. The extent of hydration of the electron is, in fact, not known exactly, hence it would be more proper to write [(H*O),.]-, but that is superfluous here. (22) (a) E. J. Hart, Radlat. Res. Rev., 3,285 (1972); (b) I. G. Draganic and Z. D. Draganic, “The Radiation Chemistry of Water”, Academic Press, New York, N.Y., 1971, pp 123-168; (c) ibid., pp 54-56: (d) ibid., p 110. (23) R. L. Willson, lnt. J. Radiat. Blob, 17, 349 (1970). (24) M. Daniels and S. C. Schweibert, Biochim. Biophys. Acta, 134, 481 (1967). (25) This has been pointed out by a reviewer, whose advice we acknowledge with thanks. (26) B. M. Brown and M. J. E. Hewlins, J. Chem. SOC.C, 2050 (1968). (27) H. E. Johns, J. C. LeBlanc, and K. B. Freeman, J. Mol. Biob, 13, 549 ( 1965). (28) H. M. Kattak and J. H. Green, ht. J. Radiat. Biob. 11, 577 (1966).
Role of Crystal Imperfections in the Thermal Decomposition of Sodium Azide V. Krishna Mohan and V. R. Pal Verneker* Department of lnorganic and Physical Chemistry, hdlan lnstitute of Science, Bangalore 5600 12, hdia (ReceivedFebruary 3, 1975; Revised Manuscript Received August 19, 1975)
The effects of dopants and pretreatment, mechanical and thermal, on the thermal decomposition of sodium azide have been investigated with a view to understanding the role of crystal imperfections in the decomposition. It has been observed that allovalent ions, Ba2+ and Sod2- ions, as well as precompression, sensitize the decomposition while preheating lowers the decomposition rate. The effects have been discussed in terms of the dependence of the decomposition rate upon the concentration of gross imperfections in the sodium azide lattice.
monium perchlorate and alkali metal perchlorates results in enhanced decomposition rates. They have demonstrated The role of crystal imperfections in the thermal decomthat the increase in the density of the gross imperfections position of solids has been stressed by several worker~.l-~ in the lattice upon prior treatment is responsible for the Boldyrev3 has proposed a classification of reactions of therchanges in the thermal reactivity of these materials. mal decomposition of solids. It has been shown that various Point defects also have been shown to play a significant types of macroscopic and microscopic defects have differrole in thermal decomposition of solids.1°-12 The results of ent effects depending upon the category of reactions. The the numerous investigationsl1J2 carried out on the thermal role played by dislocations in thermal decomposition is well decomposition of the alkali and alka!ine earth metal azides exemplified by the cases of calcite,6 NaBr03,7 a-Pb(N3)2,7 clearly establish that point defects play a vital role in ionic and NH&104.s~9 Rajeshwar and Psi Verneker416 have diffusion and electron transfer processes taking place durshown that prior mechanical and thermal treatment of aming decomposition. Introduction
The Journal of Physical Chemistry, Vol. BO, No. 2, 1976
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A survey of the work done on the thermal decomposition of sodium azide reveals that, though the decomposition of sodium azide has been the subject of a number of investigat i o n ~ , the ~ ~ importance -~~ of crystal imperfections in the decomposition mechanism is not clearly understood. Most authors report a value of -36 kcal molb1 for the activation energy of decomposition. The rate-limiting step has been argued to be the thermal excitation of the electron of the azide ion to the conduction band. The fact that electron transfer reactions play an important role in thermal decomposition of sodium azide receives support from the observation of Jacobs and Kureishy15 that Fe3+ ions, which could act as electron traps, sensitize the decomposition, However, Torkar17 et al. from a study of the effect of allovalent cations and anions on the thermal decomposition of sodium azide arrived at the conclusion that the decomposition is diffusion controlled. Divalent cations have been found to enhance the decomposition rate. The sensitization was ascribed to an increase in the diffusion rate of sodium ions in Mn2+ doped NaN3 samples. However, the effect of divalent anions could be seen to be concentration dependent; sodium azide doped with 0.01 mol % Cos2- ions decomposed at a slower rate than pure sodium azide while the opposite is true in the case of 0.1 and 1 mol % C022- ions doped samples. An altogether different type of observation has been reported by McGillls from a study on the influence of univalent cationic impurities on the thermal decomposition of sodium azide. Univalent ions, for example, K+ and Cs+ ions, having an ionic size larger than that of the host ion, decrease the induction period for decomposition. The authorla speculates that this effect is due to an increase in the local strain energy on doping sodium azide with ions of greater size than the host cation. It is, thus, evident from the above discussion that the present understanding of the exact manner in which dopants affect the decomposition kinetics of sodium azide is poor. Moreover, no investigation has been carried out to date to point out the role played by gross imperfections in the decomposition mechanism of sodium azide. The present work was, therefore, carried out with a view to (i) study the effect of divalent cations and anions, and (ii) examine the effects of precompression and preheating on the thermal decomposition of sodium azide. Experimental Section Sodium azide used in the present investigation was supplied by Riedel, De Haen Ag Seelze, Hannover. All the samples were reprecipitated twice from double distilled water by acetone. Doped materials were prepared by coprecipitation from an aqueous solution of sodium azide containing the desired concentration of the dopant. The dopants used were barium azide and sodium sulfate, both of Analar quality. The concentrations studied were 0.01 mol % (Ba2+ and S042ions) and 0.05 mol % (Ba2+ ions). No attempt was made to determine the actual concentration of the dopant in the host lattice. The effect of precompression on the thermal decomposition of sodium azide was studied by pelleting the samples a t 200 and 300 kg/cm2 in an uniaxial hydraulic press. After pelleting, the pellets were ground and then passed through sieves to obtain the desired particle size. Experiments on the precompressed samples were carried out on the same day t o preclude the effects of ageing. The kinetic studies of the thermal decomposition of The Journal of Physical Chemistry, Vol. 80, No. 2, 1976
V. Krishna Mohan and V. R . Pai Verneker
NaN3 samples were followed in the temperature range 240-300°C in a constant volume vacuum line with an initial Torr. The pressure rise was measured pressure of 1 X using a McLeod gauge. All kinetic runs on the pure and doped materials were carried on samples having the particle size range 149-177 Fm. Infrared spectra of the sodium azide samples were taken on Carl-Zeiss UR 10 spectrophotometer, Results The isothermal decomposition of pure and doped sodium azide has been studied in the temperature range 240300OC. Typical plots of the fractional decomposition, a , against the time, t , of heating are shown in Figure 1. The kinetic plots have been fitted to Avrami-Erofeev equation with n = 2. Table I gives the values of the rate constants for the thermal decomposition of pure and doped NaN3. The activation energy values remain unchanged at 36 kcal mol-l for both the samples. It could be seen from Table I that allovalent ions, both cationic and anionic, sensitize the decomposition. The effect of precompression (at 200 and 300 kg/cm2) and preheating (at 150°C for 1 hr) on the subsequent thermal decomposition of sodium azide has also been investigated in the same temperature range, Le., 240-300°C. The values of the rate constants, a t a representative temperature, for the thermal decomposition of the precompressed and preheated sodium azide samples are also given in Table I. I t could be seen that precompression enhances the decomposition rate while preheating lowers it. The activation energy for the decomposition of precompressed NaN3 was found to be the same as that for pure NaN3. Doped as well as precompressed (not shown in the figure) NaN3 show a considerable broadening of the ir peaks. To illustrate the broadening effect a typical peak corresponding to the asymmetric stretching of the azide ion is displayed in Figure 2. The preheated samples exhibit a narrowing of the peaks. Preliminary examination of the x-ray diffractograms, taken on a Phillips diffractometer, of precompressed sodium azide revealed a broadening of the diffraction peaks.
Discussion The general form of the a-t curves obtained in the present work is similar to that observed by earlier workers. The present value of activation energy, 36 kcal mol-l, is also in close agreement with the values reported in literature. Moreover, the activation energy for the thermal decomposition for pure, doped, and precompressed sodium azide is found to be the same. This indicates that the basic mechanism for decomposition remains unaltered in the case of these materials. It is evident from the present work on the thermal decomposition of pure and doped sodium azide that the effect of doping the sodium azide lattice with allovalent ions is independent of the nature of the ion, as both anions and cations sensitize the decomposition. A consideration of the ionic radii of the foreign ions constituting the dopant materials, viz., Ba2+ ions and SO42- ions, reveals that both the, ions are larger than the host ions (ionic radii of Ba2+, 1.35 A; S042-, 2.4 A; Na+, 0.95 A; Ns-, 1.7 A). As both the ions, Ba*+ and S042-, are larger than the ions constituting the host lattice, doping the sodium azide lattice with these ions results in an increased strain in the lattice. The number of gross defects would, therefore, increase as a consequence of
121
Thermal Decomposition of Sodium Azide l
I
ost
4 9' Particle size
149-177p
0
SO:-iOOlM%)doped
A
9a2+i0 O l M % ) doped NaN3
NaN3
0
Pure N a N 3
V Preheated NONS
so0
2400
1600
T i m t I ,(In mlnulesl. 01 heating 01 sodium
3200
4000
ezlb
Plots of the fractional decomposition, a,vs. the time, min) of heating sodium azide. Figure 1.
t, (in v;/2
Figure 3. Plot of the rate constant, k (in min-I), for the thermal decomposition of sodium azide against, u l / p , the band width of half absorption maxima: (0)preheated NaN3; (0) pure NaN3; (A)precompressed NaN3; (0)NaN3 doped with 0.05% Ba2+ions.
I/
NaNs doped wlth 0 05 M%, Ba2+iont
f L l 2700
I
I
I
1
I
2300
1
1900 Y
Figure 2.
I
NaNl Pre-heated at 200T
I
t
I
I
1500
cnil
infrared spectra of sodium azide samples.
TABLE I: Rate Constants for the Thermal Decomposition of Sodium Azide Type of the sample Preheated NaN, Pure NaN, NaN, doped with SO,?-ions (0.01mol %) NaN doped with BaE+ions (0.01 mol %) NaN, precompressed at 200 kg/cm? NaN, precompressed at 300 kg/cm2 NaN doped with BaE+ions (0.05 mol W )
Rate constant k (min-') at 280°C 0.0008
0.0015 0.0022 0.0021 0.0028
0.0035 0.0050
the increased strain and hence the decomposition would be sensitized. Ir spectra of the doped samples show that the absorption peaks of the azide ion undergo a considerable broadening on doping sodium azide. It has been established earliels1y5 that broadening of ir peaks arises as a consequence of an increase in the concentration of gross imper-
fections in the lattice. Rajeshwar and Pai Verneker4v5have observed that precompression of ammonium perchlorate and alkali metal perchlorates results in a broadening of the perchlorate infrared absorption peaks along with a sensitization in their decomposition characteristics. From detailed x-ray diffraction studies they have concluded that the broadening arises as a consequence of an increase in the density of gross imperfections. A broadening of the ir absorption peaks, in the case of the doped sodium azide samples, would also imply an increase in the concentration of the gross defects in the lattice. Precompression of sodium azide has also been observed to have a sensitizing effect on the decomposition. Preliminary examination of the precompressed samples shows that precompression results in a broadening of the diffraction peaks. Keating and Krasnerlg have found a similar broadening on subjecting sodium azide to mechanical grinding. They have attributed this broadening to the production of strain in the lattice upon grinding. The ir spectrum of the precompressed sodium azide shows a broadening of the absorption peaks. These observations clearly indicate that precompression results in the production of gross imperfections within the lattice. The increase in the decomposition rate upon precompression can be attributed to the larger concentration of imperfections in the precompressed samples. Further support for the dependence of the decomposition of sodium azide upon gross imperfections comes from the observation that the decomposition rate decreases when sodium azide is preheated at 15OoC for 1 hr. No decomposition has been observed during the preheating period. Preheating of the samples might result in an annealing of the defects within the crystal. The ir spectrum of preheated sodium azide shows a sharpening of the absorption peaks (see Figure 2). The change in the thermal reactivity of sodium azide bears a direct relationship to the change in the broadening of the ir bands as is evident from Figure 3. The effect can be seen to be independent of the nature of the pretreatment. The present studies, thus, clearly establish that the thermal decomposition of sodium azide is highly sensitive to the concentration of gross defects within the lattice. It is likely that point defects, especially anion vacancies, which act as electron traps, play a significant role in the decomposition mechanism but such effects are overshadowed by those produced due to gross imperfections. The Journal of Physical Chemlstry, Vol. 80, No. 2, 1976
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Farhataziz and Lewis M. Perkey
References and Notes (1) B. E. Bartlett, F. C. Tompkins, and D. A. Young, Nature (London), 179, 365 (1957). (2) V. V. Boldyrev, "Metody izucheniya klnetiki termicheskogo vazlozheniya tvevdykh verschestv", Izd-Yo Tomskogo Gosunlversitela Tomsk, 1948, Chapter 1. (3) V. V. Boldyrev, Bull. SOC.Chim. Fr., 1054(1969), (4) K.Rajeshwar and V. R. Pai Verneker. J. Phys. Chem. Solids, in press, (5) K. Rajeshwar and V. R. Pai Verneker, J. Solidstate Chem., in press. (6) J. M. Thomas and G. D. Renshaw, J. Chem. Soc., 2058 (1967); 2749 (1969). (7) J. Jach, J. Phys. Chem. Solids, 24, 63 (1963). (8) P. J. Heriey and P. W. Levy, J. Chem. Phys., 49, 1500 (1968).
(9) P. J. Heriey and P. W. Levy, J. Chem. Phys., 49, 1493 (1968). (10) J. N. Maycock and V. R. Pai Verneker, Proc. R. Soc. London, Ser. A, 307, 303 (1968). (11) P. Gray and T. C. Waddington, Chem. lnd (London), 1555 (1955). (12) P. Gray and T. C. Waddington, Proc. R. SOC.London, Ser. A, 235, 106 (1956). (13) W. Garner and D. J. B. Marke, J. Chem. SOC.,657 (1936). (14) E. A. Secco, J. Phys. Chem. Solids, 24, 469 (1963). (15) P. W. M. Jacobs and A. R. T. Kureishy, J. Chem. SOC.,4716 (1964). (16) R. F. Walker, J. Phys. Chem. Solids., 29, 985 (1966). (17) K. Torkar, H. T. Spath, and G. W. Herzog, Monatsh Chem., 98, 298 (1967). (18) W. J. McGiIi, J. S. Afr. Chem. lnst., 22, 143 (1969). (19) D.T. Keating and S. Kranser, J. Phys. Chem. Solids., 20, 150 (1961).
Pulse Radiolytical Investigation of the Reversible Reaction of Biphenyl with the Solvated Electron in Liquid Ammonia Farhataziz"' and Lewis M. Perkey RadiationLaboratory,2 Universityof Notre Dame, Notre Dame, Indiana 46556 (Recelved April 2, 1975; RevlsedManuscript Received September 10, 1975) Publlcationcosts assisted by the U.S.Energy Researchand DevelopmentAdministration
From the measured solubility of biphenyl in liquid ammonia for the temperature range 21 to -15.5O, 9.4 kcal mol-l for the heat of solution and 30 cal deg-l mol-l for the entropy of solution for a 1 M standard state are calculated. Nanosecond pulse radiolysis of biphenyl solutions in liquid ammonia show that the reaction of earn- with biphenyl produces the biphenyl anion and it is reversible. From the values of the equilibrium constant (the equilibrium concentration of the biphenyl anion divided by the initial concentration of biphenyl and the equilibrium concentration of earn-) at 21.6 and 10.5', -20 kcal mol-l for the heat of reaction and -44 cal deg-l mol-l for the entropy of reaction for a 1 M standard state are calculated. The specific rate of the reaction of earn- with biphenyl is equal to 1.24 X 10(12-1600/2.3RT) for the temperature range 21.6 to -75'. The extinction coefficient of the biphenyl anion at 0.403 K , the wavelength of maximum absorption, is 3.1 X lo4 M-l cm-l for the temperature range 23 to -76.5O. The calculated values of the standard heat of formation of earn- and the entropy of earn- for a 1 M standard state are -14 kcal mol-l and 13 cal deg-l mol-l, respectively.
Introduction The solvated electron in a mixture of methylamine and liquid ammonia reacts reversibly with b e n ~ e n e The . ~ equilibrium is interesting because in the reverse reaction an anion gives an electron to the solvent. Similar reversible reactions of the electron with C02 in isooctane, neopentane, and tetramethylsilane, and with biphenyl, B, in tetramethylsilane, have been r e p ~ r t e d . ~In? ~this communication, a study of the reversible reaction of earn- with B in liquid ammonia and various measured or calculated thermodynamic properties of species involved are reported. Experimental Section Biphenyl was obtained from Eastman Kodak. The purification of ammonia, and the vacuum line are described el~ewhere.~.~ The vessel used for the determination of solubility of B in liquid ammonia is shown in Figure 1. The volume of the vessel up to any point between marks CY and /3 is the sum of the previously determined volume up to the mark a and the computed volume of the length of precision bore capilThe Journal of Physical Chemistry, Vol. 80, No. 2, 1078
lary tube between mark CY and the point of interest. After weighing a known amount of B in the vessel, it was attached to the vacuum line and evacuated. Such an amount of ammonia was condensed into the vessel, so that at the temperature of interest the saturated solution of B would fill the vessel to a point between marks LY and p. The vessel was sealed off, allowed to warm to room temperature (-23O), and both portions were weighed to determine the weight of liquid ammonia in the vessel. By immersing the vessel in a bath (contained in a Dewar) of dissolved solid C02 in alcohol, the clear solution was cooled to crystallize out white needles of B. Except a tiny crystal of B, all B was redissolved by warming and shaking the vessel. The vessel was again immersed in a cold bath, and temperature was adjusted to give a preceptible growth of the tiny crystal of B. Most of the crystal was redissolved by warming and shaking the solution, and cooled again to a temperature a few tenths of a degree higher than previous crystal growth temperature. A fast crystal growth would indicate a repeat of above procedure. a t a higher temperature. The procedure was repeated till a temperature was found at which the
