THERMAL DECOMPOSITION OF C~~-DIAZIDOTETRAAMMINECOBALT(III) AZIDE activity of water in these dilute solutions may also be set equal to its concentration. (4) The density of these dilute solutions may be approximated by the density of pure water. Therefore, eq 13 may be written as
1%
CNaCl(H*O)j(aq)
= log K2
+ j log
CHpO
(14)
When the llogarithm of the molar sodium chloride concentration is plotted against the logarithm of the molar concentration of water for saturated solutions of NaCl in water at 600" and pressures to 392 bars, a straight line with a slope of 1.8 is obtained. A similar plot for Sourjrajan and Kennedy's 500" data gives a slope near 1.7. At 700°, the data at low densities give a value of the slope near 1.8, but a t the higher
703
densities (0.04-0.07 g cm3),a slope near 3.5 is obtained. We have assumed that the value of j in eq 13 is approximately 1.8 at low densities and high temperatures; therefore, NaCl molecules are considered to have two waters of hydration under these conditions. The value of 4.4 obtained by Styrikovich was obtained from measurements that included data at higher water densities where NaC1 probably ionizes appreciably, and therefore the average hydration number would be higher.
Acknowledgment. It is a pleasure to acknowledge the technical assistance of Wiley Jennings and the contribution of James S. Gill during a part of the experimental program.
The Thermal Decomposition of Solid cis-Diazidotetraamminecobalt(111) Azide by Taylor B. Joyner Chemistry Division, Naval Weapons Center, China Lake, California 93666
(Received August 16, 1967)
Solid cis-diazidotetraamminecobalt(II1) azide resembles the trans isomer and azidopentaamminecobalt(II1) azide in showing an ability to decompose to either CONor a cobalt(I1) complex. The CON reaction has a well-defined induction period followed by a relatively fast decomposition. Kinetic parameters derived from the induction period and the final reaction are in good agreement with each other and with the results from the trans isomer and the azidopentaammine. The resemblance extends qualitatively to the more complex cobalt(I1) system and is supported by limited rate data. The compound's lability prevents an extensive investigation of the kinetics of this reaction. A substitution reaction to triazidotriamminecobalt(II1) has also been detected.
Introduction A systematic study' of the thermal decomposition of solid cobalt(II1) azides has revealed a general similarity in the rather complex reactions of hexaamminecobalt(111) azideI2a,zidopentaamminecobalt(III) azide,3 and both crystal forms of truns-diazidotetraamminecobalt(111) azidee4 In all three cases a system of competing reactions yielding either a nitride or a cobalt(I1) complex was found. Kinetic studies on the two substituted compounds established a quantitative similarity in the reactions. It therefore seemed advisable to check cis-diazidotetraamminecobalt(II1) azide briefly for a persistence of the identity. Experimental Section Preparation and purification was by previously described rnethodss6 Two independent preparations
served as sources for samples of different particle size. Precipitation from water according to Linhard6 gave crystalline samples of red, lath-like crystals 0.1 to 0.5 mm in length. Fast precipitation from an aqueous solution by an excess of a 1:5 ethanol-ether solution gave a powder with a particle size of ca. to mm. Identification by X-ray powder patterns6 gave particular attention to possible contamination with (1) T.B. Joyner and F. H. Verhoek, J . Am. Chem. Soc., 83, 1069 (1961). (2) T. B. Joyner and F. H. Verhoek, Inorg. Chem., 2, 334 (1963). (3) T.B. Joyner, J. Phye. Chem., 69, 1723 (1965). (4) T.B. Joyner, ibid., 71,3431 (1967). (6) M.Linhard, M.Weigel, and H. Flygare, 2.Anorg. Allgem. Chem., 263, 233 (1960). (6) T.B. Joyner, D. 8. Stewart, and L. A. Burkardt, Anal. Chem., 30, 194 (1958). Volume 71, Number 8 February 1968
704
TAYLOR B. JOYNER
the trans isomer. Storage was over PZOSin an opaque vacuum desiccator. The principal kinetic data were obtained from a freshly precipitated crystalline sample within 2 weeks of preparation. The results agreed with the very brief exploratory work on crystalline material. There is, however, some evidence that powder behaves differently. The usual vacuum system' was used for expIoratory studies. I n later work, the mercury manometer was replaced by a pressure transducer. The system and procedure were otherwise unchanged. The usual 5-mg samples gave pressure increases of ca. 50 torr for eq 1 and 25 torr or less for runs involving eq 3 and 4. Once again, "normal" runs were made with the system initially evacuated and the solid exposed to only its own products. "Ammonia" runs had an initial pressure of ammonia. "Trapped" runs used a liquid nitrogen trap ca. 60 cm from the hot reaction vessel. The trap was assisted by periodic venting to the pumps during the CON reaction's induction period and then closed off when a rapid pressure increase signalled the onset of the final reaction.
Results In general behavior, the cis-diazidotetraammine is similar to the other members of the ~ e r i e s . ~ -Once ~ again, reactions to CON and cobalt(I1) are observed. The compound is more labile with fast reactions and explosions at 130 to 135" whereas previous studies reached 150". There is also a greater tendency to convert from the CONreaction with its typical induction period to the cobalt(I1) decomposition. Previously, crystalline samples reacting under normal conditions would ultimately follow the CONpath although the induction period was greatly prolonged by the small quantities of ammonia generated in the early stages of the decomposition. I n the present study, normal runs showed the cobalt(I1) reaction. Observation of the CON reaction required maintenance of ammonia a t very low levels during the induction period. Additionally, a substitution reaction yielding triazidotriamminecobalt (111)-previously observed with only one sample of the trans-diazidotetraammine under very limited conditions-was detected in residues from both crystalline samples and, apparently, as a major product of powder decompositions. The reactions will be discussed individually. The CON Reaction. Trapped runs show the cobalt nitride reaction [ C O ( N H ~ ) ~ ( N ~+ ) ~ ]CON N~
+ 4NH3 + 4Nz
(1)
with its customary well-defined induction period and relatively fast final reaction. Figures 1 and 2 give illustrative curves with n/no, the moles of gas evolved per mole of original compound, plotted against time. The closed circles (curve A) give the observed gas evolution with the trap on. The open circles report the The Journal of Physical Chemistry
1(
I
I
I
E
-
6
n
"0
4
2
0
40
1 BCDE IA
MINUTES
Figure 1. Typical reaction curves. Curve A: 120°, trapped during the induction period, normal during the final reaction. The discontinuity reflects the alteration from trapped conditions with closed symbols reporting the observed gas to normal conditions with the total gas evolution given by open symbols. Curve B: 130°, normal. Curve C: 130",50 torr of ammonia. Curve D: 130', 100 torr of ammonia. Curve E: powder sample, 120°, 100 torr of ammonia.
n "0
4
MINUTES
Figure 2. Typical reaction curves. Curve A: loo', trapped during the induction period, normal during the final reaction. Curve B: 120' normal run. Curve C: 110' normal run. Curve D: l l O o , 100 torr of ammonia.
total gas composed of the gas evolved after the trap was closed off plus the gas in the trap. The noncondensable gas lost by venting during the induction period is negligible. Runs with double-sized samples (m. 10 mg) a t 100 and 95" showed no unexpected features, reinforcing an earlier conclusion that the final reaction is independent of a m m ~ n i a . ~
THERMAL DECOMPOSITION OF C~S-DIAZIDOTETRAAMMINECOBALT(III) AZIDE The final reaction is well described by the first-order (or "unimolecular") decay law -In (1
- CY)= k,t
705
4
+ c,
(2) where CY is the! fraction reacted, t is time, and k and c are constants. Figure 3 presents illustrative curves. The resulting constants (Table I) give a good Arrhenius plot and an apparent activation energy of 24 kcal/mole (Table 11). Similarly, the duration of the induction period, 7 , once again4 taken as the time to reach a total gas evolution of n/no = 1.0, when plotted against 1/T
3
'7 E 2
-i
1
Table I: Rate Constants (k) and Induction Periods ( r ) T,
kU
O C
Conditions
130 120 110 100 95
Trapped-normal" Trapped-normal Trapped-normal Tr apped-normal Trapped-normal
135
100 torr of NHa
130 125 120 115 110
100 torr 100 torr 100 torr 100 torr 100 torr
T,
sec
1 ,350 2,160 5,118 14,004 18 ,630
80 torr of NHI
125 120 115 110
50 torr 50 torr 50 torr 80 torr
of of of of
101,
Exploded 20.1 8.86 3.76 2.56
96. Ob 173c 69.8 40.3 19.2 5.61 2.58
of NHs of NHs of NHa of NHa of NHa
130
x
11e0-1
52. 4b 104" 35.2 16.9 5.82 1.04b 2.13c
NH8 NHa NHa NHs
kl
x
104,
EBC -1
135 130 125
Normal Normal Normal
110 57.1 21.6
" Trapped during the induction period, normal during the final reaction. I' Initial portion of a broken curve, CY < 0.4. ' Final portion of a broken curve, CY > 0.4.
Table 11: Apparent Activation Energies (E.) and Preexponentials ( A ) Reac. tion
CON Co(I1)
Conditions
Treatment
Trapped" 7 Normal" Eq2 100 torr of NHa Eq 2 50 torr of NHa Eq 2 Normal Eq6
E., kcal/mole
Log A(aec-1)
23.3 f 0.9' 9.58 i0.54b 2 3 . 9 f 0 . 3 10.57f0.17 52.1 f 1.7 26.15 =k 0.92 58.9 f 1 . 2 29.91 =k 0.67 (53 f44)o (26 k2)"
" Trapped during the induction period, normal during the final reaction. All values from least-squares treatment. The limits are probable errors. These values are from the very narrow 125-135' temperature range.
'
ACD B MINUTES
Figure 3. Illustrative kinetic treatments. Curve A: 120°, initially trapped run. Curve B: looo, initially trapped run. Curve C: 120°, 50-torr ammonia run. Curve D: 120°, 100-torr ammonia run.
yields a good straight line indicating an apparent activation energy of 23 kcal/mole. These values and their mutual agreement correspond to the findings of earlier s t u d i e ~ . ~ ~ ~ The Cobalt(II) Reaction. Identification of diazidodiamminecobalt(I1) by its powder pattern establishes the presence of the cobalt(I1) reaction
[GO(NHs)4(N&]Nz
+
Co(NH3)2(Ns)2
+ 2NHa + 1.5Nz
(3) as observed in the other compounds. Qualitative similarities are also noted. At 130°, the decomposition has a conventional sigmoidal curve under normal conditions, is somewhat accelerated by moderate (50 torr) ammonia pressures, and slows with some decrease in gas evolution under higher (100 torr) pressures (Figure 1, curves B, C, and D). At lower temperatures, the gas evolution is curtailed (Figure 2, curves B, C, and D), and power patterns detect a green compound previously observed in low-temperature ammonia runs. Although usually found in mixtures with the diammine and hence never characterized with complete assurance, this is thought to be a cobalt(I1) tetraammine calling for an ideal stoichiometry. [Co("s)4(N&]N3
+Co("a)4(N&
+ 1.5N2
(4)
The presence of cobalt(I1) at 110" was substantiated by exposing the cooled residue of the 100-torr ammonia run (Figure 2, curve D) to ammonia for 3 days. The observed gas absorption of n/no = 2.0 is compatible with the formation of hexaamminecobalt(I1) azide (a reaction anticipated under these conditions)l from the green Volume 72,Number d February 1968
TAYLOR B. JOYNER
706 cobalt(I1) tetraammine. These observations all indicate a reaction system similar to those previously observed. Quantitative treatment has always been difficult with eq 3 being clean only above 130". Below 130°, eq 4 appears. Additionally, the substitution reaction [CO(NH,)~(N&]N~
---f
Co(N&)a(N3)3
+
3"
(5)
has been observed with one sample of the trans-diazidotetraammine. Nevertheless, plausible assumptions have permitted treatment of the low-temperature reactions, and good consistency with the 130-150" data has been Unfortunately, the lability of cis-diazidoobtained. tetraammine denies the important 130-150" ranges and forces reliance on the uncertain low-temperature reactions. Additionally, eq 5 is present. Powder patterns show triazidotriamminecobalt(II1) in the residues of three 100-torr ammonia runs, two with independent crystalline preparations at 130", and one with powder at 120". DiazidodiamminecobaIt(I1) was also present in the residues from the crystalline material. With powder, eq 5 appeared to be the major reaction, and hence subsequent work concentrated on crystals. Triazidotriamminecobalt(II1) was not detected in the other residues although its presence as amorphous material is possible. Although precise treatment is impossible, brief analysis is worthwhile to ascertain that a t the least, there is nothing in the rate data incompatible with the assumption of similar cobalt(I1) reactions in all three substituted compounds. As beforeJ3q4the calculation of a is based on the observed final gas evolution and assumes that the reduction to cobalt(I1) is rate determining with subsequent relatively fast adjustments of the ammoniacobalt(I1) azide equilibria responsible for variations in the stoichiometry. It also assumes-questionably for the high-temperature ammonia runs-that the substitution reaction (eq 5) is present to a minor extent and does not seriously complicate the rate data. The 100-torr ammonia runs may be described by eq 3 with considerable success (Figure 3, curve D) with only the 135" run showing a break a t a = 0.5, probably due to a significant warm-up period in this very fast run. The rate constants (Table I) give a fair Arrhenius plot and an Ea of 52 kcal/mole (Table II), in agreement with earlier result^.^ The less satisfactory 50-torr runs explode a t 135" and both the 130 and 110' first-order plots have breaks a t CY = ca. 0.5. Utilizing the final portions of these two runs gives an E, of 59 kcal/mole. This may be somewhat high since the good 115-125" plots show close agreement with the 100-torr constants. Normal runs are unsatisfactory. The curves are sigmoidal a t 125-135" (Figure 1, curve B), and asymmetric and deceleratory a t 110-120" (Figure 2, curves B and C). A linear equation )g4
a =
klt
The Journal of Physical Chemistry
+
c1
(6)
has been useful for normal run^.^^^ Applied to the 125-135" data, it fits ca. 60% of the reaction curve centered around a = 0.45 and gives an Ea of 53 kcal/ mole, a plausible but obviously inconclusive value in view of the short temperature range. The 110-120" curves are not understood. The asymmetry could result from a relatively fast reaction t o the cobalt(I1) tetraammine or diammine with a subsequent decomposition of these compounds (previously observed2) obscuring the original stoichiometry. (Accepting this and assuming eq 4 at 110 and 115" and a gas evolution of ./no = 2.0- suggested by curve B, Figure 2-at 120", the rates for a
