3872
lrOl. SO
repeat their work with HI, sp. gr. 1.60, 1.85 and 2.0, failed. At ordinary temperatures a yellow compound was formed whicli was not soluble i n water but melted a t about 165". At high temperatures a Iachrj-niator mas formed. Attempts to evtract t h e henzaldehqdc u i t h organic solvents
941
9-58 1098 Ill8
9-58 1OXO
1230 1403 1403 1< T i 3
2010 2130
2556 2659 2915 3073 3300
3037 3280
gave no better success. (Extraction with alcohol is of course not feasible as it irnniediatelp precipitates hydrazine monoiodide.) This is the only hithertr, undisputed method of preparation for this compound, and since it involves an aqueous medium it seems very unlikely t h a t the compound they obtained was other th:m the coinpourit1 we liavc iiow prepared below. In the light of our other irork tlie m.p. nf 220" Iriok.; U I I reasonable. Hydrazine Diiodide Dihydrate.-Addition of fuming HI, sp. gr. 2.0, t o a saturated water solution o f hydrazine monoiodide with cooling resultcd in the immediate formation of a white cornpound composed of sniall clear prisms, distinctly
[CONTRIRUI'IOX FROM TIIE hIINI7RAI.S
different from the needle-form of hydrazine morioiodide.'O These crystals were collected on a fritted glass filter, and pressed with a rubber dam t o remove as much liquid as possible. LVashing with alcohol or water will remove t h e second molecule of H I leaving only hydrazine monoiodide. They were placed in a desiccator over KOH pellets for about 3-4 hours and analyzed. T h e material is extremely hygroscopic. 111a sealed capi1l:iry the compound melts a t 65-66". Anel. Calcd. for N2HJ2.2H20: NZH4, 9.9; HI, 79.0; i11ol. \vt., 324. Found: X2H4, 9.8, 10.6, 9.82; HI, 78.3, 81.2, 80.5; mol. wt. by titration with 0.02 X NaOI-I, 329, 332,325. For the determination of niolecular weight, advantage was taken of the fact t h a t in all hydrazine salts containing two equivalents of acid the second equivalent can be titrated to a neutral end-point with base. This compound has not been reported previously in the literature but might be expected t o exist b y analogy t o the hydrazine dihromide dihydrate.1' Standing for 24 hours over KOH removes both the water and second molecule of H I leaving hydrazine monoiodide. T h e water of hydrazine dibromide dihydrate can be removed by standing Over P2O5,l1 b u t unfortunately this is not true for t h e iodide. If an a t t e m p t is made to dcliydrate t h e compound in this manner, HI arid water are lost simultaneously. A i l aiihydrous hydrazine diiotlide is not 01)tained. Dehydration cannot be effected with ethanol; tlehytlratiiin with dioxane was also unsuccessful. In a final attempt t o synthesize anhydrous hydrazinc cliiodide t\vo methods were used. (a) Gaseous dry 111 was passed through a slurry of hydrazine monoiodide in benzenc :ind also i n chloroform. No formation of tlie tliiodide took ])lace. (1.) .%nhydrous H I was condensecl fin hytlrazitie I l l i i l l ( J i i ) d i d c at the temperature of a Dry Ice-acetone bath. The temperature was then raised to -25' t o remove excess 111. T h c residue provcd to be unchanged liydrazine 1nonoi I iditle . ( I 0) If anliydrous ether is addeil t o a cold saturated solution o f hydrazine monoiodide in 50c/, 111, it separates first into three layers. On scratching with a rod t h e material crystallizes h u t is unfortunately a niisture of t h e hydrated crystals with some hydrazine monoiididt. 'l'lie mixture cannot he separated. (11) 12. C . Gilbert, T m S JOURNAL 67, 2 F l l (lLl35).
CORVAT,Z.IS, OREGOS
TILERMODYSAMICS EXPERIMENT STATION, REGION11,
nL?REAc OF
MINES,
ITSITED
S T A l E S 1)EPARTMENT OF TTIE INTERIOR!
Heats of Formation of Niobium Dioxide, Niobium Subnitride and Tantalum Subnitride BY
ALIA
D.
i
m
1
RECEIVED FEBRUARY 8, 1958 T h e energies of combustion of nio1,iuni dioxide ( S b 0 2 ) ,niobium subnitride ( Xb2K) and, tentatively, tantalum subnitride (TaZN) were determined by bomb calorimetry. The results, in conjunction with t h e heats of formation of t h e correspondiiig pentoxides, lead t o t h e following standard heats of formation a t 298.15OI;. (lical./lnole): Kh02, - 100.9 =t0 . 4 ; Nb?K, -61.1 =k 1.0; and TaZhi, -64.7 i 3.0 (tentative).
Previous papers froiii this Laboratory have dealt with heats of formation of the pentoxides2 of niobium and tantalum and the nitrides3 of coinposition N b N and Tax. The present paper reports energies o f combustion arid heats of formation of niobium dioxide and the subnitrides of coinposition Nb2N and Ta2N. As is usually the case with refractory nitrides, the purity of the available samples of these subnitrides leaves much to be desired. This is especially true of the tantalum coinpouiid for which only tentative values are re(1) Bureau of Mines, lJ, S. Department of t h e Interior, H rgian 71. Brrkeley, California. ( 2 ) G. I.. Humphrey. THISJUIJRNAL, 76, 978 (1954). ( 3 ) A 11 XTnh a n d N J. C.pllert, i h i c i , '78, 3 2 6 1 ( 1 Q X ) .
ported. No previous similar nicasurenieiits exist for any of these compounds. Materials.--1\Til)biuin dioxide was prepared 1)y K . C . Conway of tliis 1,aboratory frinn high purity nicibiunl petitoxide obtained froin Fansteel hIetallurgica1 Cor]). Small patches of the peiitoxide wcre reduced in hydrogen a t 9501000° for 4 hours. These were combined and heated again in hydrogen for another 4 liours at the same temperature. Completion of the reactions mas demonstrated by heating :I portion of the product t o 1460" in hydrogen and observing no weight change. Analysis b y recoiivcrsion t o the pentoxide indicated 99.90yo niobium dioxide. Analysis by dissolving, precipitating the niobium as hydroxide and igniting to constant weight showed virtually lOO.OO%, niobium tlioside. The X-ray diflractiiin pattern agreed with thnt ( I f Brauer. 4 (4) 0 Rrniter. Z n ~ t u r g allnem. Chsm
,
248. 1 (1841).
Aug. 5 , 1958
Niobium subnitride (prepared b y K. R. Bonnickson, formerly of this Laboratory) was made from niobium metal (obtained from Johnson, M a t t h e y and Co.) and purified nitrogen. After 6 hours of heating in a stream of nitrogen at 1150-1300°, the nitrogen content exceeded t h a t for t h e subnitride. Niobium metal then was added in t h e correct proportion and t h e mixture was homogenized b y heating for 9 days at 1120" in vacuo. T h e X-ray diffraction pattern of t h e product showed no niobium metal or other extraneous phase. Chemical analysis gave 92.S1y0 niobium and 6.46% nitrogen (as compared with t h e theoretical 92.99 and 7.01yo). The remainder was assumed t o be oxygen. T o correct t h e subsequently reported measurements, t h e material was treated a s if it were a solid solution containing 92.33 mole yo NbzN and 7.67 mole yo NbaO, which is in line with t h e chemical aiialysis and with t h e single phase shown b y X-ray diffraction. Tantalum subnitride also was prepared b y K. R. Bonnickson. Tantalum nitride ( T a N ) , made b y heating lathe cuttings from a Fansteel tantalum bar in purified nitrogen a t 1300", was combined with t h e stoichiometric amount of tantalum metal and homogenized b y prolonged heating a t 1120O0in m c u o . T h e X-ray diffraction pattern of t h e product was sharp and gave no evidence of urireacted tantalum. The pattern checked t h a t reported b y Brauer and Zapp6 for tantalum subnitride. T h e nitrogen content of this sample was not satisfactorily obtainable b y t h e Kjeldahl method. Consequently, analysis was made b y the vacuum fusion method at t h e Boulder City, Nev., Station of t h e Bureau of XIines. T h e results obtained were 3.48% nitrogen and 0.72% oxygen, the tantalum content b y difference being 95.80%. (The theoretical analysis is 96.2iy0 tantalum and 3.73y0 nitrogen.) T o correct the subsequent measurements, t h e material was assumed t o be 91.35 mole Yo Ta2N, 5.34 mole yo Ta and 3.31 mole % TasOj. As will be noted later, t h e uncertainty in composition of this sample permits nbtairiing o n l y a tentative heat of fnrrnation value.
Experimental T h e combustion calorimeter was descrihed previously b y Humphrey.6 T h e mean calibration value obtained with National Bureau of Standards benzoic acid sample h-0. 39g was 32495.3 =!= 0.01% cal./ohm. All weights are corrected t o vacuum and heat values are in terms of t h e defined calories (1 cal. = 4.1540abs. joules). The combustions were conducted under 30 a t m . pressure of oxygen. T h e combustion samples were held on disks of t h e corresponding pentoside supported b y a platinum sheet. The snmples showed no weight increase upon standing in 30 ntm. of oxygen for several hours. Ignition was b y means of :UI electrically heated platinum spiral and a filter paper fuse. Oxygen deficiencies in the combustion products were calculated from t h e weight increases during combustion and prnper corrections were made. T h e bomb gases after comhistion contained only negligible amounts of osides of nitrogen. In the niobium dioxide experiments most of t h e combustion product remained on the disk; only about O.OS$& was c!eposited on t h e bomb ~valls. X - R a y diffraction patterns showxl t h a t the combustion product on t h e disk and t h e disk material were the high temperature modification of niobium pentosidc listed in t h e ASTM catalog. T h e wall deposit appeared amorphous. No correction mas made for this as the amount was so small. T h e portion of combustion product remaining on t h e disk i I the niobium subnitride esperiments averaged 717,. XRay diffraction showed this t o be t h e same high temperature modification as found in t h e dioxide combustions. T h e X-ray diffraction pattern of the wall deposit, however, greed ivith a different :nodification of :iicbium pentoxide, ilso listed in the ASTM catalog. S o correction was made for the difference in heat coiltent between the two forms. However, the ccrrection must be small as no trend in t h e hear of combustion was noted with amounts of wall depcsit varying from 18 t o 36y0. In the tantalum subnitride combustions, the amount of wall deposit ranged from 0.2 to 0.8yc. X-Ray diffraction of the 99.2 to W.S? portion remaining on the disk showed i t to be the same form of pentoside as t h a t cmployed in pre(5) C:. (1S54).
3873
HEATSOF FORMATION OF NIOBIUMDIOXIDEAND SUBNITRIDE
nrnucr and K , II. Zapl),
Z. n i i o y R . a l l ~ ~ mC /. I P > I 277. I.,
( 6 ) G. I,, I I u m p h r e y , THISJ O U R N A L , 73, 1687 (1951).
129
vious low temperature heat capacity' and high temperature heat content8 measurements.
Results The experimental data for the niobium compounds are in Table I, in which the successive columns give the mass of substance burned, the total energy evolved, the energy of ignition, the oxygen deficiency of the combustion product, the correction for incompletion of combustion and finally the energy of combustion per gram. TABLE I ESERGY OF COMBUSTION AT 30" Cor - ... -f n..r Mass of substance, g.
4.99748 5.00551 5.01532 5.00882 5.00694 5.01178
1,00019 1.00019 1.00010 1.00010 1.00048 1.00008 1.00017 1.00043
Total energy evolved, cal ,
1434 . 3 0 1434 .02 14368.29 1433 .48 1434 . 1 5 1432 . 5 9
1950.73 1948.75 1941.45 1937.33 1940.88 1940.08 1940.39 1940.42
g.
incomplete combustion, cal.
10.68 10.39 12.70 9.94 10.51 9.93
NbOz 0.00773 .00866 .00896 .00888 ,00905 ,00912
35.27 39.51 40.88 40.52 41.29 41.61
291.9 292.3 292.0 292.3 292.6 292.2
Mean
292 3 f.0 . 2
11.69 10.05 10.01 10.14 10.05 10.94 10.43 10.65
Nb2N 0.00218 ,00218 ,00373 ,00448 .00388 ,00451 ,00433 ,00426
Energy from EIt, fuse, cal.
01 deficiency,
9.95 9.95 17.02 20.44 17.70 20.58 19.76 19.44
Mean (Cor. for impurities)
- IUB, cal./g.
1948.6 1948.3 1948.3 1947.4 1947.6 1949.6 1919.4 1948.4 1948.4f.0.6 1 6 . i =t3 . 4 1965.1 r t 3 . 5
The mean energy of combustion value for niobium dioxide corresponds t o AE303.15 = -36.50 kcal.1 mole under bomb conditions. Corrections to standard conditions of unit fugacity of oxygen (-23 cal.), a constant pressure process (-151 cal.), and 298.15'K. (-1 cal.) results in A H 2 9 8 . 1 5 = -36.67 + 0.10 kcal./mole for the standard heat of combustion of niobium dioxide. Employing Humphrey's2 heat of formation of niobium pent0.6 kcal./mole), there oxide ( A H 2 9 8 . 1 5 = -455.2 is obtained A 1 1 2 9 8 . 1 5 = -190.9* 0.4 kcal./inole as the standard heat of formation of niobium dioxide from the elements. The literature contains no previous directly determined value of the heat of formation of niobium dioxide. The present result may be conipared with - 189.0 1.0 kcal./mole calculated by Brewerg from hydrogen reduction equilibria of the pentoxide. The mean value of the energy of combustion of niobium subnitride corresponds to AE3,13.15 - 392.7 kcal./mole under bomb conditions. Corrections to unit fugacities of oxygen and nitrogen ( - 184 cal.) to a constant pressure process ( - 1205
*
*
(7) K. K. Kelley, ibid., 62, 818 (1940). ( 8 ) R . L. Orr, ibid., 76, 2808 (1953). (9) L. Brewer, Chem. Revs., 62, 1 (1953).
3874
JUI
H. WANGA N D EVERLY R.FLEISCHER
cal.) and to 298.13’R. ( - 10 cal.) leads to AI120815 = -394.1 f 0.8 kcal./iiiole for the standard heat of combustion. The estimated uncertainty takes into account the impurities in the sample as well as the uncertainties involved in the combustions and calibration. Again, combining with the heat of formation of niobium pentoxide gives AH29815 = - G 1 . 1 =t 1.0 kcal./mole as the standard heat oi formation of niobium subnitride froni the elements. No previous heat of formation value of niobium subnitride exists. The present result is 4.3 kcal / mole more negative than the heat of formation of NbN (-56.8 =t 0.4 kcal./niole).d Because of uncertainty regarding the coinposition of the sample, only a tcntative value is offered for tantalum subnitride. A tentative value
[CONTRIBUTION NO. 1455 F R O M THE
1701. SO
appears justified :IS no previous value exists. Four combustions gave a mean of 1123.5 i 2.0 cal./g., after correction for inconipletion of combustion (average, 36.8 cal.) and for impurities (39.5 cal.). This corresponds to = -422 7 kcal. ’mole for combustion under bomb conditions. CorrecJ ~ ~ ~ tions to standard conditions gives l I l ~ = -424.1 i. 3.9 kcal./mole. Combining with Ilul11phrey’s? heat of formation of tantalum pentoxide (-4S8.S i. 0.5 kcal./mole) gives AH2Jd 15 = - 04.7 =t 3.0 ltcal./mole as the tentative heat of formation from the elements. This value appears reasonable in magnitude, being 2.7 kcal. mo!e more iietative than the heat oi forinition of T‘iS (-60.0 i: 0.6 kcal./mole).3 I~LRKEI.CY4, CALIFORNIA
DEPARTMEST O F
CIIEMISTRY, YALE USIVEKSI’TY]
Tracer Studies on the Mechanism of Combustion of Carbon, Sulfur and Mercuric Sulfide Bs JUI 13. WANGAX‘D EVERLY U.FLEISCHER RECEIVED FEBRUARY 21, 1958
+
+
T h e mechanisms of the reactions C Op C 0 2 and S O1 -+ S3., respectively, were studied with 018018 as tracer. It was concluded t h a t the two oxygen atonis i n each COI or SO, i n ~ l ~ c ~produced ilr came from differelit 0,muleculei. --f
The elucidation of the mechanism of combustion of solid fuels through kinetic studies is often hampered by the difficulty of reproducing the surface conditions. However, valuable information on the mechanism of combustion can sometimes be deduced from the result of tracer studies. For example, when solid carbon, sulfur and mercuric sulfide, respectively, are burned in excess of I)iire oxygen, the main over-all reactions are
c + 0 2 +co* s + +so,
HgS
+
0 2
0 3
+H g
+ SO,
One may ask, “Are the two oxygen atoms in each of the COZ or ,302molecules produced in the above reactions from the same oxygen molecule or from two different oxygen molecules?” The answer was deduced from the result of the present study with 018018 as tracer. The principle of our method, which has already been described,2 is very simple. Suppose oxygen gas is prepared by the thermal decomposition of 018-enriched silver oxide. If the isotopic atomfraction of 0l8in this oxygen gas is X , the molefraction of 01*01* must be approximately equal to X 2 . (The small isotope effect due to the difference in zero point energies of light and heavy 0-atoms in O2 may be neglected for the present purpose.) When each mole of this 018-enriched oxygen gas is blended with q moles of ordinary oxygen gas, the isotopic atom fraction of Oi8and the in the mixed 0 2 become mole fraction of 018018 [X g(0.0020)]/(1 q) and [ X 2 q(0.0020)*]/
+
+
+
(1) T h i s work was supported in p a r t by a research grant (USPHSRG-4483) from the Division of Research Grants, Public Health Service. ( 2 ) K . C laruayin and J H \Vdng, THISJ O U R N A L , 80, 786
(1Q.58)
(! + y ) , respectively, where 0.0020 is the aton1 fraction of Old in ordinary oxygen gas. Tlie isotopic distribution in this blended oxygen gas is unnatural, because according to natural probabilities q(O.O020)]’ if the atom fraction of OlS is [ X (1 q ) , the mole fraction of Oi80i5 should be q(O.O02G)]’, (1 q)’. Xow approxiniately [X if this 0’8-enriched oxygen gas with unnatural isotopic distribution is usecl to burn solid carbon, sulfur and mercuric sulfide, respectively, the determination of O’*-distribution in the combustion product (CO, or SO,) could lead t o the answer of the above question. Thus if both 0-atonis in each COn (or SOy)molecule produced are from the same O? molecule, the isotopic mole fraction of Cc>lyO” (or SO1sO”) sliould sl.i!l be approximately equal to [ X 2 4(0.0020)’],/(1 (1). But if the two 0 atoms in each CO,(or SOn) molecule arc froni different 0. molecules, the reaction would involve a reshuffling of 0-atoms and yield an isotopic mole fraction of C015018(or S01801s)approxiIllately equal to [ X -/- q(0.0020)]2,’(1 q)’ in accordance with natural probabilities. Experimental
+
+
+
+
+
+
+
Preparation of the Labeled Oxygen Gas.-Labeled oxygen gas was prepared by therinal decomposition of 0’8-labeled silver oxide. The labeled silver oxide was precipitated froni its saturated solution in 0’8-enriched water (10.7 atom % in 0 1 8 ) with a concentrated solution of KOH in 018-enriched water. The precipitate, about 1.4 g., was washed with 20 ml. of ordinary water ten times, vacuunl dried and :hen lcft in a drying oven a t 105” for three days. It was stored in a desiccator before use. The oxygen gas so prepared contained 5.8 atom 70 0 ’ 8 and 0.336 mole yo of O‘80’8. Combustion Experiments.-The combustion was carried out in a n enclosed Pyrex tube which was approximately 25 cm. long and had a total inside volume of about 12.5 ml. I n each experiment about 120 t o 140 mg. of labeled silver uxidc prepared ahovi. I V : I ~wi:ighed out and placed xt one
