HEATOF FORMATION OF AMMONIUM DICHROMATE
Oct., 1957
1429
THE HEAT OF FORMATION OF AMMONIUM DICHROMATE BY CONSTANTINE A. NEUGEBAUER AND JOHN L. MARGRAVE Department of Chemistry, University of Wisconsin, Madison, Wisconsin Received June $4, 1967
The heat of formation of (NHa)&r207was determined calorimetrically as -429.1 i 0.3 kcal./mole through the reaction (NH&Cr2O7(s) = CrZOa(s) Nz(g) + 4Hz0(1). From solution data on chromates and dichromates which can be related to this heat, one deduces a heat of formation for CrO,(s) of -142.1 i 1 kcal./mole.
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Literature values for the heat of formation of ammonium dichromate are in poor agreement. The National Bureau of Standards Circular 500 lists -425.5 kcal./mole as the standard heat of formation based on the work of Berthelot' and Moles and Gonzales. Kapustinskii and Shidlovskiia have reported the heat of formation as -430 =I= 6 kcal./mole. Since the intramolecular combustion is a simple reaction with well-defined products, a very precise determination should be possible. Experimental
traces of unreacted material were found. The solution produced in the reaction was titrated for ammonia with 0.01 N acid to the phenolphthalein end-point. The product gases were analyzed mass spectrometrically. The nitrogen produced in the reaction was assumed to behave like oxygen for the purpose of the non-ideality corrections. Thus, ( ~ ~ E / Wwas ) T taken as -1.574 ca1.jatm.l mole. This is an entirely valid approximattion since the total correction from this source in calories is only of the order of 0.5 cal. All corrections for the heat capacity of the contents before and after the reaction were made according to the method outlined by Hubbard, Scott and Waddington.7
Results and Calculations
The apparatus used exclusively in this investigation is essentially that described in various previous publi~ations.~--B The results of four determinations of the heat of A rotating bomb calorimeter was used, but no use was made reaction 1 are given in Table I. of the rotating mechanism, since, In these experiments TABLE I large quantities of solution were not produced. R243 R244 R 241 R242 The reaction whose heat was measured was identical to that of previous investigators, and can be represented as Moles of dichromate, n 0.067749 0.066935 0.068049 0.0072740
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(NH&CrzO-i(s) = CrzOds) Ndgl 4HzOCI) (1) The reaction did not proceed entirely as written and was accompanied by the side reaction 1/2 Nz(g) 3/2 HzOW = NHdaq) 3/4 02(g) AEO = +82.9 kcal./mole Tt should be noted that the mechanism of the production of ammonia and oxygen is probably entirely different, but for the purpose of thermodynamic calculations this reaction is stoichiometrically correct. The procedure in making the thermal measurements and the analysis for the extent of the reaction will be described. Sixteen to eighteen grams of the ammonium dichromate was weighed accurately into a clay crucible. The dichromate waa Merck reagent quality and was free frommaterials which would interfere in the self oxidation-reduction. The rrucible was put in place in the loop electrode of the conventional double valve Parr bomb and a short coil of nichrome knition wire was just placed under the surface of the dichromate. One ml. of water was then put into the bomb, which was closed and flushed and filled with high purity helium to a pressure of 10 atm. to prevent a leak in the sealing mechanism of the bomb. The bomb was now placed in the calorimeter as described in previous publications. The reaction was initiated by pnqsing a current of 3 amp. at 25 v. through the wire for about 1 sec. After the reaction the solid residue was analyzed for residual dichromate by precipitation with barium; only
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(1) M . Berthelot, Ann, chim. phys., 1, 92 (1884). (2) E. Moles and F. Gonzales, Anales $ 8 . y quim. (Madrid),21, 206 (1923). (3) A. F. Kapustinskii and A. A. Shidlovskii, Izseat. Sektora PEatiny i Drug. Blaoorod. Metal., Ins.?. Obshchei i Naoro. Khim., Akad. Nauk., S.S.S.R., S O , 31 (1955); see also C.A , , SO, 9849c (1956). (4) C. A. Neugebauer and J. L. Margrave, THIS JOURNAL, 60, 1318 (1956). .+4 (5)'C. A. Neugebauer and J. L. Margrave, J . Am. Chsm. SOC.,19, 1338 (1957). (6) C. A. Neugebauer, Ph.D. Thesis, Univ. of Wis., 1967.
Moles NHs AT9 " C . AEb, oal. C(cont.) AT, oal. AE cor. for "3, cal. AEipnroal. AEnon id,, cal. nAE8 cul. AE2 kcal./mole
traces 1.0577 7771.49 6.82 0 4-14.4 +0.51 7753.8 114.45
0.000410 0,000204 traces 1.0604 1.1298 1.0405 7791.64 8311.40 7655.35 8.95 7.14 6.59 0 34.1 16.9 +18.0 C13.45 +17.9 +0.51 +0.51 f0.51 7780.4 8338.7 7680.4 114.33 114.64 114.99
Here A E b stands for the uncorrected heat evolved in the calorimeter, and AEOo is the standard heat of reaction at constant volume at 25". Run 242 was discarded since an unusually large amount of oxygen was evolved here, a d not enough ammonia was found t o account for it. The average of the remaining runs is AEoo = -114.47 f 0.09 kcal./ mole, where the error quoted is the standard deviation from the mean. Applying the AnRT correction, AHco = -113.87 0.09 kcal./mole. Then, with the heat of formatiop of Crz03equal to -269.7 kcal./mole, one calculates the standard heat of formation of ammonium dichromate at 25' to be -429.1 -I: 0.3 kcal./mole. This is to be contrasted with a value of -425.0 kcal./mole recently deduced by Muldrow and Hepler* from heat of solution 'data on dichromates and the heat of formation of Cr03(s). Since the only heat'with any large uncertainty used in their calculations was that for Cr03(s), it appears that the heat of formation of CrOa is"probab1y -142.1 + 1 kcal./mole, i.e., about 4 kcal./mole more stable than previously believed.
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(7) W. N . Hubbard, D. W. Scott and G. Waddington, THE JOUR152 (1954). .~ (8) C. N. Muldrow, Jr., and L. G . Hepler, J . Am. Chsm. SOC..79, 4045 (1957). NAL, 68.
1430
C. F. JEFFERSON AND EDGAR F. WESTRUM, JR.
Vol. 61
AN INVESTIGATION OF THE PHASE COMPOSITIONS OF AN IRON-RICH NICKELZINC FERROSPINELl BY C. F. JEFFERSON AND EDGAR F. WESTRUM, JR. Contribution f r o m the Departments of Electrical Engineering and of Chemistry of the University of Michigan, Ann Arbor, Michigan Received June 67,1.967
The composition of the system Ni,Znl- zFez04-Fez03in equilibrium in air a t one atmosphere pressure has been investigated. The composition of the spinel phase can be represented as a Ni,Zn~_.Fe~Oa,bFe~O~.cFe~0~. The value of b/a, which is the solubility of magnetite in the nickel-zinc ferrospinel, has been found between the temperatures 700 and 1300'. The spinel phase also contains some Fez03which is assumed to be present as the 7-phase. The value of b / ( b f C) a t the boundary line between the spinel field and the spinel-hematite field has been found to be independent of the value of a.
Introduction Because of the importance of nickel-zinc ferrospinels as components in electronic circuitry, several recent scientific and technical endeavors have been directed toward the correlation between the composition and the electromagnetic and chemcal thermodynamic properties.2-s The ferrospinel Ni,Znl-,Fez04 is formed when NiO, ZnO and Fez03 are mixed in stoichiometric proportions and sintered. Kat0 and Takeigattributed the magnetic properties of sintered ZnO and Fez03 containing a large amount of Fez03,either to the formation of Fea04 in solid solution with the ZnFez04 or t o the formation of Fez03as a magnetic second phase, depending upon the method of preparation. Kushima and Amanuma2 investigated the same system and concluded that, while some Fez03 is converted to Fe304,the magnetic properties of the Fe203-rich material were due to Fez03 in solid solution with ZnFez04. Bergera investigated the same system and determined the solubility of Fez03in zinc ferrite by measurement of the lattice constant. He found the composition of the stable phase to be 76 mole yo Fez03 a t 1400°, 64 mole % at 1200O and about 61 mole yo at 1000°. From density considerations he concluded that the solid solution contained lattice holes, which could be accounted for by assuming the presence of y-Fez03. Smolenski4 investigated the solid solutions of ( N ~ O . ~ Z ~ O . ~ ) F ~and ~ Oattempted ~ - F ~ Z Oto~ explain the magnetic properties on the basis of Ndel's theory. He found that with increasing Fez03 content, the saturation magnetization increased initially and then decreased; the Curie temperature increased and the magnetic permeability decreased. (1) This work was done through the Engineering Research Institute of the University of Michigan under the sponsorship of the U. 8. Army Signal Corps. (2) I. Kushima and T. Amanuma, Mem. Fao. Eng., Kyoto Univ., 16, 191 (1954). (3) 8. V. Berger, "Rontgenunderokningar Av Spinellfasen I System Zn0-FerOa," Feetskrijt Tellagnad J . Amid Hedvall, 31 (1948). ( 4 ) G. A. Smolenski, Izuesl. Akad. Nauk S. S. S . R., Ser. Fiz., 16, 728 (1952). (6) N. A. Toropov, L. I. Rabkin, E. Zh. Freidenfeld and E. Sh. Epshtein, Zhur. Tekh. Pi+.,2 3 , Issue 9 (1941). (6) E. F. Westrum, Jr., and D. M. Grimes, THISJOURNAL, 61, 761 (1957). (7) D.M. Grimes, 8. Legvold and E. F. Westrum, Jr., Phya. Rev., 106,866 (1957). (8) D.M.Grimes, L.Thomassen, C. F. Jefferson and N. C. Kothary, J . Chem. P h y s . , 23,2205 (1955). (9) Y.Iiato and T. Takei, Trana, Amer. Electrochem. Soc., 57, 297 (1930).
Toropov, Rahkin, Freidenfeld and Epstein6 investigated the system NiO-ZnO-FezOs and attempted to correlate the phase composition with the magnetic properties. They plotted a triaxial diagram of the system Ni0-Zn0-Fez03 showing the area of solid solutions for firing temperature of 1350-1400O. The purpose of this investigation is to extend these studies and to determine the equilibrium phase compositions of the system Ni,Znl-.FezO~Fez03 in air a t one atmosphere. The value of x was arbitrarily held constant a t 0.474 while the amount of Fe203 in the initial composition was varied. Methods used to identify the composition and phase of the sintered oxides were (1) direct chemical analysis, (2) visual microscopic examination, (3) Curie temperature determinations, and (4) X-ray diffractional analysis.
Experimental All samples were prepared by ball-milling the C.P.oxides in acetone for six hours, decanting the acetone, and airdrying the oxides at 100'. The dried oxides were then pressed into toroids and fired in air within an electric fuf;nace, the temperature of which was stabilized to f 5 . Firing temperatures were determined with a calibrated plat10% rhodium thermocouple. inum versus platinum The samples used for these studies were fired with time on temperature varying from three hours at 1400' to five days a t 1000". Samples for chemical analysis at equilibrium temperatures less than 1000° were prefked for four hours at 1150' and then crushed in a "Diamond mortar" and refked as a powder at the desired temperature for 24 hours. All samples were quenched to room temperature by rapid immersion in water. In order to ensure the reliability of these data as representing equilibrium values, it was essential to ensure that such factors as mixing, time of firing and possible oxidation during cooling to room temperature were properly controlled. Quantitative experiments indicated that a rapid quench is necessary. This was obtained by immersing the samples in water. As might be expected, samples allowed to cool in air from 1200' indicated upon analysis only 5 t o 50% the ferrous content of water quenched samples. The ferrous content was clearly dependent upon the porosity of the pressed cores. Other experiments revealed that at a firing temperature of 1200°, .SO.% of the ferrous iron present after 15 hours was formed within 3 minutes and 99% within 20 minutes. Ferrous iron determinations were made by dissolving the ferrite in a solution of 0.05 M SnClz in concentrated HCl under an atmosphere of oxygen-free COz. The amount of ferrous iron was then determined by titration with KzCrzOr. The standardization of the SnClz solution was done by an identiFa1 analysis on a solution of FeC4 containing no ferrous iron. This modification of the usual procedure was adopted because the ferrite is difficult to dissolve in the absence of the SnClz. The samples were prepared for microscopic examination by mounting in bakelite and polishing
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