HE.\TOF FORM~TION OF G E R V . ~ NOXIDE IC
NOV. 20, 1952
and AH& = 30,100 cal./mole, AF2098 mole, AS,",, = 41.0 cal./deg.-mole.
A H o = 32,500 - 8 T
+
PF' = 32,500 8 T I n T - 94.6T A S " = 86.6 - 8 I n T
[CONTRIBUTIOX FROM
THE
5T;i'T =
14,900 cal.,
BERKELEY, CALIFORNIA
DEPARTMENT OF CHEMISTRY AND CHEMICAL ENGIXEERING, A N D RADIATIOSLABORATORY, OF CALIFORNIA, BERKELEY ] UXIVERSITY
The Equilibrium Ge(s)
+ GeO,(s)
BY WILLIAML.
=
2GeO(g). The Heat of Formation of Germanic Oxide
JOLLY' AND
D'ENDELLX I . LATIMER
RECEIVED JULY 7 , 1952 The vapor pressure of GeO(g) over germanium and germanic oxide has been measured over the range 758-859OK. and the results have been compared with similar data obtained by Bues and v. Wartenberg. Vapor pressure data for GeO(s) obtained by the latter investigators have been discussed. The heat of formation of GeO?(s) has been determined calorimetrically to be AH," = -129.2 f 2.0 kcal./mole.
+
The Equilibrium Ge(s) GeOn(s) = 2GeO(g).Bues and v. Wartenbergs have reported vapor GeOz and GeO pressures of GeO(g) over both Ge in the neighborhood of 1000'K. Some inconsistencies in their data have been noticed and will be discussed here. For the reaction GeO(s) = GeO(g), Bues and v. Wartenberg give the equation
+
4.57 log Pas, = -63,000/T
and for the reaction 'lnGe(s) GeO(g), they give the equation 4.57 log Pat, = -54,800/T
T,
+ ','nGeO*(s) + 42 0
=
+
+
+
(1) Taken from a thesis presented by William L Jolly for partial satisfaction of the requirements of t h e Ph D. degree, Kiniversity of California, 1952. (2) W r)tlcr nnA FT v Wartenheru, Z ~ ~ ( P Y flllglm S Chrm , 8 0 0 , 1 R 1
Pressure, atm.
OK.
770 788 816 835 758 816 790 859
+ 57.9
These data give for the reaction '/zGe(s) '/zGeO2(s) = GeO(s) a t I00OoK., A H o = -8.2 kcal./mole, AFO = +7.7 kcal./mole and AS" = -13.9 e.u. Such a large entropy change is difficult to imagine for such a proportionation; one would expect the entropy change to be very close to zero or slightly positive. If we admit the existence of solid GeO, then it appears t h a t Bues and v. Wartenberg have erred in drawing a straight line through their log P vs. 1/Tplot for GeO. In general, it is more satisfactory to estimate entropies and heat capacities when interpreting vapor pressure data. We have estimated AC; = - 3 cal./deg. mole and AS&8 = 42 e.u. for both sublimations. From the equation AH: = AF' - G.9T log T G2T, we have calculated values of AH" for the two sets of data of Bues and 1'. Wartenberg. For the reaction Ge(s) 1/zGe02(s) = GeO(g), the average AH," = 54.1 kcal./mole. For the reaction GeO(s) = GeO(g), the calculated values of AH," ranged from 43.8 to 49.0 kcal./mole. Using a Knudsen effusion cell, we have measured the vapor pressure of what we thought t o be GeO(s) over the temperature range 738-859°K. In Table I we have listed our experimental results and the calculated values of AH:. The average AH:, 55.1 kcal./mole, is in fair agreement with the value calculated from Bues and v. Wartenberg's data for the reaction '/zGe(s)
0961)a
TABLE I OBSERVED VAPORPRESSURES AXD CALCULATED VALUESOF AHi3 1.3 X 1.8 X 2 . 6 x 10-6 1.06 x 10-5 6.7 x 1 . 5 x 10-6 3 . 7 x 10-7 7 . 7 x 10-8
AH,?, cal./mole
.53,1t3O 53,790 55,030 53,940 56,790 55,930 56,440 55,960
+
1/2Ge02(s) = GeO(g). We therefore feel that our solid phase was a mixture of germanium and germanium dioxide. By averaging the two values of AH,", we calculate the equations for the sublimation of GeO from a mixture of Ge and GeOn
+
4F0 = 54,600 6.9T log T - 62T (cal./mole) AH" = 54,600 - 3T(cal./mole) AS" = 59 - 6.9 log T (cal./deg. mole)
At 298"K., AH" = 53.7 =k 1.0 kcal./mole. It is possible t o calculate the heat of this reaction from independent data. Brewer and Mastick4 have shown that linear Birge-Sponer cxtrapolations of vibrational levels give the correct dissociation energies for CO, SiO, SnO and PbO. Hence we feel confident in using the value AH" = 159 kcal./rnole for the dissociation of GeO(g) (calculated by Herzbergj using a linear Birge-Sponer extrapolation). In Table I1 we have presented the latter heat, the heat of sublimation of germanium,6 the dissociation energy of oxygen' and the heat of formation of Ge02(s).8 From these data we calculate A H & = 50 kcal./rnole for l / 2 (3) For experimental details, t h e reader is referred t o University of California Radiation Laboratory Report lfi38. "Some Problems in the Chemistry of Germanium," January, 1952. (4) L. Brewer and D. hlastick, J. C h e m . Phys., 19, 834 (1951). ( 5 ) G. Herzberg, "Molecular Spectra and Molecular Structure, I. Spectra of Diatomic Molecules," D. Van h'ostrand Co., Inc., h'ew York, N. Y.,1950. (6) A. W. Searcy, University of California Radiatiorl Laboratory Report 1403 (1951). (7) National Bureau of Standards Selected Values of Chemical Thermodynamic Constants, Washington, D C . . lrl47, e1 yep ( 8 ) Cf, the second p a r t of this papet.
C,e(s) 4- '/tGe02(s) = GeO(g), as compared with 3 X i kcal. iiiole determined experinientally. The agreement is excellent, in view of the uncertainty i l l the rlissoci,ttion energy of (>eo(::I. r A B L I , 11 ITEAISOF R ~ A C I I O \
Redctic,n
(k.(g)
-+ O(g\ = (;eo(gi
1Ilaq8 fkcal rnole)
139
x.5 2
GC(i) = ( k ( p ) '/,02(g) = OI g I ",GeO,(',) = 2(:c(s) 4- I/.O:(g) --- - __ -'/?Ck(S) -t l/2(;eo2(\) = GeO(g)
,-,cl 2
'
61 2 ,?(J
Bues and v. Wartenberg have calorimetrically determined AIIO = - X . 3 kcal./mole for the reaction Ge(s) CeOg(s) = 2GeO(s) a t 298OK. This result c:tnnot be reconciled with their vapor pressure data without assuniing an unusually large entropy for the reaction. From their observation that GeO(s) tlisr~roI~ortionatesto Ge and GeOz a t temperatures around GOO', we conclude that their calorimetric data are spurious and that GeO(s) is thermodynamically unstable at all temperatures helow GOO". This instability of germanous oxide is analogous to the instability of Si09 and SnO.' From the values of AI]," which we have calculated from Bues and v. K'artenberg's and our vapor pressure data, we calculate that CeO(s.1 is unstable by about 7 kcal. 'mole.
+
mount Chemical Co. (purest As free). It was dried before use by heating at 900" for several hours. Some spectroscopically pure germanium powder was obt:tiriccl froin the Belinont Chemical Co. That which passed through a 400-niesh sieve was used in the calorimetric expcriincnts. The metal completely dissolved in the calorimeter iii about 20 minutes; this rather long time is responsible for most of the uncertainties in heats. The sieved metal was analyzed by oxidizing with hydrogen peroxide and then igniting to the oxide. The entire impurity is assumed to be GeO:. The results of four separate Ck: 89.7,80.7,89.1 and 90.1. We shall :iiialyses gave for uqe 89.f.55 f 0.2570. The heats were therefore corrected assiiming 1 0 . 3 % of the material t o be GeO?. The calorimetric experiments were carried out at 25 f 1' :mtl are reported in terms of the defined calorie (1 cal. = 4.1833 i n t . joules). The same calorimeter was used as in previously reported studies.'? Calculations.-From the experimental data in Table 111, we ralculate the average AH3 2= -160.2 =k 1.8 kcal./mole. The Sational Bureau of Standards? gives -27.83 kcal./ mole for the heat of formation of HClO (1000 H20). Pitzeri3 tahalated 3.32 kcal./mole as the heat of ionization of HClO; helice An," = -24.5 kcal./mole for C10-. Vsing AH? = -40.112 kca1./mole7 for Cl-, we calculate AH? = -129.2 f 2.0 kc:rl./'mole for the heat of formation of GeO?(s). Within esperimental error, this value agrees with the values obtained by Becker and Roth'o and Hahn and Juza." We c:dculate as a weighted average of all three values, AH? = -128.5 rt O.+?krnl./mole.
r0
TABLE 111 CALORIMETRIC DATA hlaterial dissolved
The Heat of Formation of Germanic 0xide.-
C,e
The heat of oxidation of germanium to germanic oxide by oxygen has been measured by Becker and RothIo and Hahn and Juza." These investigators found the heat of formation of Ge02(s) to be - 12S.1 and - 128.6 kcal./mole, respectively. I t was considered desirable to check these values by measuring the heat of oxidation of the metal by some aqueous oxidizing agent. It was found that when germanium nietal is very finely pulverized, an alkaline solution of hypochlorite effects complete solution in a relatively short time. High alkalinity is probably necessary in order to dissolve any oxide which forms on the surface of the metal.
(;e
Experimental Procedures.-The hypochlorite solution was prepared by passing chlorine into approximately 0 8 molar potassium hydroxide. The hypochlorite content wa4 then determined volumetrically using a standard thiosulfate solution. Two such batches of hypochlorite solution were prepared; each batch was used for a ieparate series of runs 111each of these series, two measurements n ere made of the heat of solution of metallic germanium and one measurement was made of the heat of solution of g e r m ~ n i coyide These heats refer to the reactions A H t . Ge(s) 2C10-(soln.) = GeOp(so1n ) 2CI-(soln ) A H o GeO?(s) = GeO?(soln.) The first heat minus the second gives AI13, the heat of the reaction ~ r 3 G~e .( s ) XIO-(soln.) = GeO?(s) 2CI-(soln.) In subsequent calculations, the heats of formation of C10-(soh.) and Cl-(soln.) are taken to be the same a? those in infinitely dilute aqueous solution The germanic oxide used was obtained from the Fair__
+
+
+
+
(9) Russell K. Edwards. University of C a l i f o r n i ~Radiation Laboratory Report 1639 (1952) (10) G Becker and W A. Roth Z.p h y s i k Chcm , A161, 09 (1932) ( 1 1) H Hahn and It Juza, E . anorp. alltern. Chrm., 144. 120 (1940).
(:e02 (;e (>e
Hypochlorite solution hlolarity c > f hliilarity of KOlI CIO-
0.8 .8 .8
AH1
AHa
AH,
o.ll39
-170.0 -170.5
,049
.x
,039 .os1
.R
,081
.s
(:eo?
Corrected heats, kcal./mole
-161.0 -161.5 -9.0
-165.5 -170.7
-156.5 -161.7 -9.n
,081
By appropriate combination of heats determined by Jolly a n d Latirner'2bL4it is possible to evaluate the heat of formation of GeO?(s) indirectly. In Table IV are listed five independent reactions and their corresponding heats at 298OE;.
The heat corresponding to the sum of these reactions may he combined with known heats of formation? for H t , I-, I f 2 0 and IS- to yield AH? = - 136.9 kcal./mole for the heat of formation of GeOi(s). The experimental uncertainties in the heats listed in Table IV are sufficient to explain the discrepancy between this value and that determined more directly. TABLE IV HEATSOF R E A C T I O X ~ * ~ ~ ~
+
AH& kcal./ mole
Reaction
2GeI4s) = Ge(s) CkI,(g) GeId(g) = GeI,(s) CkI,(s) 3H20 = H2Ge03 4TV 41II2Ge03 = H20 GeOz 1018 H + = 2GeIt(s) 2132GeO4s) 4Hz0
+ +
(:eO?(s)
+
+ +
+ 4 H + + 61-
+
+
=
Ge
+ 2H20 + 211-
+
30.1 -20.1 - 9.6 - 3.3
58.6 55.7
The authors wish to thank Prof. Leo Brewer for his helpful suggestions with regard to the problem of the stability of GeO. BERKELEY 4, CALIFORNrA (12) W. I.. Jolly a n d W. hl. Latimer, THISJOURNAL, 74, 5752 (1952). (13) K. 9. Pitzer, i b r d , 69, 2368 (1937). (14) W. L. Jolly and W. M. Latimer. ibid.. '74, 6764 (19121,
