BERNARD DEWITTAND HARRYSELTZ
3170
Vol. 61
[CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY, CARVEGIE INSTITUTE OF TECHNOLOGY ]
A Thermodynamic Study of the Zinc-Antimony System. The Thermodynamic Properties of the Intermetallic Compounds: ZnSb, Zn,Sb, and Zn,Sb, BY BERNARD DEWXTTAND HARRVSELTZ
I n a previous paper' the authors have reported the results of an electromotive force study of
mole fractions in Table 11, and the activities are plotted in Fig. 1 and the and AH values in Fig. 2 . The activity curves show deviations from Raoult's law similar to Go0 those encountered in the cadmiumantimony system, except that here ti they are even more pronounced. The a1 values a t higher mole frac2 500 tions of zinc exhibit definite positive 8 deviations and at lower mole fract-c tions very marked negative deviations. As stated el~ewhere,~ in the 400 binary systems of which antimony I is a component there seem to be opI I l , I posing influences at work, one tend10 30 50 70 90 ing to produce positive and the other Weight percentage of zinc. negative deviations. A theoretical Fig. la.-Equilibrium diagram for the zincantimony system. explanation of this behavior is not liquid cadmium-antimony solutions and have as yet forthcoming. The abnormal forms of the calculated the thermodynamic properties of the and AH curv~'salso are worthy of note. intermetallic compounds CdSb and Cd3Sb. By similar methods, zinc-antimony solutions have TABLE I E m f,at dE/dT Temp. now been investigated and the thermodynamic ai X 10' 823.loK., LI, range, properties of the three intermetallic compounds, mv. 823.1OK. volt/'C. cal. OC. 142 480-560 :lg8 1.68 0.954 ZnSb, ZnSbz and ZndSb3, which appear as stable, solid phases, have been determined. The equi(jg73 1871 9.49 4.74 .875 ,765 3.91 1.35 1049 295 510-585 540-610 librium diagram (Fig. la) forthissystemhasbeen 6451 14.67 .661 4,60 1070 555410 established accurately by the exhaustive studies 5826 25.23 .49i 4.18 425 565-620 2 . 3 8 - 1031 565-620 4940 44.11 .288 of and his solubility curves have been used 535-595 63.24 .168 2.97 -1788 in this work. 3893 Experimental Results The electromative force measurements were made on evacuated cells of the type Zn(l)/ZnClp in LiCl
-+ KCl(1) 'Zn (in Zn-Sb
liquid sohtions)
and the results are given in Table I along with the calculated activities of zinc, ul, and relative heat contents, El. From these al and ZI values for zinc the corresponding a2 and Z;Zvalues for antimony have been ckculated along with the A H of forkation of a mole of the liquid melts from the pure liquid metals. These results are given at round number (1) Seltz and DeWitt, THISJOURNAL, 60, 1305 (1988) P2) Takei, Sci. Reo. Tohokti Vzw , 16, 1031 (1927)
,2741 ,1507
81.73 107.45
.lo0
.048
6.06 11.27
-1468 - 679
530-590 580-620
TABLE I1 N1
At 823.1"K. at m/Ni az
1.0 1 0 .9 949 8 ,883 .770 G ,525 5 .292 4 179 113 2 062 024 OOo
1.0 0 1.054 o.036 1.104 .053 1, , 079 0.875 ,152 , 5 8 4 .324 .448 .485 .377 .621 .310 .751 .240 ,884 ,170 , 0
ae/Nn
El
-
AH,
LI cal./mole
0 850 140 400 275 - 645 1030 -2870 600 -2205 - 950 - 275 - 1730 390 -lW0 375 .939 - 960 150 988 - 230 25 + 220 0
0.400 .356 .264 .264 .381 .647 .So8 egg7
(31 Seltz and DeWitt, THISJOURNAL, 61, 2694 (1939)
0 166 91
- 140
-522 -613 -458 -235 - 7% 0 0
Nov., 1939
3171
THERMODYNAMIC STUDY OF THE ZINC-ANTIMONY SYSTEM
0
0.2 0.4 0.6 0.8 1 0 Mole fraction of zinc. Fig. 1.-Activity curves a t 823.1OK.: 0, experimental; 0, calculated.
Calculations In the calculations which follow, the heats and 0 0.4 0.8 Mole fraction of zinc. free energies of fusion of antimony and zinc at Fig. 2.-Relative heat content and heat of various temperatures are required. Equations for formation. these quantities have been set up from the fusion and heat capacity data recommended by K d e y 4 ideal, as shown for the Pb-Bi system,6 and the and are given below is given by the mole fraction composition, Nz(s), relation Nz(s) = &/a&). In Table I11 the data Sb(s) = Sb(1) and solubilities are given a t temperatures between AH = 4015 + 1.64T - 0.89 X 10-$T2 (1) AF" = 4015 - 1.64T In T f 0.89 X 10-8T2 5.914T the melting point and the eutectic. (2) TABLE I11 Zn(s) = Zn(1) Equil. NPW solid Temp,, Na A H = 555 f 2.34T - 1.075 X 1 0 - T 2 (3) OK. aa soln. a2(s) Equil. liq. AF" = 555 - 2.34T In T f 1.075 X IO-3T2 f 13.49T
+
(4)
The Solid Solubility of Zinc in Antimony.-From equation 2 the activity of pure solid antimony, a2(s), relative to pure liquid as standard state, can be calculated a t any temperature by the relation, AFO = -RT In a2(s). From the solubility or freezing point curve2 the activity of antimony in the equilibrium liquid a t a given temperature is calculated from the experimental a2 and ZZvalues corresponding to this composition. The E z values are considered constant over the temperature range. If the &(s) and a2 values thus determined are equal, the equilibrium solid phase is pure antimony. If, however, the value of a2is less than aZ(s), the equilibrium solid must be a solid solution of zinc in antimony. For small solubilities the solid solution can be considered (4) Kelley, Bureau of Mines Bulletins 383 (1936) and 393 (1936).
865.1 823.4 784.6 778.1 (eutectic)
0.900 .800 .800 .682
0.890 .773 .670 .653
0.883
.751 .627 .609
0.992 .972 .936 ,933
Thermodynamic Properties of ZnSb, ZnJSbz and Zn,Sb,.-At any temperature where a liquid phase is in equilibrium with a solid intermetallic compound such as ZnSb, we can write for the equilibrium constant KZ&b =
1
where a1 and are the activities of zinc and antimony in the equilibrium liquid and where the activity of the solid is unity. Hence, for the formation of the compound from the two pure liquids, we write 1 Zn(1) + Sb(1) = ZnSb(s); AF" = -RT In a1 X a2
( 5 ) Strickler and Seltz, THISJOURNAL, 611, 2084 (1936).
BERNARD DEWITTAND HARRYSELTZ
3172
points lie satisfactorily on straight lines, the AH values of the rcxactions. Taking the mid-temperature for each coapound the following relations obtain
Similarly for the Zn3Sbzand ZniSh+ :;,( (1.2
--1114
!--
K
1123
Zn(1) f Sb(1)
Using the solubility curves of Takei and the experimental activities and relative heat contents from this investigation, the values of AF" of formation of the compounds a t different temperatures are given in Table IIr. For ZnBSbzand Zn4Sbs the
TErnP,
K
778 791 804 810
4Hsw= -14,800 cal. 4%n(l)
3Sb(l) = Zn4Sbs(s);
AF'320.1
=
-113,445;
Combining these values with those calculated from equations (l),(a), (3) and (4) a t the corresponding temperatures, and calculating to 298. 1°K. by the usual thermodynamic methods, with the assumption that AC* is zero, we obtain
+ Sb(s) = ZnSb(s); AF"*sa.l = -3361; AHzg~.l = -3069; A S ~ Q = S . 0.98 ~ e. u. :Zn(s) + 2Sb(s) = Zn3Sb2(s); = -3700; = -51; AS2gs.l = 12.25 e. a. ~ Z I I ( S )+ 3 S b ( ~ ) Z~Sba(s);AF'3ys.i -13,581; %n(s)
AF'298.1
AH2gs.l
AH2ss.l = -13,092;
AS2ss.~ = 1,63 e.
11.
These values of AS298.1 with the entropies of Sb(s) and Zn(s)6 lead to the following entropies for the compounds
9,449 - 9,62S - 9,x2 --
%nSb(s),S298.1 = 21.43 e . u.; ZnSb2(s); SZQS.~ = 63.65; Zn&ba(s); S 2 ~ 8 . 1 = 73.48 e. u.
- 13,BO-i
819 1 823 1 S2B 1 8%. -1 832 3 836 1
.+
AHs2~.l = -34,400 cal.
- 9,243 - 9,313
837 1 tu3 1 806 0 771 2 7iiO 1
ZnSb(s); 1FOsw.i = -3551; AHso4.1= -9570 cal.
AFO
4 1 3 816 7
=
+ 2Sb(l) = Zn3Sbl(s); A.F0sw = -9449;
Xn(1)
- 4,085 - 3,957 - 3,88t - 3,838 - 3,xla
1
Vol. 61
- 13,51.5
It should be noted that these thermodynamic quantities refer to the forms of the solid Zn$bz and Zn4Sb3as they exist a t temperatures of the liquid-solid equilibria and do not consider any transformations which have been reported at lower temperatures. There is still some uncertainty as to the nature of these changes and no thermal data are available for them. Of the three compounds, ZnSbz is the only one which melts congruently and the heat of fusion can be evaluated from the L values and the heat of formation. For the change
- 13,445 - 13,335 - 13,294 - 13,183
slight solid solubilities are negligible; their activities are taken as unity at each temperature.
I l l
+
3Zn(l) 2Sb(l) = ZnrSbe(1iq. s o h N1 = 0.6) AH = 3z1(Ni = 0.6) 2z2(N1 = 0.6) = 3 X 600 2( -2205) = -2610 cal.
+
+
Combining this value with the AH of formation of the compound from the pure liquid metals, we obtain the heat of fusion ZnaSbt(s) = Zn~Sbp(1);A H = 12,190 cal. I
1.19 119
I
21
123
I
125
1
127
L
1.29
I
Summary ZnjSb1, The activities and partial molal relative heat contents of zinc and antimony in their liquid melts have been determined over the entire concentration range.
1.31 1.33ZnSb,
1 2 2 1 % Z&ba 117' x 103. Fig. ?.--Plots of A F o / l ' against 1/T. 1.20
I
1.2t
Plots of A F " / T against i / T for the three compounds are shown in Fig. 3 , where it iS seen that
(G) I; K
B~~~~~~of Mines Bulletin 394.
Nov., 1939
MOLECULAR STRUCTURES
OF meSO AND
2. The solid solubility of zinc in antimony has been calculated. 3. The free energies and heats of formation and the entropies of ZnSb, ZnaSbz and Zn4Sb3
RACEMIC 2,3-DIBROMOBUTANES
3173
have been evaluated. 4. The heat of fusion of ZnaSbz has been calculated. RECEIVED SEPTEMBER 1, 1939
PITTSBURGH, PENNA.
OF CHEMISTRY, [CONTRIBUTION FROM THE GATESAND CRELLINLABORATORY INSTITUTE CALIFORNIA OF TECHNOLOGY, No. 7271
The Electron Diffraction Investigation of the Molecular Structures of the meso and Racemic 2,3-Dibromobutanesl BY D. P. ST EVENS ON^
AND
The recent electron diffraction investigations of the structures of some ethylene halides by Beacha and co-workers have provided considerable information regarding the magnitude of the repulsive forces acting between halogen atoms attached to adjacent carbon atoms. In order to extend the work of Beach to the study of the interactions of the methyl group with other methyl groups and with bromine atoms attached to adjacent carbon atoms, meso and racemic 2,3-dibromobutane were selected for electron diffraction investigation. It was hoped that the quite different chemical properties of this pair of molecules4 might result from an appreciable difference in the relative orientations of the “isopropyl bromide” groups about the 2,3 carbon-carbon single bond, inasmuch as these molecules would be expected to show structural differences in only this respect. Experimental.-The meso and racemic 2,3bromobutanes used in this research were portions of samples prepared for another investigation. It was estimated by Dr. Winstein that neither compound contained more than 2% of the other and that both were otherwise very pure. Inasmuch as the racemic and meso compounds are structurally very similar, such a quantity of one in the other would have no effect on the electron diffraction photographs. The electron diffraction apparatus used in this investigation has been described by Brockway.G The wave length of the electrons, determined in (1) The results of this investigation were presented at the meeting of the A. A. A. S. in Stanford, June, 1939. (2) National Research Fellow. (3) (a) J. Y. Beachand J. K . Palmer, J . C h c n . P h y s . , 6,639 (1938); (b) J. Y. Beach and A. Turkevich, THISJOURNAL, 61,303 (1939). (4) The rates with which the n c s o and the racemic dibromobutanes react with iodide ion are very different: cf. Dillon, Young and Lucas, ibid., 61, 1953 (1930),and Young, Pressman and Coryell, ibid., 61, 1640 (1939). (5) S. Winstein, D.Pressman and W. G. Young, ibid., 61, 1645 (1939). (6) L. 0.Brockway. Rev. Modern Phys., E , 231 (1936).
VERNERSCHOMAKER
the usual way from transmission photographs of gold foil, was 0.0611 A. Photographs were taken with two camera distances, 10.86 and 20.21 cm. As the compounds are not very volatile (b. p. ca. 150°), it was necessary to use the high temperature nozzle7 in order to obtain sufficient gas pressure. Photographs were taken with the liquid in the boiler a t 100 to 130’. Interpretation.-The photographs of each molecule showed 13 rings, and were practically identical qualitatively. Only small quantitative differences of doubtful reality were found. Curve C of Fig. 2, to be discussed below, gives a good representation of the appearance of the photographs. The observed values of s SO = -i; sin 2 4= for the maxima and minima as well as the visually estimated intensities, I,are given in Table I. Radial distribution functions* were calculated using the formulas
(
sin s,, I
D(1) = n cn
=fun,
y-J-
Sn)
”>
(1) (2)
The values of C, were chosen in accordance with the recommendations of Schomakereand are given in column 4 of Table I. The curves of Fig. 1 are plots of the radial distribution functions calculated from the measurements of the maxima and the minima for the meso and the racemic compounds. The two curves for each compound are in satisfactory agreement with respect to the two peaks a t -2.8 and -4.6 A., the precise location being for the meso-maxima curve, 2.83 and 4.61 A., the mesominima 2.85 and 4.62, the racemic-maxima 2.82 and 4.58 A.,and the racemic-minima 2.87 and (7) L. 0.Brockway and R. J. Palmer, THISJOURNAL, 59, 2181 (1937). (8) L. Pauling and L. 0. Brockway, ibid., 67, 2684 (1935). (9) Verner Schomaker, A. C. S. meeting, Baltimore, Md.. April, 1939.
