H. W. ZIMMERMANNAND W. M. LATIMER
1554
values have been used with the estimated entropies of the acid and its ions to calculate
[CONTRIBUTION FROM
provisional values of the free energies. BERKELEY,CALIFORNIA
LABORATORY O F THE
THE CHEMICAL
VOl. 61
RECEIVED MARCH20, 1939
UNIVERSITY O F CALIFORNIA ]
The Heat of the Reaction of Thiosulfate with Triiodide BY H. W. ZIMMERMANN A N D W. M. LATIMER The heat of the reaction of thiosulfate with triiodide was redetermined in order to obtain a more reliable estimate of the potential of the thiosulfate-tetrathionate couple. Calorimeter.-The calorimeter was that described by Latimer and Zimmermann.' Materials.-C. P. chemicals were used without further purification. Analysis showed that the sodium thiosulfate was a t least 99.8% NazSz03. 5Hz0. TABLE I HEAT OF SOLUTION OF Na~S20a.5H20 AT 25' No.
NszSz03.5H20 moles
1 2 3
0.00930 .00968 .00883
4H, tal,
q3
cal.
-10.5. 11 - 109.21 - 99.93
Average AH Total volume in each case 870 cc.
N a 2 S z O 3 . 5 H Z O ( s ) = 2Na+
l1l3O2 11,282 11,317
* 6o * 60 * 60
11,300
* 35
+ S203= + 5Hz0; AHzg8.1 = 11,300
*
Table I1 shows the heats measured when this salt was dissolved in dilute triiodide solutions. TABLE I1 ~ N ~ z S ~ O ~ ~ ~ HIsZO =(&Oe= S)
+
No.
35
Na2SOa. m H 2 0; AH~Q, = -330,760 Na2SOa.5200H20; AHzyl = -330,630
Applying this correction we find
+ 5HeO;
AH'zm
L =
11,170
In correcting this result to infinite dilution both initial and final solutions should be considered. On the average we may write the concentrations of initial solution:
final solution (Na&Oe) (KIB) (KI) (NaI)
= = = =
0.0052 mole/liter 0.00045 niole/liter 0.0164 mole/liter 0.0103 mole/liter
The average amount of NazSz03.5Hz0 used was 0.0103 mole/liter . If we consider the initial solution to be made up of 0.0056 0.0112 = 0.0168 mole/liter of potassium iodide the heat of dilution will be a trifle high. In the list of Bichowsky and Rossini2 we find
*
100
KI'3300HzO KI.mHzO
Thomsen3 reported for this reactioii AI€"zyi = 11,300
.1ff3300H20
while Berthelot" found AH0284
&oa-6873 * 70 7117 f 70 6987 * 70 6966 * 70 6987 * 70 6986 * 30
+
AH, - AHSZOOH~O= -130 cal.
S203-
4H, cal. /mole
q, cal.
(KI3) = 0.0056 mole/liter (KI) = 0.0112 mole/liter
18O
+
Excess la-, moles
0.008730 0.00481 0.00089 -59.99 3 .009001 .00481 ,00062 64.06 4 ,008773 .00481 ,00085 61.30 5 ,008710 ,00478 ,00086 60.68 6 .009534 ,00506 ,00058 66.61 Average AH Total volume in each case 870 cc.
We estimate the heat of dilution by assuming that thiosulfate behaves like sulfate a t these concentrations. Bichowsky and Rossini2 give the heats of formation of sodium sulfate solutions a t
NazSz03,5H~O(s)= 2Na+
+ 4NaC + 31- + 5Hz0
Initial solution h'azSzOa. 5Hz0, la-, moles moles
= 10,800
(1) w. M. Latimer and H. W. Zimmermann, THISJ U T J R N A I . , 61, 1550 (1939). (2) Bichowsky and Kossini, "Thermochemistry of the Chemical Substances," Reinhold Pub. Corp., New York, 1936. ( 3 ) Julius Thomsen, "Thermochemische Untersuchungen," J . A. Barth, Leipzig. 1882, Vols. I , 11. (4) Rrrthelot, A m i . chi7it. p h y s . , [6] 17, 462 (188'3).
O 33 - A H C O H ~=
AHm = -73,607
-73,640
x
0.0168/0.0103 = $54 cal./mole
The final solution we assume to contain only 0.00045
+ 0.01635 + 0.0103 + 2 X 0.00515 = 0.0324
inole/liter NaI
The substitution of sodium iodide for potassium iodide and potassium triiodide will make the correction a little high while this substitutioii for
June, lW!J
THERMODYNAMIC PROPERTIES O F
O.5NazSzO4tends to give a lower result. Again we find from Bichowsky and Rossini NaI.mHsO AH291 = -70,848 NaI. 1500H20 -70,805 AHrnrr20
-
Applying these corrections to our experimental result and then doubling we find for the reaction
+
+
+ 31- + 13,760
* 200
and combining this with the heat of solution 2&0s’
+ Is-
= S4O6-
+ 31-;
AHigss.1
-8580
* 250
The heat of formation of IS- (as) listed by Bichowsky and Rossini, AH291 = -12,140, does not agree with the experimental data given in the second section of their book. 3H = -12,350 seems a more probable value for this quantity. Using this figure we find I ~ ( s )+ IIs-; AHm = 820 and, neglecting the small temperature difference 2&03= + Ip(s) = SlOa” + 21-; AH’2g8.1 = -iT60 * 250 Thomsen found for this reaction AH291 = -7951
Berthelot reported
+ + + + Is(s) = S4Oe’
2S203- Is- = S4OS” 31-; I&) I- = Ia-; 2S.08-
AH2sa = -8480 - 260
I
21-;
-1Hu3 =
-8740
1555
Obviously he made a mistake in sign in the second reaction and his result should read = -8220
AH283
A H i w o ~ ~=o -44 X 0.0324/0.0103 = - 160 cal./mole
2Na2Sz03.!5H~O(s) 13- = s406’ 4Na+ 10H20; AHOzes.1
CARBON TETRABROMIDE
Potential of the Thiosulfate-Tetrathionate Couple.-The entropies of SZO~=and S406s are probably about 8 and 35, respectively. These estimates seem reasonable in comparison with the and HS03-, i. e., 3, 4.4, entropies of SOa=, SO?=, and 32.6, respectively, as given by Latimer.6 Taking the entropies of Iz(s),27.9, and of I-, 25.3, also from Latimer, we find 2S2Oa” f
Ia(S) = S406‘
$- 2I-;
ASS8.l = 42 AF2)Fs.i = -20,200 Eo = 0.44
Combining with the potential of the I- - I? couple 21- = I ~ ( s )+2e-;
we find 2S2O8= = S406-
+ 2e-;
Eo = -0.5345 Eo = -0.10
Summary The heat of oxidation of thiosulfate by triiodide has been measured. This determination, with the estimated entropies of thiosulfate and tetrathionate, leads to an approximate value for the potential of the thiosulfate-tetrathionate couple. ( 5 ) W. M. Latimer, “The Oxidation States of the Elements and their Potentials in Aqueous Solutions,” Prentice-Hall, lnc., New York, N. Y.,1938.
BERKELEY, CALIFORNIA
RECEIVED MARCH20, 1939
l C O N T R I B T J T I O N FROM TIIR C H E M I C A L IdABQRATORY O F THR 1 T N I V K R S I T Y O F C A L I F O R N I A ]
Specific Heats and Heats of Fusion and Transition of Carbon Tetrabromide BY K. J. FREDERICK ANI) J. H. H I L D E B R A N ~ ‘Ihe subject of intermolecular forces and solii hility has directed our attention particularly to a detailed study of the tetrahalides, for these substances are fine examples of molecules having widely differirig volumes and intermolecular fields but with a common highly symmetrical structure. However, iii many cases the evaluation of solubility data has been unsatisfactory on account of lack of accurate knowledge of specific heats and heats of fusion. This paper presents the results of determinations of the specific heats and heat of fusion of carbon tetrabromide, whose polymorphic nature, like that of carbon tetrachloride, makes it particularly interesting. The method of mixtures employed in this work
enabled LIS to determine the heat of the solid transition. A thorough examination of the literature reveals only meager information concerning carbon tetrabromide. The phase diagram has beeti studied carefully by Roozeboom and others’ and they have established definitely the fact that there are two solid forms of carbon tetrabromide with a temperature of transition within the range of 46.6 to 46.9’. There exists also a high pressure modification which need not concern us here. The heat of decomposition of carbon tetrabromide into bromine, hexabromoethane, (1) €3. Roozeboom, “Heterogene Gleichgewichte,” Vol I , 1901, p. 127; W. Wahl, Proc. Roy Soc (London), A87, 152 (l!ll3) Phil. Tmirs., A214, 117 (1913).
