570
INDUSTRIAL AND ENGINEERING CHEMISTRY
CORRESPONDENCE Thermodynamics. of Carbon Disulfide Production SIR: In a recent paper on the “Thermodynamics of Carbon Disulfide Production” ( 7 ) , Ptull has calculated equilibrium data for the reaction: C (graphite)
+ 2S*(g)+C&(g)
0.9
0.7 V
8
66
0.5
6 LL
W
=
i
0.3
LITERATURE CITED
(1932). ( 2 ) Cross, P. C., J . Chem. Phys., 3, 168 (1935). (3) Ibid., p. 823. (4) Klemm, W., and Kilian, H., Z. physik. Chem., 49B, 279 (1941). ( 5 ) Montgomery, C. W., and Kassel, L., J . Chem. Phys., 2, 417 (1934). (6) Preuner, G., and Schupp, W., 2. physik. Chem., 68, 129 (1909) (7) Stull, D. R., IXD. ENG.CHEM., 41, 19F8 (1949). KO.1 LABORATORY COURTAULDS LIMITED COVENTRY, ENGLAND
I. F. TROTTER
+ SA(g)$CS2( g)
was required a t rather closer temperature intervals than had been calculated by Cross (S), and this was done using the published free energy functions of graphite ( I ) , diatomic sulfur ( d , 6 ) , and carbon disulfide (S),together with the zero point energy change evaluated by Cross (S).
d
Stull. The agreement is good and the slight discrepancy is probably due to the fact that Stull averaged the vapor pressure data of Preuner and Schupp, and of Klemrn and IGlian ( 4 ) ~In view of the agreement obtained, it seems likely that the true reactant is Sz(g). In comparing his results TT ith those of Cross ( S ) , Stull stated that Cross’s results led to a negative free energy change at all temperatures, vhich was contrary to experience. When, however, allowance is made for the varying partial pressure of diatomic sulfur, these results, as bhoir n above, agree well with those of Stull.
(1) Clayton, J. O., and Giauque, W.F.. J . Am. Chem. SOC.,54,2610
where the symbol S* (g) represents the equilibrium sulfur vapor containing 1 gram atomic weight of sulfur at 1 atmosphere total pressure distributed among the forms Ss.53e5fSn. In connection with the problem of the nature of the reacting species, it is of interest to compare Stull’s results with some calculations carried out in this laboratory in which the reactant was considered to be S&). For the purpose of the calculation, the equilibrium constant for the reaction:
C (graphite)
Vol. 42, No. 3
SIR: The calculations of Trotter confirm very nicely the ones in my article. This agreement stems from the fact that the basic principles of the calculations are the same. Trotter raises t,wo points that I should like t’odevelop a bit. 1. In my article (page 1973) I state, “the true reactant in both of these reactions. . . .may really be Sz(g).” Trotter’s comment is: “In view of the agreement obt,ained, it seems likely t’hat the true reactant is Sz(g).” The fact that our calculations lead to substantially the same result does not prove that the true reactant is Sz(g). We both strongly suspect it, but in m y opinion this does not constit,ute proof. That it is impossible t o deduce a kinetic mechanism from thermodynamic considerations results from the fact that thermodynamics treat the true equilibrium condition whereas kinetics deal 15-ith the time rate of approach to that’equilibrium. 2. In his last paragraph, Trot>tercalls attention to the fact that’ while Cross [ J . C h e w Phys., 3, 825-7 (1935)l considered the & ( g ) to exist at one atmosphere pressure over the whole temperature range (n-hich is not the case at lower temperat,ures), leading to a free energy change contrary to experience a t the lox-er temperatures, if one employs the actual partial pressure of the Si(g) the resulting free energy change in forming carbon disulfide from sulfur and carbon agrees with experience. Thus, the calculated equilibria predict the actual behavior of the system. D.4NIEL It. f h T L L DO--C H E M I C A L C O M P A K Y
0.1
MIDI~AZD MICH. ,
600
900 TEMPERATURE,
1900 KELVIN
1500
Thermodynamic Properties of Nitroparaffins
Figure 1. Mole Fraction of Carbon Disulfide as a Function of Temperature
------
Results of resent author Results of &ull
The partial pressure of diatomic sulfur was calculated over the temperature range 700’ to 1500’ K. using the vapor pressure data of Preuner and Schupp ( 6 ) , and was combined with the equilibrium constant calculations to give the mole fraction of carbon disulfide a t a series of temperatures. The results are shown graphically in Figure 1, together with those obtained by
SIR: The following additions arid corrections should be noted for the article, “Thermodynamic Properties of K’itroparaffins” [IKD.ENG.CHEM.,41, 2788 (1949)j. In Table 11, the units of the standard deviation, af,are calories per gram, On page 2791, the average error of estimate associated with the method of Andersen, Beyer, and Watson should read 1.08% instead of 1.18%. DYSART E. HQLCQMIB 132 DOGWOOD STREST
PaRK
FOREST, ILL.
