199. On the equilibrium and the heat of the reaction C2H4+ H2= C2H6

Abstract

has been carried out by Frost (Compt. rend. U.R.S.S., 1933, lSl), who obtained agreement with the experimental results then known, the difference between theory and experiment being 15% at 700" Abs., rising to 25% at 1000" Abs. Since the appearance of his work new experimental data have been published for the equilibrium and for the reaction heat, and also for one of the inertial moments of ethylene. Repeating the calculation with the newer data, we find a much greater discrepancy, the calculated value of K p being 2.43 times larger than the experimental value at 863" Abs. This change results mainly from the difference between the newer value for the reaction heat (-AH = 32,575 & 50 cals. at 298" Abs.) obtained by Kistiakowsky, Romeyn, Ruhoff, Smith, and Vaughan ( J . Amer. Chem. Sac., 1935, 57, 65) and the earlier value of 30,630 j , 300 found by von Wartenberg and Krause (2. physikal. Chem., 1930, A , 151, 105). The calculation is more sensitive to changes in the reaction heat than in the other quantities involved, and it appears that rather a large change in some of the molecular constants would be required in order to remove the discrepancy now revealed.For reasons given in Section 3, it seems to us that the newer equilibrium measurements by Travers and Pearce ( J . SOC. Chem. Ind., 1934, 53, 322; also Travers and Hockin, Proc. Roy. SOC., 1932, A , 136, 1) cannot be rejected. On the other hand, some of the molecular constants (viz., the forces acting between the methyl groups in ethane, some of the inertial moments, and some of the vibration frequencies) are known only by indirect methods. Very detailed consideration is necessary in order to decide what uncertainty is thereby introduced into a statistical mechanical correlation of the heat of reaction determined at the ordinary temperature and the equilibrium constant determined at relatively high temperatures. . ;Our main object is to investigate whether such a choice is possible of those constants which are not known exactly as will reconcile the thermal data of Kistiakowsky et al. with the equilibrium data. To this end we have calculated an upper limit for the reaction heat by making certain (maximum) assumptions concerning such of the frequencies in the ethylene and ethane molecules as are not fixed spectroscopically, and concerning the potential energy of the res,tricted rotation of the methyl groups in ethane. These assumptions are the most extreme which we believe to be possible within the limits set by the spectroscopically observed frequencies and the measured specific heats.The conclusion from the calculation is that, if we accept the equilibrium data within limits of error suggested by a consideration of all the measurements (see Section 3), then the reaction heat must lie at least 900 cals. below the lower limit allowed by Kistiakowsky and collaborators.The heat of reaction which we derive from the absolute value of the equilibrium constant by means of the most probable values of the molecular constants (see Section 2) is -AH = 31,050 If 300 cal...

Heat of hydrogenation of ethylene

“…The value thus deduced by Teller and Topley [7] for the heat of reaction (1) was LlHo=-31.05 ± 0.30 kilocalories per mole at 25° C. This uncertainty of ± 0.30 kilocalorie per mole was considered to be the limit of "probable" error, with the limit of "possible" error being ± 0.60 kilocalorie per mole. These limits of error were evaluated by T eller and Topley from a very critical analysis of the fallible factors entering into their calculations, namely: (1), the error in the value of the free energy from equilibrium data; (2), the error in the moments of inertia of ethylene and ethane; (3) the error in the vibrational frequencies of ethylene; (4), the uncertainty in the magnitude of the potential restricting the free rotation of the methyl groups in ethane; and (5), the uncertainty in the vibrational frequencies of ethane.…”

“…Teller and Tapley [7] (see also Frost [19], Kassel [17], and Smith and Vaughan [11]) made statistical calculations of values of the thermodynamic function, (FO-E~) / T, for gaseous C2H 4 and C2Hs, combined these with similar values computed for gaseous H2 by Giauque [14], and obtained a value for the property (I1FO-llE~) i T for reaction (1). Combination with the "best" value of llF o for 863 0 K, as obtained from the equilibrium data, then yielded a value for I1E~.…”

“…There is also a remote possibility, as discussed on page 633 that the values of Kistiakowsky, Romeyn, Ruhoff, Smith, and Vaughan may be subject to a small correction upward (numerically). [19], Smith and Vaughan [11], and Teller and Topley [7]. It is possible to combine, according to equation 5, the value of the change in entropy, I1S, for reaction (1), obtained from these statistical calculations, with the value of the change in free energy, I1F, as determined from equilibrium measurements, to obtain a value for the change in heat content, llH, for reaction (1).…”

“…Teller and Topley [7] (see also Frost [19] and Smith and Vaughan [11]) reviewed the experimental data of Pease and Durgan [20], Travers and Pearce [21], Vvedenskii and Vinnikova [22], and Frey and Huppke [23] on the equilibrium conditions for reaction (1), and deduced a very concordant, and apparently quite reliable, value for the free-energy change for the reaction of hydrogenation at 863 0 K . Teller and Tapley [7] (see also Frost [19], Kassel [17], and Smith and Vaughan [11]) made statistical calculations of values of the thermodynamic function, (FO-E~) / T, for gaseous C2H 4 and C2Hs, combined these with similar values computed for gaseous H2 by Giauque [14], and obtained a value for the property (I1FO-llE~) i T for reaction (1).…”

“…In addition to these five values there is one recently derived by Teller and Topley [7] by a combination of the statistical calculations of the thermodynamic properties of the molecules, hydrogen, ethylene, and ethane, with the experimental data of others on the equilibrium represented by reaction (1). Their procedure in evaluating the heat of hydrogenation is substantially that of utilizing the relation, t:..H=t:..F+Tt:..S, (5) where t:..F is the change in free energy, evaluated from equilibrium data, and t:..S is the change in entropy, calculated by statistical methods.…”