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Heats-Of-Combustion and of Formation of the Normal Paraffin Hydrocarbons

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Heats-Of-Combustion and of Formation of the Normal Paraffin Hydrocarbons U.S. Department of Commerce National Bureau of Standards RESEARCH PAPER RP692 Part ofJournal of Research, of the Rational Bureau of Standards, vol 13, July 193^ HEATS OF COMBUSTION AND OF FORMATION OF THE NORMAL PARAFFIN HYDROCARBONS IN THE GAS- EOUS STATE, AND THE ENERGIES OF THEIR ATOMIC LINKAGES By Frederick D. Rossini abstract From the data recently obtained in this laboratory on the heats of combus- tion of methane, ethane, propane, normal butane, and normal pentane, reliable values are deduced for the heats of combustion of all the other normal paraffin hydrocarbons in the gaseous state. For normal C wH 2n+2 (gas), with n>5, the heat of combustion, at a temperature of 25 C and a constant total pressure of 1 atmosphere, in gaseous oxygen to form gaseous carbon dioxide and liquid water, is 60.40 + n(157.00±0.08) k — cali 5 per mole. (This unit of energy is a defined one and is derived from the fundamental international joule by means of the factor 1.00040/4.1850.) The following generalization is made: In any organic molecule containing a normal alkyl group of more than 5 carbon atoms, the addition of a CH 2 group to the normal alkyl group to form the next higher normal alkyl group results in an increase in the heat of combustion of the organic molecule in the gaseous state, at a temperature of 25 C and a constant total pressure of 1 atmosphere, of 157.00 ± 0.08 k -cali 5 per mole. From these and other data values are computed for the heats of formation, from hydrogen and /3-graphite and diamond, respectively, of all the normal paraffin hydrocarbons in the gaseous state at temperatures of 25 C and Kelvin. The energy of dissociation, at zero Kelvin, of a gaseous normal paraffin hydrocarbon, C nH 2n +2 , into gaseous carbon and hydrogen atoms, is shown to be a linear function of n above, but not linear below, n— 6. The deviations from linearity for the latter are in the direction of greater stability of the molecule and have the following values: Methane, 4.81 ±0.08; ethane, 1.59 ±0.14; pro- pane, 0.83 ±0.19; normal butane, 0.46 ±0.27; normal pentane, 0.13 ±0.37, k-cal per mole. It is shown that for the normal paraffin hydrocarbons, and for carbon and hydrogen atoms in the normal state, the energy evolved in the reaction C (gas) + 2H(gas) + C wH 2n+2 (gas)=C n+ iH 2n+ 4 (gas), for n>5, at Kelvin, is — AH%= (20.8 ±0.9) + Dco k-cali 5 per mole (Dco is the energy of dissociation of CO at Kelvin) ; and any assignment of values for Dco , for the energy of excitation of the carbon atom to the 5S state, and for the energies of the 1 C-C and 2 C-H bonds formed in the reaction must satisfy this relation. CONTENTS Page I. Introduction 22 II. Heats of combustion 23 III. Heats of formation from the elements 26 1. At 25 centigrade 26 2. At Kelvin = 29 IV. Change in heat content for the reaction nCO (gas) + (n+l)H 2 (gas) n/2 2 (gas) = C„H 2n+2 (gas) 30 V. Energies of the atomic linkages 30 VI. References 35 21 22 Journal oj Research of the National Bureau of Standards [vol. is I. INTRODUCTION In this paper the writer utilizes the data on the heats of combustion of methane, ethane, propane, normal butane, and normal pentane 1 recently obtained in this laboratory (1, 25) for the following pur- poses: First, to predict accurate values for the heats of combustion of the normal gaseous paraffin hydrocarbons containing more than 5 carbon atoms; second, to compute values for the heats of formation of all the normal paraffin hydrocarbons in the gaseous state; and third, to test the accuracy of the postulate of the constancy of the energies of the atomic linkages in these molecules. Because of the increasing industrial importance of vapor phase reactions involving the paraffin hydrocarbons, there is great need for values of the heats of formation of these substances in the gaseous state, for making calculations of the chemical equilibria involved. The heats of combustion of hydrocarbons have been of considerable interest to the physical chemist for many years. Speculation con- cerning the influence of structure and of the number of carbon atoms upon the energy content of hydrocarbon molecules began as early as 1869, when Hermann (2) utilized for this purpose the data on heats of combustion obtained by Favre and Silbermann (4) about 15 years earlier. From the results of his own measurements, reported in 1886 in the fourth volume of his monumental treatise on thermochemistry, Thomsen (3) showed that, in general, the addition of a CH2 group to any molecule in the gaseous state resulted in an increase in its heat of combustion of about 158 kilocalories per mole. And, in particular, Thomsen's values for methane, ethane, propane, isobutane, and tetramethylmethane showed constant successive differences in the heats of combustion, at constant pressure and a temperature of 18 C, of 158.34 ±0.20 kilocalories per mole. Except for the meager data of Berthelot and Matignon (5) on ethane and propane, and the isolated value of Koth and Machlett (6) on pentane, no data on the heats of combustion of the gaseous paraffin hydrocarbons above methane were reported during the half century following Thomsen's measurements. Thus it was only natural that Thomsen's data were used to substantiate the almost universally accepted postulate of the constancy of the energies of the C-C and C-H linkages in the saturated hydrocarbon molecules. Many calculations have been made in recent years concerning the postulate of the constancy of bond energies in the paraffin hydro- carbons. In his recent book on "The Covalent Link in Chemistry", published in 1933, Sidgwick (7) writes: " * * * and so the values of the heats of formation of the [atomic] links [in the paraffin hydro- carbons] should be independent of the number of atoms in the mole- cule, as in fact they are found to be." In particular, one may cite the recent, 1932, calculations of Pauling 2 (8) who showed that, using the values given by Kharasch (9) for the heats of combustion of methane, ethane, propane, butane, hexane, and heptane, the energies of formation from gaseous atoms calcu- lated on the assumption of constant energies for the C-H and C-C 1 Figures in parentheses here and throughout the text refer to the references at the end of this paper, p. 35. 2 The values given by Kharasch (9) for methane, ethane, propane, and butane are based on the calori- metric data of Thomsen (3). Rossini] Heats of Combustion and Formation 23 bonds were practically identical with the values computed from heats of combustion plus some auxiliary data. 3 The differences between the values calculated by the two methods were such as to be equiva- lent to errors of less than 0.1 percent in the heats of combustion, or about 0.2 kilocalorie per carbon atom. This agreement is quite ex- traordinary because it borders on the very limit of the calorimetric precision obtainable by Thomsen (3), and there must be superim- posed upon his calorimetric uncertainty errors due to the large cor- rections for impurities in the samples of hydrocarbons burned by him. This latter uncertainty is probably much greater than Thomsen's calorimetric error because his various samples had the following empirical formulas: Ethane, propane, C2 H6 . 54, C2 H6 . C 3H . 33; 7 991 ; isobutane, ; tetramethylmethane ; . C 4H 9 . 748 , C4H9 . 899 C5Hn.78 , C 5 Hn.64 Thomsen assumed that his last sample of pentane was, in mole per- cent, 78.5 CfiHw + 21.5 C4H8 . Because of these facts, it has seemed desirable that an accurate test be made of the postulate of the constancy of the energies of the atomic linkages in the simple paraffin hydrocarbon molecules, by discovering, if possible, any real deviation from linearity in the heats of combustion of the members of the series. With the data on heats of combustion recently obtained in this laboratory (1), it is now possible to submit the postulate to a more rigorous test. II. HEATS OF COMBUSTION According to prevailing ideas, the heats of combustion of the paraffin hydrocarbons in the gaseous state should show constant differences between successive members of the series. That is to say, the heat of combustion will be a linear function of the number of carbon atoms, Qc = A + Bn (1) where Qc is the heat of combustion of a gaseous normal paraffin hydro- carbon containing n carbon atoms, B is the constant increment per carbon atom or CH2 group, and A is a constant. This relation may be expressed also as (Qc-A)/n = B (2) The following are the data recently obtained in this laboratory (1) on the heats of combustion of the gases at a temperature of 25 C and 4 a constant total pressure of 1 atmosphere, in k-cali 5 per mole: Methane 212. 79±0. 07 Ethane 372. 81±0. 11 Propane 530. 57±0. 12 Normal butane 687. 94±0. 15 Normal pentane 845. 27±0. 21 Examination of these values shows that the successive differences are 160.02, 157.76, 157.37, and 157.33, respectively. These differences are quite significantly not constant, but are apparently approaching a 3 The nature of the calculation is such that the use of different values for the subsidiary data changes only the actual values of the C-H and C-C bond energies, not their constancy from one molecule to another.
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