3606
W. D. GOODAND B. L. DEPRATER
The Enthalpies of Combustion and Formation of the 1-Alkanethiols. The Methylene Increment to the Enthalpy of Formation1
by W. D. Good and B. L. DePrater Contributwn No. 146 f r m the Thermodynamics Laboratory of th6 Bartlesville Petroleum Research Center, Bureau of Mines, U . 8. Department of the Interior, BartEesville, Oklahoma (Received June 17, 1966)
The enthalpies of combustion of 1-hexanethiol, 1-heptanethiol, and 1-decanethiol were measured by the rotating-bomb method. Enthalpies of formation of these compounds in the liquid state were derived. Values of the enthalpies of vaporization of 1-hexanethiol and 1-heptanethiol were derived from vapor pressure measurements of this laboratory. The enthalpy of vaporization of 1-decanethiol was estimated. The following values, in kilocalories mole-’, are reported for the standard enthalpy of formation, AHfo298.15J of these compounds in the gaseous state from graphite, rhombic sulfur, and gaseous hydrogen: 1-hexanethiol, -30.90 f 0.23; 1-heptanethiol, - 35.73 f 0.23; and 1-decanethiol, - 50.65 0.35. These values of enthalpy of formation were combined with earlier enthalpy-offormation studies of methanethiol, ethanethiol, 1-propanethiol, 1-butanethiol, and 1pentanethiol to compute values of the methylene increments to the enthalpy of formation of the 1-alkanethiols. These methylene increments were compared to those for the nparaffin hydrocarbons.
*
Introduction The Bureau of Mines has made thermodynamic studies of a selected group of organic sulfur compounds as part of American Petroleum Institute Project 48A. Information obtained about the change in thermodynamic properties with molecular shape or size has been used in calculating, by incremental methods, tables of data for homologous series of sulfur compounds. For example, earlier studies on 16 compounds were used to prepare comprehensive tables of data for 100 linear thiols, sulfides, and symmetrical disulfides. This paper reports basic experimental data for enthalpy-of-combustion measurements on l-hexanethiol, 1-heptanethiol, and 1-decanethiol, compounds for which thermodynamic properties were predicted in ref 2. Values of the enthalpy of combustion were used to derive values of the enthalpy of formation in the liquid state. Enthalpies of vaporization of 1hexanethiol and 1-heptanethiol were derived from vapor pressure measurements of this laboratory.3 The enthalpy of vaporizat.ion of 1-decanethiol was estimated. Values of the enthalpy of formation in the ideal gaseous state were derived. The Journal of Physical Chemistry
Methylene Increments to the Enthalpy of Formation of 1-Alkanethiols Earlier enthalpy-of-formation data for methanethiolJ4 e t h a n e t h i ~ l ,l-propanethiol,6 ~ 1-butanethiol,’ and 1pentanethiols were adjusted to conform to the 1961
(1) This investigation was part of American Petroleum Institute Research Project 48 on “Production, Isolation and Purification of Sulfur Compounds and Measurement of Their Properties,” which the Bureau of Mines conducts at Bartlesville, Okla., and Laramie, Wyo. (2) D. W. Scott and J. P. McCullough, U. S. Bureau of Mines Bulletin 595, U.S. Government Printing Office, Washington, D. C., 1961. (3) A. G. Osborn and D. R. Douslin, “Vapor Pressure Relations of 36 Sulfur Compounds Present in Petroleum,” to be published. (4) W. D. Good, J. L. Lacina, and J. P. McCullough, J. Phys. Chcm., 65, 2229 (1961). (5) J. P. McCullough, W. N. Hubbard, F. R. Frow, I. A. Hossenlopp, and G. Waddington, J . A m . Chem. Soc., 79, 561 (1957). (6) W.N. Hubbard and G. Waddington, Rec. Trav. Chim., 7 3 , 910 (1954). (7) W. N. Hubbard, W. D. Good, and G. Waddington, J . Phys. Chem., 62, 614 (1958). (8) W. N. Hubbard, C. Kats, and G. Waddington, ibid., 58, 142 (1954).
ENTHALPIES OF COMBUSTION AND FORMATION OF ~-ALKANETHIOLS
Atomic Weight Scales and to the current best valuesl0 of the enthalpies of formation of carbon dioxide, water, aqueous sulfuric acid, and gaseous S2. Enthalpies of formation of these C1 to C5 1-alkanethiols are tabulated along with those from the present research in Table I. The methylene increment to the enthalpy of formation of these compounds (after 1-butanethiol) is about 4.95 kcal mole-’. The increment to the enthalpy of formation of the normal paraffin hydrocarbons” (after nhexane) is about 4.93 kcal mole-’. The enthalpies of formation of the paraffin hydrocarbons are based on atomic weights of 1.0080, 12.010, and 16.000 for hydrogen, carbon, and oxygen, respectively, and on values of -68.3174 and -94.0518 kcal for the molar enthalpies of formation of liquid water and gaseous carbon dioxide; however, the values of the methylene increment would be changed very little by conversion to 1961 atomic weights and current best values of the enthalpies of formation of carbon dioxide and water. Table I: Methylene Increments to the Molal Enthalpy of Formation of Gaseous 1-Alkanethiols at 298.15% AHfozes.is,a
Compound
Methanethiol Ethanethiol 1-Propanethiol 1-Butanethiol 1-Pentanethiol 1-Hexanethiol 1-Heptanethiol 1-Decanethiol
koa1 mole-1
-20.69 A 0.154 -26.29 i:0.156 -31.48 A 0.16’ -36.31 f 0.28’ -41.36 f 0.28* -46.24f0.24 -51.07 z t 0 . 2 4 -65.99 i 0 . 3 6
+
Increment
5.60 5.19 4.83 5.05 4.88 4.83 4.97 x 3
+
a For t h e reaction: aC(c, graphite) b/2Hz(g) l/&(g) = C,HbS(g). Uncertainty is t h e “uncertainty interval” equal t o twice the final over-all standard deviation.
3607
The samples of 1-hexanethiol and 1-heptanethiol were dried by means of a vapor pass over molecular sieve, and the 1-decanethiol was dried by a liquid contact with activated silica gel. Samples for the jndividual combustion experiments were sealed in Pyrex13 ampoules. The hydrocarbon oili4 and the cotton thread fusel5 used to initiate the combustion reactions have been described. The value of AEco/M for the oil, as determined in experiments chronologically near those with the 1alkanethiols, was -10,984.12 f 0.51 cal g-I (mean and standard deviation). The value of A E c o / M for the cotton thread was -4050 calg-’. Apparatus and Techniques. The combustion experiments were performed in a rotating-bomb calorimeter, laboratory designation BMR 111, that is essentially identical with calorimeter BMR 1114already described. Platinum-lined bomb Pt-3b,’5 internal volume 0.3494 l., was used for all experiments. Experimental procedures for the combustion calorimetry of organic sulfur compounds have been described.* The bomb initially contained 10 ml of distilled water. One atmosphere of air was left in the bomb, and the initial pressure before a reaction was 30 atm (of air and purified oxygen). Units of Measurements and Auxiliary Quantities. All data reported are based on the 1961 International Atomic Weightsg and fundamental constants16 and the definitions: 0°C = 273.15”K, 1 cal = 4.184 (exactly) joules. The laboratory standard weights had been calibrated at the National Bureau of Standards. The values of density, p , specific heat, cp, and ( ~ E / ~ P ) T given in Table I1 were used in reducing weights in air to in vacuo, in converting the energy of the actual bomb process to the isothermal bomb process, and in
Experimental Section The basic procedures used in this research for the combustion calorimetry of organic sulfur compounds have been described.8 Only details pertinent to the study of the three 1-alkanethiols will be reported here. Materials. The samples of the three 1-alkanethiols were purified a t the Laramie Petroleum Research Center of the Bureau of Mines. The method of purification and useful physical properties of 1-hexanethiol and 1-heptanethiol have been published,12 and similar information for 1-decanethiol will be published later. The purities of the three compounds, as determined by unpublished studies of the melting point as a function of the fraction melted were 1-hexanethiol, 99.97 f 0.01 mole %; 1-heptanethiol, 99.97 f 0.01 mole %; and 1-decanethiol, 99.88 0.02 mole %.
*
(9) A. E. Cameron and E. Wichers, J . A m . Chem. Soc., 84, 4175 (1962). (10) D. D. Wagman, W. H. Evans, I. Halow, V. B. Parker, S. M. Bailey, and R. H. Schumm, “selected Values of Chemical Thermodynamic Properties,” National Bureau of Standards Technical Note 270-1, 1965. (11) “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Substances,” American Petroleum Institute Research Project 44 at the Carnegie Institute of Technology, Carnegie Press, Pittsburgh, Pa., 1953. (12) J. C. Morris, W. J. Lanum, R. V. Helm, W. E. Haines, G. L. Cook, and J. S. Ball. J . Chem. Eng. Data, 5, 112 (1960). (13) Reference to specific brands is made for identification only and does not imply endorsement by the Bureau of hlines. (14) W. D. Good, D. W. Scott, and G. Waddington, J . Phys. Chem., 60, 1080 (1956). (15) W. D. Good, D. R. Douslin, D. W. Scott, A. George, J. L. Lacina, J. P. Dawson, and G. Waddington, ibid., 63, 1133 (1959). (16) F. D. Rossini, J . Pure A p p l . Chem., 9,453 (1964).
Volume 70, Number 11 November 1966
W. D. GOODAND B. L. DEPRATER
3608
Table I1 : Physical Properties a t 298.15”K cal deg
(aE/aP)T, oal atm-1
g -1
g -1
0.4663 0.4686 0.4793
-0.00609
CP, PI
g
l-Hexanethiol l-Heptanethiol l-Decanethiol
ml-1
0.83826 0.83912 0.84075
- 0.00585 - 0.00549
reducing to standard states.” Values of the density of l-hexanethiol and l-heptanethiol were from ref 12. Density data for l-decanethiol are from unpublished measurements of t8he Laramie Research Center. Values of c, are from unpublished measurements of this laboratory. Values of ( ~ E / ~ Pwere ) T calculated from
I11 (l-decanethiol). Two experimental results for CeHi4S(1) 1102(g) 84HzO(1) = 6COdg) H2SO4.90H20(1) (I) C7HieS(l) 2b/z02(g)f 93H2O(1) = 7COz(g) H2S04*100H20(1) (11) CioH22S(l) 1702(g) 13OHzO(1) = lOCOz(g) f HzS04*140H20(1) (111) each compound were rejected because of incomplete combustion evidenced by soot deposits. Recoveries of sulfuric acid in the combustion products of the three compounds were l-hexanethiol, 99.7 f 0.2%; l-heptanethiol, 99.6 f 0.3%; and 1decanethiol, 99.8 A 0.2%. Previous research indicates that these recoveries are normal.8
+
+ +
+
+
Table I11 : Energy of the Idealized Combustion Reaction“
m’(compound), g m”(oil), g m’”(fuse), g
+
Atc = t f - t i Atcor, deg &(Calor.)(-AL), cal &(Cont.)(-Ato),*cal AEign, cal A E f d e c (“Os HNOz), cal AE (cor to standard states),’ cal -m”AEc’/M(oil), cal m”’AEc’ /?@(fuse), cal
+
-
m’AEc / M (compound), cal APEcO/M(compound), cal g-l a
LHexanethiol
l-Heptanethiol
0.81464 0.03672 0.00102 2.05073 - 8256.79 -28.48 0.67 10.28 3.10
0.79874 0.03938 0.00090 2.05110 -8258.28 -28.34 1.16 10.00 3.49
0.76953 0.04278 0.00111 2.04966 - 8252.48 -28.44 1.30 8.12 4.14
403.34 4.13
432.56 3.65
469.90 4.50
-7863.75 - 9653.04
l-Decanethiol
-7835.76 -9810.15
The symbols and abbreviations of this table are those used in ref 17, except as noted. E Items 81-85, 87-91, 93, and 94 of the computation form of ref 17.
-7,792.96 -10,126.91
Ei(Cont.)(ti
- 25) + Ef(Cont.)(25” - tf +
At,,,).
the temperature dependence of density by use of the T T(bV/bT)p. approximation: ( ~ E / ~ P=) Calibration. The energy equivalent of the calorimetric system, E(Calor.), was determined by combustion of benzoic acid (National Bureau of Standards standard sample 39h with a certified enthalpy of com0.003 abs. kjoule/g of mass under bustion of 26.434 certificate conditions). Ten calibration experiments spaced among the experiments with the l-alkanethiols gave the value, &(Calor.) = 4026.27 f 0.12 cal deg-l (mean and standard deviation).
*
Calorimetric Results Typical calorimetric experiments with l-hexanethiol, l-heptanethiol, and l-decanethiol are summarized in Table 111. The results for all experiments are summarized in Table IV. Values of A E c ” / M refer to reactions I (l-hexanethiol), I1 (l-heptanethiol), and The Journal of Physical Chemistry
Derived Results Derived results for the three l-alkanethiols are given in Table V. The values of AEco298.15 and AH ~ ~ ~ are ~ 8for . 1 the ~ idealized combustion reactions I, 11, and 111. The enthalpies of combustion and values of the enthalpies of formationlo of C02(g), H2O(l), and HzSOl(aq) were used in computing values of the enthalpy of formation of the liquids according to reaction IV. Values of the enthalpy of aC(c, graphite)
+ b/2Hz(g) + S(c, rhombic) =C,H,S(l)
(IV)
vaporization of l-hexanethiol and l-heptanethiol were (17) W. N. Hubbard, D. W. Scott, and G. Waddington, “Experimental Thermochemistry,” F. D. Rossini, Ed., Interscience Publishers, Inc., New York, N. Y . , 1956, Chapter 5 , pp 75-128.
ENTHALPIES OF COMBUSTION AND FORMATION OF ~-ALKANETHIOLS
Table IV : Summary of Experimental Results: Values of A E c " / M (in cal g-l at 25")
Mean S M dev
1-Hexanethiol reaction I
1-Heptanethiol reaction I1
1-Decanethiol reaction I11
-9655.06
- 9810.15
9652.33 9653.94 9653.65 9653.04 9655.16 9652.86 9656.39 9653.45
-9810.18 -9811.84 - 9811.45 - 9810.76 9809.52 -9809.89 -9810.53
- 9653.99
- 9810.54 0.28
-10,126.91 -10,123.45 -10,126.94 -10,128.19 -10,128.57 -10,128.47 -10,126.90 -10,126.22 -10,127.86 -10,129.35 -10,127.29 0.52
-
0.43
3609
bic sulfurlo was used to compute the values of the enthalpies of formation of the 1-alkanethiols from &(g) given in Table I.
Discussion The enthalpies of formation reported supersede all earlier values. The total change from earlier published values is of the order of 0.1 kcal mole-' when the reference state for sulfur is gaseous Sz and a few hundredths kea1 mole-' when the reference state is rhombic sulfur. Minor changes were caused by the use of 1961 International Atomic Weightss and by the recently selected "best value'' of the enthalpy of formation of aqueous sulfur acid.I0 The largest single change was caused by the newly selected "best value" of the en-
Table V:" Derived Results a t 298.15"K (kcal mole-') Property
AEc" AHc" AHj" AHv AHf"
State
Liquid Liquid Liquid Gas
1-Hexanethiol
-1141.51 f 0 . 1 8 -1144.48 f 0.18 -41.84 f 0.22' 10.94rt0.05 -30.90 f 0.23'
1-Heptanethiol
1-Decanethiol
-1297.64f0.18 -1300.90f0.18 -47.82 f 0.22' 12.09f0.05 -35.73 f 0.23'
-1765.70f0.29 -1769.85f0.29 -66.07 f 0.34' -15.42 f 0 . 1 -50.65 f 0.35'
a Uncertair ies are the "uncertainty interval" equal to twice the final over-all standard deviation of he mean. for sulfur is S (rhombic).
derived from Cox'* equations fitted to the experimental vapor pressure data of ref 3, the exact form of the Clapeyron equation, and estimated values of the second virial coefficient. These values of the enthalpy of vaporization are strictly the enthalpy of vaporization to the real gas, AHu298.15, but are not significantly different from the standard enthalpy of vaporization, AHv0298.16. The standard enthalpy of vaporization of 1-decanethiol was estimated. Values derived for enthalpy of formation refer to rhombic sulfur. The enthalpy of formation of Sz(g) from rhom-
' Reference state
thalpy of formation of gaseous Szl0 from rhombic sulfur. The enthalpies of formation of 1-hexanethiol, 1heptanethiol, and 1-decanethiol are, within experimental error, the values that had been predicted by the incremental method.2 The value of the methylene increment derived from the enthalpies of formation of the 1-alkanethiols is in good agreement with the value of the methylene increment derived from the enthalpies of formation of the hydrocarbons. ~
~~
(18) E.R.Cox, Ind. Eng. Chem., 28, 613 (1936).
Volume 70,Number 11
November 1966
