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Adventures in Thermochemistry

Adventures in Thermochemistry. James S. Chickos * Department of Chemistry and Biochemistry University of Missouri-St. Louis Louis MO 63121 E-mail: jsc@umsl.edu 4. Gateway to the West. Previously we concluded the following:.

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Adventures in Thermochemistry

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  1. Adventures in Thermochemistry James S. Chickos* Department of Chemistry and Biochemistry University of Missouri-St. Louis Louis MO 63121 E-mail: jsc@umsl.edu 4 Gateway to the West

  2. Previously we concluded the following: • 1. Vaporization enthalpies at the boiling temperature are predicted to approach a limiting value • 2. Boiling temperatures appear to converge to a finite limit. • Critical temperature and boiling temperatures appear to converge as a function of the number of repeat units. • Critical pressures appear to converge to some small finite pressure (~1 atm) as the number of repeat units  . Can any of this be experimentally verified?

  3. If vaporization enthalpies at the boiling point attain some maximum value, then vaporization enthalpies of homologous series as a function of temperature shouldshow some curvature as the size increases. How do known vaporization enthalpies of the n-alkanes at T = 298.15 K behave as a function of the number of carbons?

  4. Literature Vaporization Enthalpies of the n Alkanes C5 to C20 at T = 298.15 K lgHm(Tm) = (5.01±0.007)N + (1.487±0.1) r2 = 0.99997 Ruzicka, K.; Majer, V. Simultaneous Treatment of Vapor Pressures and Related Thermodynamic Properties Between the Triple Point and Normal Boiling Temperatures for n-Alkanes C5-C20. J. Phys. Chem. Ref. Data 1994, 23, 1.

  5. Applications of gas chromatography The measurement of vaporization enthalpies A. Calorimetric B. Vapor pressure dependency on temperature The measurement of vapor pressure A. Various static method B. Effusion methods C. Transpiration methods both properties depend on pure samples, moderate quantities (> mg) Our group has been interested in developing a method that could circumvent the requirement of sample purity and quantity and could be applicable in the sub-pascal region

  6. A series of isothermal runs. The compounds are n-alkanes

  7. Basic Considerations in Using Gas Chromatography In gas chromatography, the time a compound spends on the column (ta) is inversely proportional to the compounds vapor pressure on the column. Therefore, the vapor pressure p of a compound is proportional to 1/ta. The amount of time a compound spends on the column, ta, (the adjusted retention time) is obtained by subtracting the retention time of an non-retained reference (often the solvent) from the retention time of each analyte. If 1/ta is proportional to vapor pressure, then for chromatograms run isothermally, a plot of ln(to/ta) versus 1/T (K-1) over a 30 K temperature range, where to is the reference time, 1 min, should result in a straight line with a negative slope equal to the enthalpy of transfer from the stationary phase of the column to the gas phase divided by the gas constant, slngHm(Tm)/R. slngHm(Tm) = lgHm(Tm) + slnHm(Tm) Both terms are predicted to have the same dependence on size. Coiling of the n-alkane decreasing intermolecular interactions will lead to an attenuation of both slngHm(Tm) and lgHm(Tm).

  8. A plot of ln(to/ta) versus 1/T (K-1) From top to bottom: docosane tetracosane hexacosane nonacosane dotriacontane tetratriacontane hexatriacontane octatriacontane ln(to/ta) = -slngHm(Tm)/RT +C to = 1 min

  9. Enthalpies of transfer measured at Tm = 520 K vs the number of carbon atoms from C21 to C38 slngHm(520 K) = (3005±13.1)N+(3054±287); r2 = 0.9997 slngHm(Tm) = lgHm(Tm)+ slnHm(Tm)

  10. Individual n- alkanes are available commercially for most even n-alkanes up to C60. In addition, alkanes derived from oligomers of polyethylene are available up to ~C100

  11. Even Alkanes from Polywax1000 C60 C86

  12. A plot of slngHm(T) against the number of carbon atoms, N for N = 42 to 76. The equation of the linear fit: slngHm(653 K) = (299913)N + (3039286); r2 = 0.9997. The equation of the line fit by a second order polynomial is given by: slngHm(653 K)(-8.775)N2 +3200.8N - 20915; r2 = 0.9999.

  13. Enthalpies of transfer kJ/mol as a function of the number of carbon atoms from C50 to C92 to a third order polynomial triangles: slngHm(653 K) = (2.12±0.02)N + (16.18±0.73); r2 = 0.9989 triangles: slngHm(653 K) = -(8.37±0.96)10-3N2+(3.45±0.14)N –(29.6±5.3); r2 = 0.9998 circles: slngHm(676 K) = (2.12±0.016)N + (12.43±0.92); r2 = 0.9909 circles: slngHm(676 K) = -(5.64±0.56)10-3N 2+(2.93±0.08)N –(15.1±2.8); r2 = 0.9998 squares: slngHm(676 K)= (2.12±0.018)N + (11.42±0.78); r2 = 0.9989 squares: slngHm(676 K)= -(7.47±0.42)10-3N 2+(3.24±0.06)N –(29.8±2.3); r2 = 0.9999

  14. Conclusions: Based on the data available, it appears that enthalpies of transfer at temperatures below the boiling temperature do show some curvature as a function of carbon number. Whether this is due to changes in lgHm(Tm) or slnHm(Tm) or both is not known from these results.

  15. Richard Heinze Tom Murphy Hui Zhao William Hanshaw Patamaporn Umnahanant (T) Hui Zhao William Hanshaw T

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