Shomate coefficients for n-octane

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SUMMARY

The discussion centers on the absence of Shomate coefficients for n-octane (C8H18) in the NIST Webbook, which provides gas phase thermochemistry data but lacks these specific coefficients. Users can extrapolate enthalpy figures using the provided Cp data, but challenges remain in accurately modeling entropy. The conversation suggests that reliable Shomate coefficients for n-octane may not exist due to insufficient measurement data, and recommends using values from pentane and hexane as a workaround.

PREREQUISITES
  • Understanding of the Shomate equation
  • Familiarity with gas phase thermochemistry
  • Knowledge of Cp (heat capacity) data interpretation
  • Basic skills in data extrapolation techniques
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  • Research the Shomate equation and its applications in thermodynamics
  • Explore the NIST Webbook for gas phase thermochemistry data
  • Learn about data extrapolation methods for thermodynamic properties
  • Investigate the thermodynamic properties of pentane and hexane for comparison
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Chemical engineers, thermodynamicists, and researchers in fuel chemistry who require accurate thermodynamic data for n-octane and related hydrocarbons.

manderson99
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Hello! I've been looking for coefficients to use with the Shomate equation for n-octane (C8H18), but I have been unable to find any. The NIST Webbook lists some gas phase thermochemistry data:

http://webbook.nist.gov/cgi/cbook.cgi?ID=C111659&Mask=1#Thermo-Gas

but it omits the Shomate coefficients that are present for some other compounds (methane, for example). I could extrapolate enthalpy figures using the Cp data included in the NIST entry, but I'd be at a loss as to what to do about entropy at anything but ATP.

It could be that one can't model Cp/H/S properly for octane using the Shomate equation, though I'm too ignorant to know whether or not that is the case.

So, can anyone help clear up this matter for me? Thanks!
 
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You are probably looking at the price of data; i.e., it's not been measured. You might be able to "kluge" together a set from extrapolation of the pentane and hexane values that are in the tables plus a little "Kentucky windage."
 
D'oh! Oh well, thanks for letting me know. I'll do my best with what's available.
 

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