Hi Wayne,
In general, there is no “absolute” correct value for terms like internal energy or enthalpy. All values are calculated relative to a reference state, and the value is set to zero at that reference state.
This is the standard approach taken across multiple fields; peruse the tabulated values in the back of any thermodynamics textbook and you will very likely encounter negative values for a number of species.
This approach works because it is not the absolute value of thermodynamic quantities that matter, but the change in thermodynamic quantities from one state to another, or one location to another, which drives processes such as transport, heat
release, etc.
Hope this helps, and please let me know if I am misinterpreting your question or if my response is unclear.
Best,
Steven
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Steven C. DeCaluwe, Ph.D | Associate Professor of Mechanical Engineering
COLORADOSCHOOLOFMINES
Brown Building W410B
Golden, CO 80401
Twitter: @CORESresearch
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