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I'm unsure why $\Delta_\mathrm{f}H^\circ$ within the NIST-JANAF Thermochemical Tables [1] is temperature-dependent. Wouldn't the $\Delta_\mathrm{f}H^\circ$ represent the enthalpy of formation at the reference temperature (298.15 K) by default? If not, what does it represent?

The following screenshot of the online page for water is provided as an example:

JANAF H2O Table Screenshot

EDIT: I believe my point was missed, so I will provide another example. The following is Equation 13.17b within Fundamentals of Engineering Thermodynamics $$ Q - W = \sum_P n \left( \bar{h}_f^\circ + \Delta \bar{h} - \bar{R} T_P \right) - \sum_R n \left( \bar{h}_f^\circ + \Delta \bar{h} - \bar{R} T_R \right) $$

If I wanted to evaluate the $\bar{h}_f^\circ + \Delta \bar{h}$ term for gaseous water in a hypothetical reaction at a temperature of 800 K, how would I do so?

Would it be (-241.826 + 18.002) kJ/mol or (-246.443) kJ/mol?

Reference

  1. Thomas C. Allison. NIST-JANAF Thermochemical Tables - SRD 13, 2013. DOI: 10.18434/T42S31.
  2. Michael J. Moran “Reacting Mixtures and Combustion.” Fundamentals of Engineering Thermodynamics, Wiley, S.l., 2020, pp. 519–519.
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