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The theoretical adiabatic flame temperature of Hydrogen and Fluorine in their stoichiometric mixture is often quoted at around $\pu{4300K}$, such as in this presentation on page 2, or this paper which is shown entirely in the pubs preview. However, this thermodynamics table from JANAF makes it seem as though the temperature should be much higher. I suspect I'm missing something, but after quite a lot of searching I can't find it! (So it's probably something very basic)

My work: The Gibb's energy for the reaction is quite negative even out to $\pu{6000K}$, so the combustion should happen effectively to completion regardless of temperature ($K = 307 = \exp\{--285.7\times1000/(8.314\times6000)\}$. The formation enthalpy is listed $\pu{-273kJ/mol}$, and we could either do the temperature integral of $C_p$, or scan down the H-H(Tr) column to find this puts us way beyond $\pu{6000K}$, let alone $\pu{4300K}$.

Anhydrous $\ce{HF}$ gas apparently does form dimers, trimers, etc. but these should only increase the adiabatic flame temp if present, and they (understandably) don't appear to be stable beyond a couple thousand K.

Thanks in advance for the help!

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    $\begingroup$ Unless I'm missing something, your assessment looks correct. $\endgroup$ – Chet Miller May 22 at 16:01
  • $\begingroup$ Glad it's not just me, thanks for giving it a look! There's a mystery afoot. $\endgroup$ – Michael Irving May 22 at 17:50

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