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In my data book, there's a list of common compounds and their molar enthalpies of formation -- at 298.15 K. What's the meaning of this given temperature value (is it the final temperature of the compound in question?) and how does it affect the molar enthalpy of formation of a substance?

What, for example, does it mean when the molar enthalpy of formation of H2O(g) at 298.15 K is given as -241.8 kJ/mol, even though water should be in a liquid state at 298.15 K? And what would the molar enthalpy of formation of H2O(g) be at, for instance, 500 K?

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  • $\begingroup$ Seems you forgot about water vapor. $\endgroup$ – Mithoron Sep 17 '15 at 23:41
  • $\begingroup$ Sorry, I still don't understand. Isn't water vapour H2O(g)? $\endgroup$ – the real deal Sep 17 '15 at 23:45
  • $\begingroup$ Yes, so there's no problem with it at 25C $\endgroup$ – Mithoron Sep 17 '15 at 23:47
  • $\begingroup$ So is the standard temperature listed in these molar enthalpy tables the temperature of the system upon losing/gaining the given amount of enthalpy? $\endgroup$ – the real deal Sep 17 '15 at 23:50
  • $\begingroup$ It's simply enthalpy in this temp. you have to pick temp. so... Oh whatever, I need to go to sleep $\endgroup$ – Mithoron Sep 17 '15 at 23:56
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Most enthalpies (or themodynamic observables whatever their type) are not constant across the temperature range — not even across the range of one phase. So the enthalpy of vapourisation of water is different a $273\,\mathrm{K}$ from what it is at $295\,\mathrm{K}$ or $350\,\mathrm{K}$.

Of course, standard conditions have been defined. However, the definition of standard conditions does not explicitly include temperature! Likely because some wanted $273\,\mathrm{K}$, others wanted $295\,\mathrm{K}$. So the temperature at which a certain entropy is valid must always be specified.

Some books put a ‘standard temperature’ at the top and call that their specification.

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