Standard heat of formation

Quick question anon: how is heat of formation measured/calculated for phases that don't exist at STP? i.e., how is the heat of formation of H2O(g) measured? Much appreciated.

Sorry, let me rephrase my question:

To my understanding, Enthalpy = internal energy + P*deltaV. The standard enthalpy of formation is then the total heat content of the new molecule under STP conditions, relative to its constituent elements. The concept of standard heat of formation makes sense to me when applied to reactions that actually occur at STP - it can be used to predict the outcome of reactions when used in conjunction with Hess's law.

Now, say we start out with liquid water. We heat the liquid, which increases the enthalpy of the system prior to any phase transitions taking place. Because of this, the actual enthalpy of the liquid after it transitions into the gas phase will always be higher than what is predicted by its standard heat of formation. So my question is: what is the purpose of quoting this hypothetical value for molecules in hypothetical phases? Will the enthalpy of reaction predicted by the addition/subtraction of these hypothetical heats of formation at STP be useful in describing the actual reaction taking place under realistic conditions?

H2 + O2 -> H2O(l)
H2O(l) --> H2O(g)

add together and then heat capacity * temperature for gas

>>3

Thanks, but please read my rephrased question.

not 100% on it, but the ideal gas equation PV=nRT could play a role in it, if one knows that PV is the work function for a gas. Divide that by n and you have J/mol; divide by 1000 and you have kJ/mol. that's the best I can figure

I'm feeling really keen, for some of that good ol' green

Marijuana MUST be legalized.