How to determine enthalpy of formation

Norseman

Background:
I've seen many different values for enthalpy of formation on Wikipedia, such as the following: MgO: -602 kJ·mol⁻¹, Fe₂O₃: -826 kJ·mol⁻¹, NO: 90.29 kJ·mol⁻¹.

I'm curious about these values, but I have no idea how they were determined and I've only taken an introductory course in chemistry. However, my understanding is that these values can be used to determine (or at least estimate) how much energy will be released (or consumed) by a chemical process which converts one set of compounds into another.

Question:
Is it possible to calculate the enthalpy of formation for a given compound, or is it only possible to determine that based on an experiment? If so, are there any good, free reference materials for that kind of information?

Borek

You can try to estimate them using bond formation energies, but the result is never very precise.

Norseman

Okay. How do I estimate using bond formation energy? Also, how imprecise are you talking about? ±50%? An order of magnitude? More?

Borek

Tens of percent usually.

http://www.google.com/search?rls=en&...ng+bond+energy

DrDu

Some enthalpies can be obtained by measuring heat in a calorimeter. A very precise way to determine enthalpies is via the van't Hoff equation e.g. from the change of electric potentials of electrolytic cells with temperature.