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?