Enthalpy of formation | Thermodynamics | AP Chemistry | Khan Academy

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How come Enthalpy of Formation is calculated differently from the typical chemical reaction under non-standard conditions (DeltaH)? For the latter, all one needs to do in order to find a reaction's change in enthalpy is re-arrange the additional (intermediate) chemical reactions (if needed) (altering their Delta H too) and then simply add their enthalpies.

The way Enthalpy of Formation is calculated also seems to, in a way, not follow Hess's law and the fact that energy is a state function. Maybe, because I learned how to solve for enthalpy change of reactions under non-standard conditions first I haven't considered the entire meaning of Enthalpy. But, how come it is not correct to just add all the enthalpy of formations (with their potential alterations) to determine the main reaction's change in enthalpy of formation?

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