substances in their elemental state have enthalpies of formation equal to zero,why?
As you have stated - it is DEFINED to be that. We could define it differently. Definition selects a reference point.I am sorry but isn't standard enthalpy of formation defined as the change of enthalpy that comes from the formation of 1 mol of the compound from its elements at the most stable state at 1 atm?
Certainly --- and ΔH for such a process is zero. However, what would I accomplish by breaking the bond in the first place? The mole of O2 doesn't need its bonds broken before I use it in whatever process I have in mind --- it's the zero point from which all measurements are made.But don't you obtain the same energy again when the two bonds are formed again?