substances in their elemental state have enthalpies of formation equal to zero,why?
Convention. There has to be an agreed upon zero, and this is as good as any.
so no reason or concept behind this?
The "concept" is that measurements have to be made relative to some reference point, or "zero." "How far is Omaha?" "7." "7 what from where?"
It is not some sort of convention.
It is just that elements are not formed, they exist like they are. That's it.
You don't form an element, element just exists.
Anyway, it's important to say that you are referring to elements in their standard state. For example, carbon is at standard state in form of graphite. Carbon as diamond has a ΔΗ of formation that's not 0.
Yes it is.
That doesn't make their standard enthalpy zero. That's just our choice.
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?
CO2 is formed by C + O2: they are its elementary components, for example. How do you form O2 from its constituent elements if the costituent element at the most stable form is O2?
You take one mole of O2 and turn it into one mole of O2.
If you break the bonds between the two atoms, of course it takes energy.
But don't you obtain the same energy again when the two bonds are formed again?
As you have stated - it is DEFINED to be that. We could define it differently. Definition selects a reference point.
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.
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