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I know that the standard enthalpy of formation of an element in its most stable form is zero, but does that mean that the element begins in an unstable form and goes to its stable form, ex: N2 (l) ==> N2 (g)? Or would the element begin in its most stable form and then stay in its most stable form, ex: O2 (g) ==> O2 (g)?
I think it implies that the element would begin in its most stable form
When it is 0 it means the element began in its most stable from like the O2 example you described, the N2 example would not be 0
The standard enthalpy of formation for an element in its most stable form is zero because, being already in its most stable form, it requires zero enthalpy of formation to get to its most stable form. There is no need for a change in heat to reach a form that it already exists in. Usually, we use this principle in our work because the elements are almost always found/used in their most stable form. When they are explicitly not in their most stable form, we would need to consider their enthalpies of formation which would then be non-zero values.
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