[lala56]

hi? why is it that the standard heat of formation of an element is always 0? for instance, O2 must take some energy to form . right? wrong?

dang, this is confusing stuff . if H2O has a heat of formation, then there darn well better be one for O2 as well .

[Borek]

hi? why is it that the standard heat of formation of an element is always 0?

By convention.

[lala56]

ah ... but there must be some reason?

[Borek]

It is a good selection as it makes calculations easier. Note that there is no simpler form of the element at STP, so it is a good starting point (although sometimes you must account for allotropic forms) for calculations.

Whenever you need some point of reference, you should select it in such a way that your calculations are as easy as possible (but not easier ).