Standard Enthalpies of Formation of Diatomic Molecules

[Chris Lamb 1G]

Why is the standard enthalpy of formation of O2 gas 0kJ/mol?
(This is from example 8.11 on page 296).

[Julie Steklof 1A]

All elements in their natural states have an enthalpy equal to zero because there is no change involved in their formation.

[Meredith Steinberg 2E]

O2 is the most stable form of oxygen and therefore has an enthalpy equal to zero. This is the same for other diatomic elements, such as nitrogen (N2), chlorine (Cl2), etc.

[Chris Lamb 1G]

So, this rule would only count for the naturally diatomic elements: H, O, F, Cl, Br, I, N?

[miznaakbar]

No this rule applies to all elements, because in their standard state they are in their most stable state, so enthalpy is zero. For instance, Carbon's standard state is a solid as graphite, so it's enthalpy would be zero. However, Carbon in its solid state as a diamond would have a nonzero enthalpy because it is not the standard state.

[Ashley Garcia 2L]

The standard enthalpy of formation for diatomic molecules such as O2 (g) equals 0 because no change occurs in the reaction O2 (g) --> O2 (g). The definition of the standard enthalpy of formation is that the standard enthalpy of formation of an element in its most stable form equals 0. O2 (g) is the most stable form of oxygen, and no change occurs in the reaction, therefore the standard enthalpy of the reaction equals 0.

[David Zhou 1L]

It's also of note that standard enthalpy refers to when the pressure is at 1 atm and 25 degrees C (298 K).