Question

Why is the enthalpy of formation of oxygen zero?

Answer 1

Hint: The standard enthalpy of formation of a compound refers to the change in enthalpy which is calculated when the formation of one mole of the compound from its pure elements in their standard states takes place. The standard enthalpies of formation are always given in units of $kJmo{l}^{-1}$ of the compound formed.

Complete answer:
According to IUPAC, the enthalpy of formation for an element in its elemental state or naturally occurring state will always be zero because it takes no energy to form a naturally occurring compound. In simple words, the standard enthalpy of formation is zero for each of the elements in its stable or designated form.
Now, for the case of oxygen, it consists of its elements which are already in the standard state that means oxygen gas naturally exists as $O_2$ gas in atmosphere and thus, the standard enthalpy of formation of oxygen in its gaseous phase is zero. Alternatively, we can also say that the standard enthalpy of formation of oxygen is zero because diatomic oxygen in its gaseous phase is its most stable form as compared to mono-atomic form $(O)$ and triatomic form $\left(O_3\right)$ .

Note:
It is important to note that the standard state should not be confused with standard temperature and pressure for gases (STP) or with the standard solutions which are used in analytical chemistry. A standard state is a reference point which is used to calculate a material’s properties under different conditions whereas STP is used while performing calculations on gases like gas density.