Why the Standard Enthalpy of Formation of O2 Equals Zero

To understand standard enthalpy of formation of  O2  Equal to Zero, you need to understand the definition of standard enthalpy of formation. This is the change of enthalpy when one mole of a substance in its standard state is formed from its elements under standard state conditions of 1 atmosphere pressure and  298K temperature. Oxygen gas consists of its elements already  in the standard state, so there isnt any change here. Oxygen (the element) at standard state is O2. The same is true other other gaseous elements, such as hydrogen and nitrogen, and solid elements, such as carbon in its graphite form. The standard enthalpy of formation is zero for  elements in their standard states.