Why Is the Standard Enthalpy of Formation of O2 Equal to Zero?

Understanding Standard State and Enthalpy

Enthalpy change does not occur for gases in standard state.
Enthalpy change does not occur for gases in standard state. PM Images, Getty Images

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 standard state, so there isn't 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.

Table of Heats of Formation of Common Compounds