Science, Tech, Math › Science Why Is the Standard Enthalpy of Formation of O2 Equal to Zero? Share Flipboard Email Print Enthalpy change does not occur for gases in standard state. PM Images, Getty Images Science Chemistry Physical Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated August 02, 2018 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 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.