The standard enthalpy of formation (ΔHf°) of oxygen gas (O₂) is defined as zero because it is the element's most stable form at standard conditions (298 K and 1 atm).
Here's a breakdown:
-
Standard Enthalpy of Formation Definition: The standard enthalpy of formation is the change in enthalpy when one mole of a substance is formed from its elements in their standard states.
-
Standard State of Oxygen: The standard state for oxygen is diatomic oxygen gas (O₂). This means O₂ is the naturally occurring, most stable form of oxygen under standard conditions.
-
Formation from Itself: Since O₂ is already in its standard state, forming O₂ from O₂ involves no change in energy. It's already "there." Therefore, the enthalpy change for this process is zero. Think of it like this: O₂ → O₂; no reaction, no enthalpy change.
-
Reference Point: The zero value for elements in their standard states acts as a reference point for calculating the enthalpy changes of reactions involving other compounds. It provides a baseline for comparison.
In Summary:
The standard enthalpy of formation of O₂ is zero because it's defined as the stable, elemental form of oxygen under standard conditions. There's no energy change involved in "forming" it from itself.
