enthalpy of formation

[inaccensa]

why is the enthalpy of formation of elements in its standard state equal to zero. i do realize that the enthalpy of a substance in the standard state is zero. O2 is in its standard state and the enthalpy is zero, but why?

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[RogueUnicorn]

why is the enthalpy of formation of elements in its standard state equal to zero. i do realize that the enthalpy of a substance in the standard state is zero. O2 is in its standard state and the enthalpy is zero, but why?

it's because enthalpy is not an intriniscally measurable quantity. we can't just take any random molecule and "measure" in any way how much enthalpy is there. all we can do is measure the CHANGE of enthalpy caused by a reaction. therefore, we need an arbitrary starting point for a "base" value of enthalpy, so we assign that to elemental forms of the various elements.

 

[inaccensa]

it's because enthalpy is not an intriniscally measurable quantity. we can't just take any random molecule and "measure" in any way how much enthalpy is there. all we can do is measure the CHANGE of enthalpy caused by a reaction. therefore, we need an arbitrary starting point for a "base" value of enthalpy, so we assign that to elemental forms of the various elements.

i actually have another question

The change in enthalpy for a reaction lets say

H2+O2 =H202, now if the reaction is exothermic, it simply means that that the products are more stable than the reactants. Although the Enthalpy of both H2 and O2 is zero, U can still end up with an enthalpy change based on the stability of the molecules.