Standard heats of formation (enthelphy, free energy)

[renob]

Quick question: why are the standard heats of formation 0 for pure elements?

 

[sjb-2812]

Simply by definition.

 

[spectre101]

Ya, it pretty much means there is no change in enthalpy when a pure element (in standard state) is formed. And by definition the standard heat of formation of a substance has to be measured for the reaction where the substance is formed by its constituent elements in their standard states. Obviously a pure element is only made of itself so the change in enthalpy is non-existent.

 

[alxm]

Ya, it pretty much means there is no change in enthalpy when a pure element (in standard state) is formed.

Sorry but that's gibberish. Formed from what?

Which is the point. There's no such thing as an absolute enthalpy (or any other energy). It's a relative quantity. If you want to talk about the energy required to form a certain compound, then it must always be relative whatever compound you're forming it from. What particular compounds are chosen to be zero isn't important and doesn't imply anything at all about those compounds.

Obviously a pure element is only made of itself so the change in enthalpy is non-existent.

That's totally misleading. If two atoms combine to form a molecule, then that certainly involves a change in enthalpy!