How do I know if compound is in "most stable state"??

[userman]

In the Berkley Review book examples for Hess's Law

The problems are calculating the change in standard enthalpy.
But the reactions don't include values for O2 and H2
apparently they are not applicable because they are in their most stable state...



The book doesn't say anything about why though...
So I ask, how do you know if something is in its most stable state?

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[UCB05]

It's not that they aren't applicable because they are in their most stable state. The standard enthalpy of formation is defined as the enthalpy change of 1 mol of substance to be formed from its elements. Therefore, the standard enthalpy of basic elements in their normal phase is 0 by this convention. As for how you know they are actually in their standard state, it has to be a pure elemental substance, and it has to be in the right phase at standard pressure/temp.

e.g. Br2 is a liquid at 25degreesC and 1atm
Br2(l) has a standard enthalpy of 0kJ/mol
Br2(g) has a standard enthalpy of 31kJ/mol

As for the standard phase of an element, you should either know it (maybe from gchem lab) or hope the passage tells you what it is. I suppose BR expected you to know the O2(g) and H2(g) were in their standard states.