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How do we know the most stable form of an element, such that standard enthalpy of formation would be zero for that element? For instance, I would assume the most stable form of water would be H2O(l) because it is found in most commonly in nature, but its enthalpy of formation is -285.83. Is the most stable form ice because of a structural reason?
On a test, we would probably be given the standard enthalpies of formations we would need to solve the problem so with that information, we could probably deduce which ones are zero. In terms of understanding though, you bring up a good point with your water example. My guess is that it is due to structural reasons, because ice has more stable hydrogen bonds between water molecules than liquid water does.
For it to be the most stable form, it must be the one that is most pure. Some of them you just have to memorize. But there are some that you can figure out. All noble gases are most stable in the element phase as a gas. All diatomic gases (such as N2, O2, Cl2, F2, H2, etc.) are the most stable in this gaseous form. Carbon is most pure as graphite as a solid. The only two metals that are liquid in their standard state are mercury and bromine. The rest of the metals are solids. Based on everything I have given, water is not part of this list as it is not the most pure state. Consequently, it will not be 0. Just because water and ice are common, doesn't mean either is the most pure state. Therefore, neither will have a bond enthalpy of 0.
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