How can a substance have a negative enthalpy of formation? Wouldn't that mean it has a negative amount of stored energy?


The confusion here is due to sloppy semantics. The sloppy semantics are the fault of chemists, however, not students. Chemists often use the term "enthalpy" when they really mean "change in enthalpy." A substance certainly can never have a negative amount of stored energy; thus, a substance's absolute enthalpy can never be negative. However, as I mentioned in that long, difficult discussion, we cannot measure absolute enthalpies. There is just no way we know of to determine the amount of potential energy in a substance.

We can, however, measure the enthalpy of one substance RELATIVE to another. For example, when I measure the heat released or absorbed in the following reaction:

Ca (s) + S (s) -> CaS (s)

it is called the "enthalpy of formation" for CaS (s). This isn't the total amount of stored energy in CaS (s). Instead, it is the DIFFERENCE in stored energy between CaS (s) and the elements that form it. Even though we call it "enthalpy," we really should say it is the "change in enthalpy of the formation reaction." Chemists, however, are just too lazy to write all of that out.

So, since "enthalpy" really means "change in enthalpy," then it is easy to understand why some enthalpies are negative. For example, if the enthalpy of formation of a substance is negative, that just means the elements which form the substance have more potential energy than the substance does. If the enthalpy is positive, however, we can conclude that the substance has more stored energy than the elements which make it up

Tags: Chemistry
Last update:
2017-11-17 21:23
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Sue
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