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I am a little bit confused as to when I should include phase change enthalpies in my delta H calculations. From the lecture, I would think that it is only included when the state change involves a gas, and not when it is between a liquid and a solid. This doesn't make sense to me, so I'm wondering if anyone can either 1) clarify for me what was actually meant or 2) explain why only vaporization and sublimation enthalpies should be included, but not melting.
I think Professor Lavelle mentioned phase changes in delta H calculations because the table of bond enthalpies refers to breaking bonds for gases, so you would need to calculate phase change in order to find delta H via bond enthalpies.
All the state change enthalpies must be included. I believe Professor Lavelle was simply mentioning that the table given in the textbook gives enthalpies for gases, however if a reaction has another type of state change we must take that also into account.
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