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Enthalpy is defined as the amount of heat released or absorbed at a constant pressure. You are correct in speculating that it is related to exothermic (releasing energy) and endothermic (requires heat) reactions, and that relates to phase changes. In a phase change, energy is required to break the chemical bonds so that a new phase may be attained. In the phase transitions we have discussed in class (vaporization, melting/fusion, sublimation), the reactions are endothermic and require heat.
Remember that Lavelle said that enthalpy is a state function, meaning its value depends on the phase of the substance which you are referring to. However, enthalpy change is a two-state function so wouldn't even be considered a state function.
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