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BioPhysicist
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Those are the definitions of each. They don't assume anything about the system and can always be applied. You are getting mixed up with the scenarios in which they are usually applied since nice things happen. For example, for a system at constant pressure $\Delta H=Q$, where $Q$ is the heat that enters or leaves the system.

BioPhysicist
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  • 19
  • 116
  • 193