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My chemistry textbook states the following "general enthalpy" formula for the change in enthalpy when the internal pressure is not constant:

$∆H= ∆U + ∆(PV)$

How is the internal pressure involved in this equation? The equation for enthalpy change under constant internal pressure accounts for the heat energy that was used to increase the volume, and in this case the work done by the system only depends on the external force and the difference in height. So what does the ∆P term mean here?

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  • $\begingroup$ Enthalpy at variable pressure keeps being the state function, just its change is not equal to exchange heat at the same temperature. State functions need not to have particular physical meaning. You can invent one, that is totally useless. $\endgroup$
    – Poutnik
    Oct 14 '21 at 23:24
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This has nothing to do with any process or process path. Enthalpy is a function of state (a property of the material comprising the system), and depends only on the internal energy, pressure, and volume in the initial and final states. For certain kinds of processes(such as constant pressure), the change in enthalpy between the initial and final states can be related to the heat involved. For most other processes, no.

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