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?