# Physical meaning of enthalpy change when internal pressure Is not constant [closed]

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?

• 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. Oct 14 '21 at 23:24