What does the relationship ∆U = ∆H - p∆V indicate?

The relationship ∆U = ∆H - p∆V indicates that the change in internal energy (∆U) of a system is related to the change in enthalpy (∆H) and the work done by the system due to volume change against external pressure (p∆V). This equation shows that enthalpy change encompasses both the change in internal energy and the work associated with volume expansion or compression.

In thermodynamics, enthalpy is defined as a state function that combines internal energy with the product of pressure and volume (H = U + pV). When a process involves expansion work (such as a gas expanding against a constant external pressure), some energy goes into doing that work, which needs to be accounted for when relating changes in energy.

Thus, if a system undergoes a process where it exchanges heat with the surroundings at constant pressure, the change in enthalpy reflects the heat exchanged while considering the work done due to volume change. Therefore, the statement accurately captures how enthalpy accounts for this volume work effect, which is essential in understanding energy changes in chemical processes under constant pressure.