Enthalpy (H)

Enthalpy, symbol $H$, is a thermodynamic property of a system defined as the sum of its internal energy and the product of its pressure and volume:

Physically, enthalpy reflects both a system's capacity to do mechanical (P-V) work AND its capacity to release heat. Its real usefulness shows up under one specific, very common experimental condition: when a process occurs at CONSTANT pressure, the heat involved -- whether released or absorbed -- is exactly equal to the change in enthalpy of the system.

Since enthalpy is built entirely from the state functions $U$, $P$ and $V$, it is itself a state function, and is normally expressed as a CHANGE, $\Delta H$, for a process running between an initial and a final state at constant pressure:

Provided the system does no work beyond ordinary P-V expansion work, this change in enthalpy equals the heat supplied at constant pressure:

$\Delta H = q_p$ …