Enthalpy (H = U + PV) is a thermodynamic state variable constructed by adding pressure-volume energy to internal energy, so that its value at any equilibrium state depends only on that state, not on the path taken to reach it. Under the constraint of constant pressure, the first law of thermodynamics reduces so that the change in enthalpy equals the heat added to the system, making enthalpy a valid, path-independent stand-in for "heat content" specifically in constant-pressure processes. This concept belongs to classical thermodynamics and is foundational to chemistry, since most chemical reactions occur at constant (atmospheric) pressure.
Enthalpy (H = U + PV) is a thermodynamic state variable constructed by adding pressure-volume energy to internal energy, so that its value at any equilibrium state depends only on that state, not on …