Enthalpy is a thermodynamic property that measures the total energy of a system. It is defined as the sum of the internal energy of the system and the product of its pressure and volume/Thermodynamics/Energies_and_Potentials/Enthalpy). Enthalpy is an extensive property, meaning that it is proportional to the size of the system. For homogeneous systems, the specific enthalpy, h = H/m, is referenced to a unit of mass m of the system, and the molar enthalpy, Hm = H/n, where n is the number of moles. Enthalpy is a state function, which means that it depends only on the initial and final states of the system, and not on the path taken to get there/Thermodynamics/Energies_and_Potentials/Enthalpy).

Enthalpy is often used to describe the heat absorbed or released during a chemical reaction or a physical change. When a process occurs at constant pressure, the heat evolved (either released or absorbed) is equal to the change in enthalpy/Thermodynamics/Energies_and_Potentials/Enthalpy). Enthalpy change is the sum of internal energy denoted by U and the product of volume and pressure, denoted by PV. Enthalpy can also be expressed as a molar enthalpy, ΔHm, by dividing the enthalpy or change in enthalpy by the number of moles/Thermodynamics/Energies_and_Potentials/Enthalpy).

Enthalpy is a useful concept in thermodynamics, particularly in chemical reactions that occur at constant pressure. It is often used to predict the amount of heat that will be released or absorbed during a reaction, which can be important for designing and optimizing chemical processes.