For any process that occurs at constant pressure the heat absorbed or evolved is equal to the enthalpy change if the only work is pressure/volume work, i.e. ΔH = ΔU + pΔV for a process of constant pressure. In chemical reactions the enthalpy change (ΔH) is related to the changes in Gibbs free energy (ΔG) and entropy (ΔS) by the equation ΔG = ΔH – TΔS.

Enthalpy is usually expressed in units of kilojoules per mole.