enthalpy enthalpy change Change in enthalpy observed in the constituents of a thermodynamic system when undergoing a transformation or chemical reaction. This is the preferred way of expressing the energy changes to a system at constant pressure, since enthalpy itself cannot be directly measured. The enthalpy change is positive in endothermic reactions, negative in exothermic reactions, and is defined as the difference between the final and initial enthalpy of the system under study: ΔH = Hf - Hi. The standard unit of measure is J. Symbol: ΔH