Enthalpy is a measure of the total energy of a chemical system. The enthalpy change of a process is the heat energy that is transferred between the system and its surroundings at constant pressure.The difference in the enthalpy of the reactants and products is called the enthalpy change ΔH.ΔH = Hproducts - HreactantsExothermic processes transfer heat energy from the system to the surroundings. The temperature increases. ΔH is negative.Endothermic processes transfer heat energy from the surroundings to the system. The temperature decreases. ΔH is positive.