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Since this measurement is at constant pressure (isobaric), the heat is equal to the change in enthalpy or \(\Delta H\). Note: be careful with the sign on \(\Delta H\). Heat out of the reaction is heat in to the calorimeter and vice versa.ince this measurement is at constant pressure (isobaric), the heat is equal to the change in enthalpy or \(\Delta H\). Note: be careful with the sign on \(\Delta H\). Heat out of the reaction is heat in to the calorimeter and vice versa.
The Î´ H is a positive change in endothermic reactions, and negative in heat-releasing exothermic processes. For processes under constant pressure, Î´ H is equal to the change in the internal energy of the system, plus the pressure-volume work that the system has done on its surroundings.f Î´ H is positive, the reaction is endothermic, that is heat is absorbed by the system due to the products of the reaction having a greater enthalpy than the reactants. On the other hand if Î´ H is negative, the reaction is exothermic, that is the overall decrease in enthalpy is achieved by the generation of heat.