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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.
Because the reaction is run at constant pressure, the change in the enthalpy that occurs during the reaction is equal to the change in the internal energy of the system plus the product of the constant pressure times the change in the volume of the system. = E + P V (at constant pressure). Substituting the first law of thermodynamics into this equation gives the following result. H = (qp + w) + P V. Assuming that the only work done by the reaction is work of expansion gives an equation in which the P V terms cancel.