The quantity was calculated in part 1. The change in energy is given by the heat released under constant volume conditions. This value is given in the question as 393.5 kJ/mol. These two quantities can then be substituted into equation (10) with the appropriate signs. The change in energy is negative. When heat is released in a reaction, the final energy of the system is lower than the initial energy of the system, and DE is negative. The enthalpy change is therefore
Since the enthalpy change is negative, the reaction is exothermic.
