In the previous section, the second of the four major questions around which the course is structured was answered. We have a method for predicting whether a reaction will be exothermic or endothermic. This and the sections remaining concern the first and more fundamental question: how to determine if a reaction will in fact occur.
Observations of natural processes led a surprising number of chemists of the late 19th century (including Berthelot and Thomsen) to conclude that all spontaneous reactions must be exothermic since:Objects roll downhill spontaneously
If this were true all we would need to predict whether a reaction is spontaneous is the enthalpy value.