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 19^th century (including Berthelot and Thomsen) to conclude that all spontaneous reactions must be exothermic since:

: Objects roll downhill spontaneously; (i.e., energy is "lost" from the system); Objects do not roll uphill spontaneously; (i.e., energy does not suddenly appear from nowhere)

If this were true all we would need to predict whether a reaction is spontaneous is the enthalpy value.