Steve Marsden’s

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
(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.