Because spontaneous reactions are so important, it is necessary for you to understand the factors that favour spontaneity. Think about a skier going downhill, a waterfall and a fuel fire at a gasoline refinery. If you study these events all of them involve a lowering, or decrease, in the energy of a system. Because these events are spontaneous, we can conclude that when a change lowers the energy of a system, it tends to occur spontaneously . Since a change that lowers the energy of a system is exothermic then we can come up with the opening sentence generalization.
Exothermic changes have a tendency to proceed spontaneously .
The word tendency should be emphasized. There are some exothermic reactions that are not spontaneous, nor is every nonspontaneous endothermic. The dissolving of salts such as NaI in water is just one example of a change that occurs spontaneously even though it is endothermic. It is a universal phenomenon that something that brings about randomness is more likely to occur than something that brings about order. Think about your room at home.