Concept of heat of reaction in a isolated system
When chemical bonds are broken potential energy increases.  When they are made potential energy decreases. A loss in PE raises kinetic energy and a gain in PE lowers KE.  Since all reactions involve bond breaking and bond making the reaction will involve an increase or decrease in PE and a decrease or increase in KE.  When KE of the system increases, a system in contact with the surroundings will lose energy as heat (exothermic).  When KE of the system decreases a system in contact with the surroundings will gain energy as heat (endothermic).  In an isolated system "exothermic" reactions just raise temperature and "endothermic" reactions just lower temperature.