If a process , whether chemical or physical one, involves the absorption or emission (taking in or exchanging) in thermal energy it’s the thermodynamics field that can explain the process. If the process absorbs heat, it’s called endothermic. If it emits heat, it is off heat, it’s called exothermic. The condition of the molecules or atoms that result from the exothermic process is that is less energy since energy was taken out of the system. In an endothermic reaction the state of molecules or atoms is with higher energy because they have added energy into the process. The chemical reaction that takes place within hand warming devices is one of those situations in which the energy gap between reactants and the products is substantial.