Before discussing thermodynamic and chemical stability it is important to recognize how energy plays a role. Heat is the most common form of energy introduced into chemical systems, but light, electrical current, microwave radiation and ultrasound are also used. The effect of heat is readily apparent when we consider that most organic compounds decompose at temperatures greater than 1000 ºC. On the other hand, at temperatures below -100 ºC, molecular species that cannot be isolated or examined under normal conditions (20 to 25 ºC) may be observed and characterized. Heat energy is the total kinetic energy of the molecules in a given sample. At absolute zero (-273.15 ºC) this energy is essentially zero, but at higher temperatures this energy increases proportionally. Although we think of ice (0 ºC) as very cold, it actually has considerable heat energy.
When a chemical reaction takes place, energy in the form of heat or light is usually released or absorbed. Such energy changes are defined by the terms exothermic and endothermic.

• If energy is given off during a chemical reaction, the reaction is called exothermic. Such reactions may be spontaneous and very fast, but many are not.
• If energy must be supplied for a chemical reaction to occur, the reaction is called endothermic. Such reactions are seldom spontaneous.

Any reaction in which bonds between atoms are broken, even though other bonds are formed, will require some energy input to proceed. This is true for both exothermic and endothermic reactions. This initiating energy component is called an activation energy. Strongly exothermic reactions may supply their own activation energy once they start, but endothermic reactions require a continuous energy source.