An exothermic reaction is a chemical reaction wherein energy is released in the form of light, heat, or sound. Reactions such as this can occur spontaneously.
Exothermic reactions result in an increase in entropy (?S > 0) and a decrease in enthalpy (?H < 0). These reactions are characterized by negative heat flow. This means the system gives heat off to the surroundings. Examples of exothermic reactions include combustion reactions involving fuels.
Opposite exothermic reactions are endothermic reactions. The reactions require the involvement of energy absorption in order to proceed. A temperature drop characterizes these reactions, with positive heat flow and an increase in enthalpy (+?H).