Many chemical reactions release energy in the form of heat, light, or sound. These areexothermic reactions. Exothermic reactions may occur spontaneously and result in higher randomness orentropy(ΔS > 0) of the system. They are denoted by a negative heat flow (heat is lost to the surroundings) and decrease in enthalpy (ΔH < 0).

eg:

An example of an exothermic reaction is the mixture of sodium and chlorine to yield table salt. This reaction produces 411 kJ of energy for each mole of salt that is produced:

Na(s) + 0.5Cl2(s) = NaCl(s)

There are other chemical reactions that must absorb energy in order to proceed. These areendothermic reactions. Endothermic reactions cannot occur spontaneously. Work must be done in order to get these reactions to occur. When endothermic reactions absorb energy, a temperature drop is measured during the reaction.

Endothermic reactions are characterized by positive heat flow (into the reaction) and an increase in enthalpy (+ΔH).

eg: Photosynthesis

sunlight + 6CO2(g) + H2O(l) = C6H12O6(aq) + 6O2(g)

exothermic reaction means- heat is evolved in that particular reaction where as endothermic reaction means - heat is absorbed in that particular reaction...