Chemical reaction is nothing but change in chemicals. During chemical reaction, energy is either absorbed or released and according to that chemical reactions are described as endothermic or exothermic reactions.
Endothermic Reactions
When energy is absorbed during chemical reaction, it is called as endothermic reaction. Any reaction to occur, energy is required. In endothermic reaction, energy is either heat energy or electrical energy.
Example: Breaking of sodium chloride into sodium and chloride ions involves the absorption of energy.
NaCl → Na+ + Cl-
Exothermic Reactions
When energy is released during chemical reaction, it is called as exothermic reaction. Energy is released in form of heat and stored in the chemical bonds of the reactants.
Example: When nitrogen reacts with hydrogen in a close vessel, ammonia is formed. This reaction releases heat and so called as exothermic reaction.
N2 + 3H2 → 2NH3