All chemical reactions are normally accompanied by energy changes. These energy changes take place because energy is needed to break chemical bonds, and energy is released when new bonds are formed. In the process of chemical reaction, rearrangement of atoms in reactants takes place, to give products.Sometimes, the energy changes associated with the chemical reactions are more significant than the products of the reaction. Most common examples in which energy is given out is combustion of fuels petrol, coal, wood etc. Energy changes accompanying chemical reactions can be in the form of heat, light, mechanical energy, electricity, etc. Therefore, the study of energy changes during chemical reactions is of great importance in our everyday life. It finds a large number of applications not only in chemistry but also in other branches of science, engineering and technology.