During a chemical reaction energy changes from one type to another. The chemical bonds break and re-form. Balance amount of heat is either given out or absorbed in from the surroundings, resulting in a change in temperature of the reaction vessel, most of the cases. On this basis, chemical reactions are classified as exothermic or endothermic.
Reactions that give out heat energy are called exothermic reactions. In an exothermic reaction, the total energy released in the formation of new bonds is greater than the total energy used to break the bonds. Therefore, the reaction (and its vessel) will heat up, overall. Exothermic reactions are very common. Here the product molecules have less energy stored in their chemical bonds than the reactants. Conversion of reactants to products is thus a stabilising process, hence intreinsically spontaneous because one f the laws of thermodynamics predicts that all reactants and products will try to contain as little energy as possible.
Combustion reactions are all exothermic. Explosions are highly exothermic reactions occurring in very rapidly. Combustion of methane is an example.
CH4(g) + 2O2(g) → CO2(g) + 2H2O (l) + (–890.3) kJ/mol
Methane and oxygen contain more energy than carbon dioxide and water. This extra energy is released in the form of heat energy when methane is burnt.
Chemical reactions that require energy are called endothermic reactions. They become cooler as the reaction proceed. Energy input to break the bonds outweighs the energy released due to new bond formation. The products of endothermic reactions contain more energy than the reactants. Reaction between barium hydroxide and ammonium chloride is so endothermic that it can freeze water.
2NH4Cl + Ba(OH)2 → 2NH3 + BaCl2 + 2H2O + 54.8 kJ/mol