In a reaction, bonds in the reactants are broken and new bonds form in the products. Breaking a bond takes in energy from the surroundings, so bond breaking is endothermic. Making a bond releases energy to the surroundings, so bond making is exothermic. Energy is not released when bonds break.
Each type of bond has a bond energy, in kilojoules per mole (kJ/mol). It is the energy needed to break one mole of that bond, and the same amount is released when it forms. To find the overall energy change, add up the energy needed to break all the bonds in the reactants, then add up the energy released making all the bonds in the products.
Energy change = energy in to break bonds minus energy out when bonds form, so subtract the total for bonds made from the total for bonds broken.
Example: H2 + Cl2 → 2HCl. The bond energies are H-H 436, Cl-Cl 243 and H-Cl 432 kJ/mol. Bonds broken: 436 + 243 = 679 kJ/mol. Bonds made: 2 × 432 = 864 kJ/mol. Energy change = 679 - 864 = -185 kJ/mol.
A negative answer means more energy is released making bonds than is needed to break them, so the reaction is exothermic. A positive answer means the reaction is endothermic.