Energy is required to break a covalent bond between two atoms to overcome the attractive force. Bond breaking is an endothermic process. The opposite is true if we want to make new bonds.

Energy in = 436 + 243 = 679 kJ/mol (this is the energy absorbed when the bonds of the reactants break). Energy out = 2 × 432 = 864 kJ/mol (this is the energy released when the bonds of the ...