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.
A database of 290,664 bond dissociation energies for small molecules (10 or fewer heavy atoms) consisting of C, H, O, or N atoms. Enthalpy calculations were carried out using the M06-2X/def2-TZVP ...