The bond dissociation energy is the energy required to break a bond between two atoms. In this case, we are given the enthalpy change for the reaction HCl g H g + Cl g , which is the same as the bond dissociation energy for the H-Cl bond in the HCl molecule. Therefore, the bond dissociation energy of the H-Cl bond in the HCl molecule is 431 kJ/mol.