The given information seems to be contradictory, as it states that the bond dissociation energy for the Cl-Cl bond in Cl2 g is both unknown and known to be 242 kJ/mol. However, I will assume that the correct information is that the standard enthalpy change for the reaction Cl2 g 2 Cl g is +121 kJ/mol, and we need to calculate the bond dissociation energy for the Cl-Cl bond in Cl2 g .The bond dissociation energy is the energy required to break a bond in a molecule. In this case, we are looking at the energy required to break the Cl-Cl bond in Cl2 g to form 2 Cl g atoms. The standard enthalpy change for this reaction is given as +121 kJ/mol. Since the reaction is endothermic positive enthalpy change , this means that energy is required to break the bond.Therefore, the bond dissociation energy for the Cl-Cl bond in Cl2 g is equal to the standard enthalpy change for the reaction, which is +121 kJ/mol.