Use bond energies to determine which halogen produces the most exothermic halogenation reaction with ethene. the c−f, c−br, and c−i bond energies are 552 kj/mol, 280 kj/mol, and 209 kj/mol, respectively.

To know the total energy you must know that bond-breaking is endothermic (+) while bondbond-forming is exothermic (-). The bond energy of ethene is 602 kJ/mol. Since we break the ethene, that would be -602 kJ/mol.

For C-F: -602 + 552 = -50 kJ/mol
For C-Br: -602 + 280 = -322 kJ/mol
For C-I: -602 + 209 = -393 kJ/mol

Since the most negative, hence exothermic, answer is C-I, the answer is C-I.