Use bond enthalpies (Tables 4E.2 and 4E.3) to estimate the reaction enthalpies for the halogenation of ethene

Question:

Use bond enthalpies (Tables 4E.2 and 4E.3) to estimate the reaction enthalpies for the halogenation of ethene by chlorine, bromine, and iodine. What trend, if any, exists in these values? TABLE 4E.2 Bond Enthalpies of Diatomic Molecules Molecule H N 0 CO F Cl Br 1 HF HCI HBr HI AHB/(kJ.mol-) 436 TABLE 4E.3 Mean Bond Enthalpies Bond C-H C-C C=C CC* C=C C-N C-F C-Cl C-Br AHB/(kJ.mol-) 412 348 612 518 837