Question

how to calculate the energy of a bond

Answer 1

Without a more specific ? I am shooting in the dark here ..But if you have a reaction and they want the bond enthalpy you take the bond energy of bonds broken minus the bond energy of the bonds formed.... Then that is your answer. You will get those numbers from the appendix in your book or some other source they've given you.

Answer 2

ΔH = ∑ ΔH(bonds broken kJ/mol) - ∑ ΔH(bonds formed kJ/mol)= X kJ
so for water 2H2 +O2--> 2H2O
H-H -->436kj/mol
O-O -->499kj/mol
O-H -->463kj/mol

((2(436)+499)) - (4(463))= -481kJ

Good Luck.