[hwgw]

Calculate the bond energy (O-H) for H2O(g) given the bond energies (O=O), 495 kJ/mol, and (H-H), 435 kJ/mol. The deltaHf for H2O(g) is -242 kJ/mol.

H2(g) + 0.5 O2(g) -> H2O(g)

The calculations turned to be

-242 kJ = (H-H) - 2(O-H)

and solve for (O-H). But I don't understand why only the (O=O) bond energy was neglected from the equation, but also not (H-H). Oxygen has its standard state as O2(g), i understand thats why we neglected it, but hydrogen has its standard state as H2(g), so shouldn't it be neglected too?

Thanks a lot in advance!

[FeLiXe]

in my eyes you have to consider both bond energies
you don't have to look at vaporisation energies since everything is already in the gas phase