[123456789]

Liquid hydrazine N2H4 is sometimes used as a rocket propellant. There is this equation (in a rocket) how it reacts: N2H4 +2H2O --> N2 +4H2O

Then you are given the enthalpy of formation for the different compounds, to calculate the enthalpy of the reaction. (a) After that you are given the bond energy of the various bonds and asked to calculate the enthalphy again.(b) This calculation part is not the problem.

There is a discrepancy between the result (a) and result (b). Thus the last part is: why is there a difference in the answers? I wrote because the reactions may not be at standard states, as it is compressed fuel, but it is wrong... Anyone has any idea why is there such a difference? Thank you!

[Rutherford]

Your equation is wrong, it has 2 oxygens on the left side and 4 on the right side.

[123456789]

Your equation is wrong, it has 2 oxygens on the left side and 4 on the right side.

Sorry just had a whole week of exams...  There is a typo: It's N₂H₄ with H₂O₂, so  N₂H₄ +2H₂O₂  N₂ +4H₂O

In any case, I would really really want to know why is there a difference between the calculated values... Thank you.

[Rutherford]

Because for bond energies, average values are used , but in reality, same bond energies differ between different molecules. For example, the O-H bond energy in water is different from the O-H bond energy in peroxide.

[curiouscat]

Because for bond energies, average values are used , but in reality, same bond energies differ between different molecules. For example, the O-H bond energy in water is different from the O-H bond energy in peroxide.

This could be it but my hunch was: If you use the bond energy approach you are essentially building up the molecules from N, H and O i.e. atoms

If OTOH you use the heat of formation approach you are building the molecules up from O2 H2 and N2. I suspect the difference in the enthalpy of reaction in the two approaches is the heat of formation of O2, H2 and N2 from constituent atoms.

[Borek]

I suspect the difference in the enthalpy of reaction in the two approaches is the heat of formation of O2, H2 and N2 from constituent atoms.

That would be easy to account for. Unfortunately, it doesn't work. Bonds can't be treated as independent entities, their energies depend on many factors and similar bond can have different energies in different molecules. As Raderford wrote, what is presented in tables is some approximate, average value.