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Because each bond enthalpy AVERAGES can be found on a table. The bond enthalpy of a specific bond depends on the molecule it is in (electrons may be shared unequally). Therefore, using average bond enthalpies will provide an inaccurate value.
It is also important to take into consideration that some reactions require a change in enthalpy due to a phase change in addition to the substances' bond enthalpies. In this way, Hess's method is more efficient and not as time costly because the enthalpy changes are already given.
Bond enthalpies of diatomic molecules (Br2, I2, Cl2, N2, O2, H2, F2) are accurate because they are measured for those specific molecules; however, all other bond enthalpies are averages from many different molecules, so they are not completely accurate.
Leila_4E wrote:Is the table we find this in the textbook or is there a general one online that is the same?
I would go with the table in the textbook for homework problems because sometimes the bond enthalpies differ slightly online. It should be roughly the same.
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