HW 7.101 Standard Enthalpies of Formation vs Bond Enthalpies

[jpevec3F]

The question asks if a chemical equation is endothermic or exothermic. I used Hess's law and got the negative of the answer in the solution manual. Why does the solution manual show the total enthalpy as being the enthalpy of CO minus the enthalpy of H2O? I thought Hess's law stated that the total enthalpy is equal to the enthalpy of the bonds broken (reactants) minus the enthalpy of the bonds formed (products) Thanks!

[Kayla Denton 1A]

The solutions manual is not using the equation "bonds breaking - bonds forming", it's using the equation "delta H reaction = delta H formation products - delta H formation reactants"!

Edit: I think the reason they don't use the equation "bonds breaking - bonds forming" is that the question does not give the change in enthalpy to convert C(s) to C(g) (the atomization of carbon). Therefore, there is not enough information for this calculation.

[Neil DSilva 1L]

Remember that there are three ways to solve any enthalpy problem. Depending on the situation, you can use Hess' law, enthalpies of formation, or bond enthalpies. In this case, using the enthalpies of formation makes the most sense (since we aren't given multiple equations and their enthalpies to do Hess' law, and - as Kayla mentioned - we don't have the change in enthalpy of the atomization of carbon to use bond enthalpies since bond enthalpies are calculated for molecules in the gas phase).

[Justin Le 2I]

In general, you don't use the bonds broken - bonds formed method unless the problem specifically says so because the other two methods are much more accurate. The values in the charts that give us bond enthalpies are just averages, which is why there are discrepancies between the bond method and the other two methods.

[Chem_Mod]

The above posts are correct. Using bond enthalpies is actually the least accurate method out of the 3. The energies in bonds are estimated and can vary greatly depending on molecule and other quantum-mechanical / electronic effects such as resonance. On the other hand, Standard Enthalpies of Formation are determined experimentally.