Calculating enthalpy using bond energies [ENDORSED]

[Natalie LeRaybaud 1G]

When calculating the enthalpy using bond energies do we multiply the bond energy by the amount of moles the molecule has within the reaction or by the amount of bonds in the molecule? For example in number 75 the solution manual did not multiply the C-H bond energy by 4 even though the initial equation has 1 mole if CH4 as a reactant. Wouldn't that be 4 C-H bonds?

[Daniisaacson2F]

In the textbook, they are only taking into account which specific bonds were broken and produced from the reactants to products. When I do these problems, I tend to write out all of the bonds and multiply by the number of them (it's easier for me), but wither way is fine.

[Chem_Mod]

1. We use bond enthalpies (not bond energies) to calculate reaction enthalpy.

2. Multiply the bond enthalpy by the number of moles of the molecule in the balanced reaction AND by the number of bonds broken (or formed) in the molecule.

3. Only one C-H bond was broken in the reactant. Therefore the answer is correct.

I will discuss this Q&A in class.