[confusedstud]

When we calculate enthalpy change, we use ΔH=-(total bond energies of reactants)+(total bond energy of products)

Is the total bond energy of the reactants the same as the total energy the reactant has? There is another formula ΔH=energy of product-energy of reactant? I know they give the same magnitude eventually. But I'm not sure if they mean the same thing because my textbook says that the first one is about breaking the bonds and forming the bonds. So I'm not too sure about it.

Thanks!

[ramboacid]

When you are given enthalpies of formation (ΔH_f) you can use the formula ΔH_rxn = Σ(ΔH_{f products}) - Σ(ΔH_{f reactants}).

When you are given bond energies the formula is ΔH_rxn = Σ(ΔH_{bonds in reactants}) - Σ(ΔH_{bonds in products}).

The reactant can also have kinetic energy among other things, but thermodynamically I think that the only energies you need to consider are the bond energies.

[confusedstud]

Oh so are they the same thing? Besides numerically? Thanks

[curiouscat]

I'll just note: Pay careful attention to your reference states if using something like heats of formation.

Absolute energies are rarely useful; differences matter.

[Zeppos10]

Oh so are they the same thing? Besides numerically? Thanks

No they are not the same thing: if the volume changes due to the reaction some work is done by the surrounding atmosphere. However the contribution of this work (+ or -) is a few kilojoule per mole reactant, while the change in bond energies is much higher in most cases.