Hess's Law vs Bond Enthalpies vs Standard Enthalpies of Formation

[Jocelyn Thorp 1A]

Hey guys, what should I look for to help me determine which way (Hess's Law, Bond Enthalpies, Standard Enthalpies of Formation) to solve a problem? I know this is old material, but I'm still pretty confused.

[Selena Yu 1H]

You use Hess's Law when you are giving the enthalpy change of the total reaction. You use bond enthalpies when they give you the enthalpy change based on bonds, for example C-H 413KJ/mol. And you use standard enthalpies of formation when they give you the enthalpy of a compound. Hess's Law is for the whole equation, bond Enthalpies is for bonds, and standard enthalpies of formation is for the enthalpy of a compound.

[PranaviKolla2B]

Can someone please define Hess's Law?

[Hui Qiao Wu 1I]

Usually, they will explicitly say to use bond enthalpy or standard enthalpy of formation in the problem. Bond enthalpies usually give you a chart with the different bond and their enthalpy values.

[Jocelyn Thorp 1A]

You use Hess's Law when you are giving the enthalpy change of the total reaction. You use bond enthalpies when they give you the enthalpy change based on bonds, for example C-H 413KJ/mol. And you use standard enthalpies of formation when they give you the enthalpy of a compound. Hess's Law is for the whole equation, bond Enthalpies is for bonds, and standard enthalpies of formation is for the enthalpy of a compound.

Thank you for this explanation!! It makes more sense now.

[Veronica Lu 2H]

Can someone please define Hess's Law?

according to google, Hess' law "states that regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes. This law is a manifestation that enthalpy is a state function."

[John Liang 2I]

You can use each method based upon the information given by the problem. If given bond enthapy values, you can use bond energies, if given standard enthalpies of formation, use that, etc. hope it helps!

[505316964]

I'm a bit confused about standard enthalpy of formation.

So the definition is enthalpy for the formation of one mole of a substance from its elements in their most stable form and the standard enthalpy of formation of an element in its most stable form is zero.

If it is always zero, why are we asked to calculate the enthalpy of rxn? Based on the definition isn't it always zero?