bonds broken minus bonds formed

[Bumbl3b33]

this is kind of a rudimentary question, but

how do you know when to do (products - reactants) versus (bonds broken) - (bonds formed) for reactions?

Is it just that if you're given Delta-H-formation then it's products-reactants?

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[Majik]

this is kind of a rudimentary question, but

how do you know when to do (products - reactants) versus (bonds broken) - (bonds formed) for reactions?

Is it just that if you're given Delta-H-formation then it's products-reactants?

If they give you heats of formation, you use the equation:
deltaH = (EProducts - EReactants) using formation values.

deltaH = (Bonds Broken - Bonds Formed) is used when Bond Enthalpies are given.

 

[Bumbl3b33]

If they give you heats of formation, you use the equation:
deltaH = (EProducts - EReactants) using formation values.

deltaH = (Bonds Broken - Bonds Formed) is used when Bond Enthalpies are given.

That's what I thought, thank you kind sir!

 

[MD Odyssey]

If they give you heats of formation, you use the equation:
deltaH = (EProducts - EReactants) using formation values.

deltaH = (Bonds Broken - Bonds Formed) is used when Bond Enthalpies are given.

Also know that these two approaches are not equal, in general.

The values for the enthalpies of the products and reactants are usually based on chemical processes. The bond enthalpies given are based on average bond energies. For example, a C-H bond in an alkane is not the same as a C-H bond in an aromatic ring. One sees this quite explicitly in IR spectroscopy.