What are the three ways to find Delta H?

[Makayla Morton 1G]

I'm a little confused by this. Thanks!

[Niharika Reddy 1D]

The 3 ways are:

1) Using Hess's Law: add the enthalpy changes at each step of a multi-step reaction.

2) Bond Enthalpies: find the sum of enthalpies of bonds broken (reactants, endothermic, energy needed, "positive" bond enthalpies) and bonds formed (products, exothermic, energy released, "negative" bond enthalpies).

3) Standard Enthalpies of Formation: (sum of standard enthalpy of formation of all products)-(sum of standard enthalpy of formation of all reactants), where the stoichiometric coefficients of the balanced reaction need to be taken into account. Also, the standard enthalpy of formation of elements in their most stable form is zero.

[Lily Kiamanesh 2G]

Hi! I remember Dr. Lavelle stating that some of these are more reliable than the others, and I was wondering which ones are more accurate and why. Thank you!

[reva_bajjuri]

Hi! I remember Dr. Lavelle stating that some of these are more reliable than the others, and I was wondering which ones are more accurate and why. Thank you!

I think the bond enthalpies method is the least stable because the bond enthalpy values for a lot of different bonds is from the averages of the bond strengths in different compounds.

[Mackenzie Van Val 3E]

Hi! I remember Dr. Lavelle stating that some of these are more reliable than the others, and I was wondering which ones are more accurate and why. Thank you!

Hi! Out of the three methods (Hess's Law, Bond Enthalpies, and Standard Enthalpies of Formation), using bond enthalpies is the least accurate way to find delta H because the bond enthalpies given in the table are averages of the same bond in several different molecules, and thus are not 100% accurate to the bond enthalpy in the molecule you are working with. However, bond enthalpies are accurate for diatomic molecules like O2 and F2 because they can be measured specifically for those molecules. Hope this helps!

[Earl Garrovillo 2L]

Hi! I remember Dr. Lavelle stating that some of these are more reliable than the others, and I was wondering which ones are more accurate and why. Thank you!

Hess' Law and Standard Enthalpies of Formation are about the same level of reliability and are the more reliable ones. These are specific to the reaction (delta Hrxn) or are specific to the reagents (standard enthalpies).
Bond enthalpies are less reliable because the bond enthalpy can actually change depending on the molecule (i.e. a C-O bond on one molecule may be weaker than one on another molecule because of the surrounding atoms affecting the bond strength)

[Gicelle Rubin 1E]

Hi! I remember Dr. Lavelle stating that some of these are more reliable than the others, and I was wondering which ones are more accurate and why. Thank you!

Hi! Out of the three methods (Hess's Law, Bond Enthalpies, and Standard Enthalpies of Formation), using bond enthalpies is the least accurate way to find delta H because the bond enthalpies given in the table are averages of the same bond in several different molecules, and thus are not 100% accurate to the bond enthalpy in the molecule you are working with. However, bond enthalpies are accurate for diatomic molecules like O2 and F2 because they can be measured specifically for those molecules. Hope this helps!

Thank you for this clarification!

[Ethan Laureano 3H]

Hess's law, bond enthalpies (least accurate), and standard enthalpies of formation.