If given a reaction, which products do we use to solve for enthalpy?

[AndresElizarraras 2A]

Textbook problem 4D.17 asks us to

"Use the data in Appendix 2A to calculate the standard reaction enthalpy for the reaction of pure nitric acid with hydrazine: 4 HNO3(l) 1 5 N2H4(l) ¡ 7 N2(g) 1 12 H2O(l)"

In their calculations for the standard reaction enthalpy, they included the enthalpy bonds of everything except the N2. Can anyone explain why this was excluded in their calculation?

Thank you!

[Chem_Mod]

Elements in their most stable form have zero enthalpy of formation by definition. This applies to N2, as well as other diatomic gas molecules.

[Layla Shapouri 2L]

Hey, just to add on to this answer, the standard enthalpy of formation of N2 would look like,

N2 (g) -> N2 (g). (because you want to have N2 g as your product, and the most stable form of it is also N2 (g). I think seeing it written out like this makes it easier to see how it would be zero.