# Consider the reaction: 2ClF_3 (g) + 2HN_3(g) -> N_2(g) + 6HF(g) + Cl_2(g). When calculating DeltaH^(o)rxn, why is the DeltaH_f^o for N_2 not important?

Jul 4, 2016

Because DeltaH_f""^@ for any element in its standard state is defined to be ZERO.

#### Explanation:

Of course, in that reaction we also ignored DeltaH_f""^@ for $C {l}_{2} \left(g\right)$, another element in its standard state. This definition may seem to be arbitrary, but since we measure the change in enthalpies, not absolute enthalpies, the treatment is consistent.