Question

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?

Answer 1

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 `Cl_2(g)`, 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.