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?

1 Answer
anor277
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 #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.

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