What is DeltaH_f^@ for NO(g)?

Nov 20, 2016

For $\text{NO}$, DeltaH_f""^@=+91.3*kJ*mol^-1; and thus here $\Delta {H}_{f} {\text{^@-=DeltaH_"rxn}}^{\circ}$

Explanation:

To account for the positive value, consider what DeltaH_f""^@ represents:

" the enthalpy associated with the formation of 1 mole of substance from its constituent elements in their standard states under given conditions....... "

So here, DeltaH_f""^@ is the enthalpy associated with the following reaction:

$\frac{1}{2} {N}_{2} \left(g\right) + \frac{1}{2} {O}_{2} \left(g\right) \rightarrow N O \left(g\right)$ ;DeltaH""^@""_"rxn"=91.3*kJ*mol^-1

In order to make the $N = O$ bond, we have to break exceptionally strong $N \equiv N$ and $O = O$ bonds. The energy shortfall is anticipated.