Draw resonance formulas of the nitric acid molecule, HNO3. What is the geometry about the N atom? What is the hybridization on N? Use bond energies and one Lewis formula for HNO3 to estimate ∆H°f for HNO3(g). The actual value of ∆H°f for HNO3(g) is –135 kJ/mol, which is lower than the estimated value because of stabilization of HNO3 by resonance. The resonance energy is defined as ∆H°f (estimated) – ∆H°f (actual). What is the resonance energy of HNO3?