Nitric acid, nitrates and the nitrogen cycle
Study HNO3 as a strong acid and oxidizer, then follow nitrate through reactions and living systems.
Learning goals: calculate nitrogen oxidation state; explain nitrate resonance and geometry; write acid–base and selected redox equations; describe nitrate uses and environmental risks; connect nitrate with the nitrogen cycle.
1. Structure and dissociation
Nitric acid, HNO3, contains nitrogen at oxidation state +5: (+1)+x+3(−2)=0, so x=+5. In water it behaves as a strong monoprotic acid: HNO3 → H+ + NO3−. The nitrate ion is trigonal planar. Three resonance contributors distribute bonding and negative charge across the three oxygen positions, so the real ion has three equivalent N−O bonds rather than one permanently fixed double bond. The ion is stabilized by this delocalization.