In the following series of reactions, the oxidation number of nitrogen changes from –3 in NH3 to +5 in HNO3. −3 +2 \[\ce{\underset{\text{Ammonia}}{4NH3}_{(g)} + \underset{\text{Oxygen}}{5O2}_{(g)}->[Pt/Rhgauge catalyst][500K,9 bar]\underset{\text{Nitric oxide}}{4NO}_{(g)} + 6H2O_{(g)}}\] +4 \[\ce{\underset{\text{Nitric oxide}}{2NO}_{(g)} + \underset{\text{Oxygen}}{O2}_{(g)}⇌\underset{\text{Nitrogen dioxide}}{2NO2}_{(g)}}\] +5 \[\ce{\underset{\text{Nitrogen dioxide}}{3NO2}_{(g)} + \underset{\text{Water}}{H2O}_{(l)}->\underset{\text{Nitric acid}}{2HNO3}_{(l)} + \underset{\text{Nitric oxide}}{NO}_{(g)}}\]
