How does nitrogen dioxide react with water in a disproportionation reaction?

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Nitrogen dioxide (NO2) reacts with water through a disproportionation reaction, a process in which the nitrogen atom is simultaneously oxidized and reduced. The specific products of this reaction depend on the temperature of the water:123

  • In cold water: Nitrogen dioxide disproportionates to form a mixture of nitrous acid (HNO2) and nitric acid (HNO3).
  • At higher temperatures: The reaction produces nitric acid (HNO3) and nitric oxide (NO). In this specific reaction, the nitrogen atom in the +4 oxidation state is reduced to +2 (in NO) and oxidized to +5 (in HNO3), as shown in the following equation:123

3NO2+H2O→2HNO3+NO