Video: Reaction of nitrogen dioxide

First, nitrogen monoxide is produced by the reduction of nitrite ion with the weak reducing agent, iron(II) ion:

NO2-(aq) + 2 H+(aq) + e- ® NO(g) + H2O(l)
Fe2+(aq) ® Fe3+(aq) + e-

Air is then drawn into the syringe to cause oxidation to red-brown nitrogen dioxide:

2 NO(g) + O2(g) ® 2 NO2(g)

Nitrogen dioxide is a mixed acid oxide, thus reaction with water gives asolution that is highly acidic.

2 NO2(g) + H2O(l) ? HNO3(aq) + HNO2(aq)