nitric acid
NaNO 3 s + H 2 SO 4 l NaHSO 4 s + HNO 3 g The Ostwald process is the commercial method for producing nitric acid. This process involves the oxidation of ammonia to nitric oxide, NO; oxidation of nitric oxide to nitrogen dioxide, NO2; and further oxidation and hydration of nitrogen dioxide to form nitric acid: 4NH 3 g + 5O 2 g 4NO g + 6H 2 O g 2NO g + O 2 g 2NO 2 g 3NO 2 g + H 2 O l 2HNO 3 aq + NO g Or 4NO 2 g + O 2 g + 2H 2 O g 4HNO 3 l Pure nitric acid is a colorless liquid. However, it is often yellow or brown in color because NO2 forms as the acid decomposes. Nitric acid is stable in aqueous solution; solutions containing 68% of the acid are commercially available concentrated nitric acid. It is both a strong oxidizing agent and a strong acid. The action of nitric acid on a metal rarely produces H2 by reduction of H+ in more than small amounts. Instead, the reduction of nitrogen occurs. The products formed depend on the concentration of the acid, the activity of the metal, and the temperature. Normally, a mixture of nitrates, nitrogen oxides, and various reduction products form. Less active metals such as copper, silver, and lead reduce concentrated nitric acid primarily to nitrogen dioxide. The reaction of dilute nitric acid with copper produces NO. In each case, the nitrate salts of the metals crystallize upon evaporation of the resultant solutions. Nonmetallic elements, such as sulfur, carbon, iodine, and phosphorus, undergo oxidation by concentrated nitric acid to their oxides or oxyacids, with the formation of NO2: S s + 6HNO 3 aq H 2 SO 4 aq + 6NO 2 g + 2H 2 O l C s + 4HNO 3 aq CO 2 g + 4NO 2 g + 2H 2 O l Nitric acid oxidizes many compounds; for example, concentrated nitric acid readily oxidizes hydrochloric acid to chlorine and chlorine dioxide. A mixture of one part concentrated nitric acid and three parts concentrated hydrochloric acid called aqua regia, which means royal water reacts vigorously with metals. This mixture is particularly useful in dissolving gold, platinum, and other metals that are more difficult to oxidize than hydrogen. A simplified equation to represent the action of aqua regia on gold is: Au s + 4HCl aq + 3HNO 3 aq HAuCl 4 aq + 3NO 2 g + 3H 2 O l .