nitric acid

Reactions with metals

Being a powerful oxidizing agent, nitric acid reacts violently with many organic materials and the reactions may be explosive. Depending on the acid concentration, temperature and the reducing agent involved, the end products can be variable. Reaction takes place with all metals except a few of the precious metal series and certain alloys. This characteristic has made it a common agent to be used in acid tests. As a general rule, oxidizing reactions occur primarily with the concentrated acid, favouring the formation of nitrogen dioxide (NO2).

Cu + 4 H+ + 2 NO3− → Cu2+ + 2 NO2 + 2 H2O

The acidic properties tend to dominate with dilute acid, coupled with the preferential formation of nitric oxide (NO). However, when the reaction is carried out in the presence of atmospheric oxygen, the nitric oxide rapidly reacts to form brown nitrogen dioxide.

3 Cu + 8 HNO3 → 3 Cu(NO3)2 + 2 NO + 4 H2O

2 NO + O2 → 2 NO2

Since nitric acid is an oxidizing agent, hydrogen (H2) is rarely formed. Only magnesium (Mg), Manganese (Mn) and calcium (Ca) react with cold, dilute nitric acid to give hydrogen:

Mg (s) + 2 HNO3 (aq) → Mg(NO3)2 (aq) + H2 (g)

Source : http://en.wikipedia.org/wiki/Nitric_acid#Reactions_with_metals