Most of the metals do not give hydrogen while reacting with nitric acid. Why ?
Hydrogen gas is not evolved when a metal reacts with dil.nitric acid. This is because nitric acid is a strong oxidizing agent . It oxidises the hydrogen produced to water and itself gets reduced to oxides of nitrogen such as NO , NO2 ,N2O . However, magnesium reacts with nitric acid to evolve hydrogen gas.
Metals do not give Hydrogen while reacting with Nitric acid because HNO3 is a strong oxidizing agent. It oxidizes the hydrogen evolved into water and it itself is reduced to an oxide of Nitrogen such as NO etc. However, it gives hydrogen when reacted with Magnesium or Manganese.
Nitric acid reacts with most metals but the details depend on the concentration of the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in its reaction with most metals. Magnesium, manganese and zinc liberate H2. Others give the nitrogen oxides.
Nitric acid can oxidize non-active metals such as copper and silver. With these non-active or less electropositive metals the products depend on temperature and the acid concentration. For example, copper reacts with dilute nitric acid at ambient temperatures with a 3:8 stoichiometry.
3 Cu + 8 HNO3 → 3 Cu2+ + 2 NO + 4 H2O + 6 NO3-