The reactions of metals with nitric acid are explained in 3 steps as follows:
Primary reaction:
Metal nitrate is formed with the release of nascent hydrogen
M + HNO3 → MNO3 + (H)
Secondary reaction:
Nascent hydrogen produces the reduction products of nitric acid.
Tertiary reaction:
The secondary products either decompose or react to give final products
For examples:
Copper reacts with nitric acid in the following manner
3Cu + 6HNO3 → 3Cu(NO3)2 + 6(H)
6(H) + 3HNO3 → 3HNO2 + 3H2O
3HNO2 → HNO3 + 2NO + H2O
Overall reaction
3Cu + 8HNO3 → 3CU(NO3)2 + 2NO + 4H2O
The concentrated acid has a tendency to form nitrogen dioxide
Cu + 4HNO3 → 3CU(NO3)2 + 2NO2 + 2H2O