Copper is an unreactive metal and doesn’t react in normal circumstances with dilute acids. However it does react with nitric acid. Why is this?
Nitric acid is an oxidising agent and the reaction is not the usual acid + metal reaction. The products are oxides of nitrogen instead of hydrogen. The actual nitrogen oxide formed depends on the concentration and temperature of the acid.
There are actually two equations for the reaction of copper with nitric acid. It depends on whether the nitric acid is concentrated or not. If it is concentrated and in excess then the ratio is 1:4 copper to nitric acid. If it is dilute then the ratio is 3:8.
Cu + 4HNO3 –> Cu(NO3)2 + 2NO2 + 2H2O
3Cu + 8HNO3 –> 3Cu(NO3)2 + 2NO + 4H2O
Nitric acid when concentrated is a strong oxidising agent so it makes sense that a higher oxidation state of nitrogen (IV) oxide is formed when the nitric acid is concentrated.