When a metal reacts with nitric acid it does not evolve hydrogen gas why?

Answer 1

Only certain metals are "active" enough to react with nitric acid (HNO₃) to produce hydrogen gas (H₂). And it also depends on the concentration of the nitric acid. For example, magnesium (Mg), Manganese (Mn) and Zinc (Zn) will react with dilute HNO3 as follows:

Mg + 2HNO₃ ==> Mg(NO₃)₂ + H₂

Mn + 2HNO₃ ==> Mn(NO₃)₂ + H₂

Zn + 2HNO₃ ==> Zn(NO₃)₂ + H₂

Some non-active or should I say less active metals can still be oxidized by dilute HNO₃ but not produce hydrogen gas. Copper (Cu) is an example. 3Cu + 8HNO₃ ==> 3Cu²⁺ + 2NO + 4H₂O + 6NO⁻³ Note that no H₂ gas is produced by the less reactive Cu. Here the nitric acid acts as an oxidizing agent.

I guess the short answer to your question would be that some metals don't generate hydrogen gas with nitric acid because the nitric acid is a strong oxidizer and can oxidize the free hydrogen to produce water, and this again depends on the concentration of the nitric acid. It is a stronger oxidizer than it is an acid especially when concentrated. When dilute, it will or can behave as indicated above with Mg, Mn and Zn, for example.