Question

(a) Explain why metals usually do not liberate hydrogen gas with dilute nitric acid.
(b) Name two metals which can, however, liberate hydrogen gas from very dilute nitric acid.

Answer 1

(a)Nitric acid $\left(HN{O}_3\right)$ is a strong oxidising agent.
The Hydrogen gas produced during its reaction with metal gets oxidised to $H_{2}O$, hence no hydrogen gas is produced.
Instead, it evolves nitric oxides such as $N{O}_2,NO$ etc.

For Eg:

$Cu\left(s\right)+4HN{O}_3\left(aq\right)⟶Cu\left(N{O}_3{)}_2\right(aq)+2N{O}_2(g)+2H_{2}O(l)$


(b)Magnesium and Manganese can generate $H_2$ with very dilute $HN{O}_3$ because at very low concentration $HN{O}_3$ can not oxidise $H_2$ to water