Question

Give the reason for the following statement.
Hydrogen is not evolved when metal reacts with nitric acid.

Answer 1

Hint: The molecular formula for nitric acid is $HN{{O}_{3}}$ and nitric acid is a strong oxidizing agent. Hydrogen gas is a reducing agent when it reacts with nonmetals and an oxidizing agent when it reacts with metals.

Complete Step by step solution:
Let's consider the example of a chemical reaction when metals react with nitric acid.
Considering metal is zinc. Then reaction, when zinc reacts with nitric acid, is:
$Zn+4HN{{O}_{3}}\to Zn{{(N{{O}_{3}})}_{2}}+2N{{O}_{2}}+2{{H}_{2}}O$
According to the above reaction, we can say that hydrogen is not evolved when metal reacts with nitric acid.
We know that nitric acid is a strong oxidizing agent.
$HN{{O}_{3}}$ is an oxidizing agent because the N can be readily reduced. Nitric acid actively oxidizes non-metals, metals, inorganic compounds, and organic substances. Nitric acid is a strong oxidizing agent and the nascent oxygen produced on decomposition oxidizes hydrogen to water.
So nitric acid oxidizes the hydrogen gas formed during the reaction to form water and an oxide of nitrogen $(NO,{{N}_{2}}O,N{{O}_{2}})$.

Hence due to the oxidizing nature of nitric acid hydrogen is not evolved when metal reacts with nitric acid.

Additional Information:
There is one exception to the above statement.
Magnesium and manganese react with very dilute nitric acid to liberate hydrogen gas.
The reaction when magnesium reacts with nitric acid:
$Mg+2HN{{O}_{3}}\to Mg(N{{O}_{3}})+{{H}_{2}}$

Note: $HN{{O}_{3}}$ is a strong acid, because it is a strong acid it will dissociate completely into its ions$({{H}^{+}}andN{{O}_{3}}^{-})$. Therefore we consider $HN{{O}_{3}}$ to be a strong electrolyte.