Question

Generally, when metals are treated with mineral acids, hydrogen gas is liberated, but when metals (except $\mathrm{Mn}$ and $\mathrm{Mg}$), are treated with ${\mathrm{HNO}}_3$,hydrogen is not liberated, why?

Answer 1

The reaction of $\left(\mathrm{Mg}\right)$ and $\left(\mathrm{Mn}\right)$ with nitric acid:

1. When metals $\mathrm{Mn}$ and$\mathrm{Mg}$ are treated with Nitric acid, ${\mathrm{H}}_2$ gas is produced.
2. While reacting with $\mathrm{Mn}$ and$\mathrm{Mg}$ it further doesn't reduce itself and gives ${\mathrm{H}}_2$gas as the end product.

$\underset{\mathrm{Magnesium}}{\mathrm{Mg}\left(\mathrm{s}\right)}+\underset{\mathrm{Nitric}\mathrm{acid}}{2{\mathrm{HNO}}_3\left(\mathrm{aq}\right)}\to \underset{\mathrm{Magnesium}\mathrm{nitrate}}{\mathrm{Mg}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)}+\underset{\mathrm{Hydrogen}\mathrm{gas}}{{\mathrm{H}}_2\left(\mathrm{g}\right)}$

The reaction of nitric acid with other metals:

1. Nitric acid is a strong oxidizing agent and most metals don't react with it.
2. When it reacts with other metals it reduces itself to any of the Nitrogen oxides$({\mathrm{N}}_2\mathrm{O},\mathrm{NO},{\mathrm{NO}}_2)$ after oxidizing and ${\mathrm{H}}_2$ is converted into water.
3. It gives water as an end product with other metals instead of ${\mathrm{H}}_2$gas.

$\underset{\mathrm{Silver}}{\mathrm{Ag}\left(\mathrm{s}\right)}+\underset{\mathrm{Nitric}\mathrm{acid}}{{\mathrm{HNO}}_3\left(\mathrm{aq}\right)}\to \underset{\mathrm{Silver}\mathrm{nitrate}}{{\mathrm{AgNO}}_3\left(\mathrm{aq}\right)}+\underset{\mathrm{Nitrogen}\mathrm{dioxide}}{{\mathrm{NO}}_2\left(g\right)}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

Hence, Nitric oxide is able to reduce itself further when reacting with a metal other than $\mathrm{Mn}$ and$\mathrm{Mg}$ able to produce water in place of Hydrogen gas.