Question

which of the following metals becomes passive with concentrated hno3 mg , cr, zn , al , fe , ni , co, mu

Answer 1

From WIKIPEDIA: Although chromium (Cr), iron (Fe), and aluminium (Al) readily dissolve in dilute nitric acid, the concentrated acid forms a metal-oxide layer that protects the bulk of the metal from further oxidation. The formation of this protective layer is called passivation.

From ME: contrast this with the action of hydrochloric acid, which removes the FeO layer, and makes the surface very active. This application of HCl to steel is called “pickling”, and it prepares the surface of the steel for rolling or forming, free of the FeO which would cause significant abrasion of the tools being used (rolling mill rolls, forming presses, etc.).

Answer 2

The metals that become passive with concentrated $\[HN{O_3}\]$ are $\[Cr,\,Al,\,Fe\]$ and $\[Ni\]$.

Explanation:

• The reaction of certain metals with concentrated nitric acid ($\[HN{O_3}\]$) forms oxides of the metals.
• These oxides of metals form a passive layer on the metal so that the inner metal is prevented from acid.
• When nitric acid reacts with the following metals, oxides are formed as given below:

1. $\[Cr\]$ - $\[C{r_2}{O_3}\]$
2. $\[Fe\]$ - $\[F{e_3}{O_4}\]$
3. $\[Ni\]$ - $\[NiO\]$
4. $\[Al\]$ - $\[A{l_2}{O_3}\]$

• Thus, among the given metals, the metals that become passive with concentrated $\[HN{O_3}\]$ are $\[Cr,\,Al,\,Fe\]$ and $\[Ni\]$.

#SPJ2