Question

Why does the reactivity of a piece of aluminium metal decrease if it is dipped in ?

Answer 1

We know that when metals are reacted with nitric acid, hydrogen gas is released. Nitric acid has the ability to oxidize hydrogen gas to water.

Complete step by step answer:
We have to know that hydrogen gas is liberated, when nitric acid and metal reacts.
Nitric acid is a strong oxidizing agent and it oxidizes hydrogen gas formed during the reaction to produce water and an oxide of nitrogen.
For example, let us consider the reaction of zinc with nitric acid.
When zinc is reacted with nitric acid, zinc nitrate along with nitrogen dioxide and water are the products formed. We can write the chemical reaction as,
$Zn\,\,\,\,\,\,\,\, + \,\,\,\,\,\,\,\,\,\,\,4HN{O_3} \to \,\,\,\,\,\,\,\,\,Zn{\left( {N{O_3}} \right)_2} + \,\,\,\,\,\,\,\,\,\,N{O_2}\,\,\,\,\,\,\,\,\,\, + \,\,\,\,\,\,\,\,2{H_2}O \\\ Zinc\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Nitric\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Zinc\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Nitrogen\,\,\,\,\,\,\,\,\,\,\,\,\,Water \\\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,acid\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,nitrate\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,dioxide \\\$
There is an exception that, when magnesium/manganese reacts with dilute nitric acid hydrogen gas is liberated. We can write the chemical reaction for this as,

$Mg\,\,\,\,\,\,\,\, + \,\,\,\,\,\,\,\,\,\,\,2HN{O_3} \to \,\,\,\,\,\,\,\,Mg{\left( {N{O_3}} \right)_2} + \,\,\,{H_2}\,\,\,\, \\\ Magnesium\,\,\,\,Nitric\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,Magnesium\,\,\,\,\,Hydrogen\,\,\,\,\,\,\,\,\,\,\,\,\, \\\ \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,acid\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,nitrate\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\\$
When we dip aluminium in nitric acid a layer of aluminium oxide is formed on the metal. This is because nitric acid is a strong oxidizing agent. The layer of aluminium oxide prevents further reaction of aluminium. Because of this, the reactivity of aluminium decreases.

Note:
We must remember that the oxidizing agents are that species (molecule/ion) that accepts electrons and reducing agents are species (molecule/ion) that donates electrons. A substance that is oxidized acts as a reducing agent and substance that is reduced acts as oxidizing agent.
Nitric acid is an oxidizing acid, which would dissolve most metals to produce soluble metal nitrates. It has poor oxidizing strength below 2 M but is a powerful oxidizing acid in concentrated form. We can enhance the oxidizing power by addition of chlorate, permanganate, hydrogen peroxide or bromine, as well as by raising its temperature and pressure.