Question

how why does aluminium metal become passive when heated with concentrated hn O3? write the5 strongest acid all over the world?

Answer 1

Aluminium reacts vigorously with Nitric acid once the Al2O3 layer has been breached. (HOWEVER, as Kraput pointed out, if the HNO3 is very strong, it will form a passive layer) the reaction is just like magnesium with nitric acid, except that it takes a few minutes to get started. it produces aluminium nitrate and hydrogen gas and a lot of heat, as well as nitrogen oxides fumes.

the reaction formula is:

aluminium + nitric acid —> aluminium nitrate + hydrogen
2Al(s) + 6HNO3 (aq) —> 2Al(NO3)3 (aq) + 3H2(g)

Answer 2

The position of Al in the Elechtro-chemical series shows that Al is stronger reducing agent than Hydrogen. When Al metal is added to dilute HNO3containing higher concentration of H+ions, it reduces the H+ ions in the solution to hydrogen gas and itself gets oxidized to Al(NO3)3

2Al(s)+6HNO3(dil)→2Al(NO3)3+3H2↑⏐

But when Al is dipped in conc . HNO3 where the concentration of H+ ions is very low, it reduces Nitrogen of H+5NO3 molecule to NO2and itself gets oxidized to Al2O3 which forms very thin invisible protective layer on the metal. It then resists further oxidation of Al atom below the protective layer of Al2O3.

This occurs when concentrated HNO3 is kept in Aluminium container and makes transportation of conc. HNO3 in Aluminium container possible.

The equation of possible oxidation reaction occurred during formation of the protective layer of Al2O3

2Al(s)+6HNO3(conc)→Al2O3(s)+6NO2(g)+3H2O(l)