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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 HNO_3containing higher concentration of H^+ ions, it reduces the H^+ ions in the solution to hydrogen gas and itself gets oxidized to Al(NO_3)_3
2Al(s) + 6 HNO_3(dil)->2Al(NO_3)_3 +3H_2uparrow
But when Al is dipped in conc . HNO_3 where the concentration of H^+ ions is very low, it reduces Nitrogen of Hstackrel(+5)NO_3 molecule to NO_2and itself gets oxidized to Al_2O_3 which forms very thin invisible protective layer on the metal. It then resists further oxidation of Al atom below the protective layer of Al_2O_3.
This occurs when concentrated HNO_3 is kept in Aluminium container and makes transportation of conc. HNO_3 in Aluminium container possible.
The equation of possible oxidation reaction occurred during formation of the protective layer of Al_2O_3
2Al(s) + 6 HNO_3(conc)->Al_2O_3(s)+6NO_2(g)+3H_2O(l)
