[victor]

Um, I've gotten a problem when I'm testing metals activity to oxydation...one of the meatl is Aluminium (Al). I use three types of solutions, which are hot water, High concentrated HCl, and Nitrate acid (HNO₃).
Al metal did react with HCl, oxydized and get soluted become AlCl₃, but it does nothing with the Nitrate acid. According to my chem book, nitrate acid is a stronger oxydator than HCl, then why did Al metal didn't get oxydized by HNO₃ solution??

[Albert]

Aluminium is not oxidized by HNO3 conc. because of passivation.
This means that HNO3 oxidizes a minimum part of Al, producing an invisible surface of Al2O3, which protects Al from being oxidized any further.
You can recognize this phenomenon also with Cr and Ni.

[victor]

Um, talking about the metal's shelter, Why different thing happen to Magnesium metal, Mg also can form MgO if it's oxidized. You said that, because of passivation, HNO3 oxidation will form oxide layer surrounding the metal. But Mg is totally dissapeared (I mean get oxidized totally become Mg²⁺). Can you explain that why it's only happens to Mg metal but not to Al metal?
PS: I use HNO3 solution which has the concentration for 6M.

Hey, is this reaction equilibrium correct?
Al_(s) + 2NO^-_3(aq) + 4H⁺ ---> Al³⁺_(aq) + 2NO_2(g) + 2H₂O_(l)

if the reaction above is correct, then O atom will be caught to form water....so, no chance to form AL₂O₃ then....

[Alberto_Kravina]

ey, is this reaction equilibrium correct?
Al(s) + 2NO-3(aq) + 4H+ ---> Al3+(aq) + 2NO2(g) + 2H2O(l)

Nope! The charges are not balanced: the left side is +2 and the left side is +3

[Albert]

Al2O3 is inert, MgO has basic properties and reacts with acids:

MgO + HCl -> MgCl2 + H2O

[Borek]

Al₂O₃ is not inert, it dissolves both in acids and in bases. Al is passivated only in neutral conditions or - even better - in dry air. So why does it not dissolve in nitric acid? The answer it - it dissolves. It doesn't dissolve in concentrated nitric acid. as it gets oxidised and passivated, Nitric acid - although considered strong - is a weakest of strong acids, thus in concentrated solutions concentration of H⁺ is much lower than expected.

http://www.pc.chemie.uni-siegen.de/pci/versuche/english/v44-24-1.html

http://www.pc.chemie.uni-siegen.de/pci/versuche/english/v44-24-2.html

So in general it is enough to write:

Al₂O₃ + 6H⁺ -> 2Al³⁺ + 3H₂O

and

2Al + 6H⁺ -> 2Al³⁺ + 3H₂

But if you really, really want to balance oxidation reaction using nitric acid:

Al + 3NO₃^- + 6H⁺ -> Al³⁺ + 3NO₂ + 3H₂O

or if you want it to get Al oxidised to Al₂O₃:

2Al + 6HNO₃ -> Al₂O₃ + 6NO₂ + 3H₂O

Note that above is mainly test of my new program and the new option - export as UBBC - I am testing now. I take no responsibility for the reactions itself, but the equations are nicely formatted, aren't they

[victor]

Ok..I got it..
I asked this question because I get confused that from 4 metals that I tested (Mg, Al, Cu, and Zn)...only Al metal which seems not reacting with concentrated HNO₃.
Oh, my book also said that H⁺ is not enough strong to oxidize Cu metal (this is right), so it needs a stronger oxydator which is HNO₃. Is the oxidation potential different with the concentration of H⁺ in the HNO₃?
Yup, the reaction is nicely welldone...