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Homework Help: Aluminium Hydroxide as Amphoteric Substance

  1. May 19, 2008 #1
    [SOLVED] Aluminium Hydroxide as Amphoteric Substance

    1. The problem statement, all variables and given/known data
    Aluminium hydroxide is an amphoteric substance. It can act as either a Bronsted-Lowry base or a Lewis acid. Write a reaction showing [tex]Al(OH)_{3}[/tex] acting as a base toward [tex]H^{+}[/tex] and as an acid toward [tex]OH^{-}[/tex].

    2. Relevant equations
    I know that amphoteric means the substance can act both as a base and as an acid. I also know that, to be a Bronsted-Lowry base, the substance needs to be a proton acceptor and to be a Lewis acid, an electron-pair acceptor.

    3. The attempt at a solution
    My biggest problem is that I'm not sure what they really ask. Since it mentions that aluminium hydroxide is an amphoteric substance, does that mean it reacts with itself? If yes, then I'm clueless. If not, then I attempted the following:

    [tex]Al(OH)_{3}+H^{+}\rightarrow Al^{3+}+3H_{2}O[/tex]


    [tex]Al(OH)_{3}+OH^{-}\rightarrow Al(OH)_{4}[/tex]

    However, my attempt is very much "gazing in my crystal ball" kind of science since it isn't based on anything I know but rather on a lot of unfounded assumptions and shooting in the dark which is why I need assistance :smile:

    Thanks in advance!

  2. jcsd
  3. May 19, 2008 #2


    User Avatar

    Staff: Mentor

    Google aluminates.

    Your reactions - while not that far from the correct answer - ar far from being balanced :wink:
  4. May 19, 2008 #3
    Oh, yeah...of course...see what happens when you concentrate too hard on tex :smile:

    This better?

    [tex]Al(OH)_{3}+3H^{+}\rightarrow Al^{3+}+3H_{2}O[/tex]


    [tex]Al(OH)_{3}+OH^{-}\rightarrow Al(OH)^{-}_{4}[/tex]

    and Google you reckon? Mmmmmm...:rolleyes:

    Google here I come! :biggrin:
  5. May 19, 2008 #4
    Solved! :cool:

    Thanks Borek! Three thumbs up :wink:
  6. Jun 6, 2008 #5
    Just a thought, but could the question be asking for something like this?

    Al(OH)_{3} + Al(OH)_{3}{+}\rightleftharpoons AlH(OH)_2^+ + AlO(OH)_2^-

    This is, as the question states, "a" reaction, where Al(OH)3 bevaes in both ways, one accepting a proton, the other an electron pair. Autoionisation?

    Its late, I'm tired, and this has kept me busy for too long, so pleeease tell me if im completely wrong :P
    Last edited: Jun 6, 2008
  7. Jun 7, 2008 #6


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    Staff: Mentor

    These are equilibrium systems, to some extent such reaction will proceed - along many others. So you are not completely wrong.

    What I don't like about the reaction is that it may suggest a little bit too much. It may occur, but for sure it will not dominate the solution.
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