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Amphoteric Acids and Bases

  1. Aug 28, 2011 #1
    1. The problem statement, all variables and given/known data
    I'm having problems understanding amphoteric acids and bases. I know amphoteric means that it can act as an acid or base, but when the question asks me to write the equation to support this statement, I always get it wrong :confused:


    2. Relevant equations
    HSO4^-1 is an amphotheric ion. Write chemical equations to show this ion acting as
    a) an acid
    b) a base


    3. The attempt at a solution
    For a)
    I wrote HSO4^-1 + H2O -> SO4^-2 + H3O^+
    but, on the answers it is HSO4^-1+OH^-1 -> SO4^-2 + H2O

    For b) I wrote HSO4^-1 + H2O -> H2SO4 + OH^-1
    but on the answers it is HSO4^-1 + H3O+ ->H2SO4 + H2O

    I have no idea why I'm wrong :confused: Could someone help explain this to me? :smile:
     
  2. jcsd
  3. Aug 28, 2011 #2

    Borek

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    Staff: Mentor

    I don't think you are wrong, your answers make sense to me.
     
  4. Aug 28, 2011 #3
    oh hm :shy:. So is my answer and the book's answer both correct? I'm a bit worried since I have an acids and bases test coming up.
     
  5. Aug 28, 2011 #4
    I think what the test is looking for is for the neutralisation reaction between an acid and alkali. It would be better to phrase it such that it shows the ion reacting with a hydroxide ion for the acidic property and the basic ion for the basic property.

    Both of your equations also have the same reagents. Since water can self-ionise to form hydronium and hydroxide ions and those are the ones reacting, it is wrong to write water as the water is not the ion reacting with the bisulphate ion.
     
  6. Aug 28, 2011 #5

    Borek

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    Staff: Mentor

    Both are correct - both show that HSO4- behaves like an acid and like a base. The difference is that you show it can yield both H+ and OH- when put in solution, while they show it can react with H+ and OH-. These are equivalent statements (more precisely - equivalent enough for a purpose of showing HSO4- is amphoteric).

    There is a reason why the book answer to b is better - H2SO4 is a very strong acid, chances of observing HSO4- reacting with water molecule to produce OH- are slim. It will be easier to desing an experiment in which HSO4- can be protonated to H2SO4. That's all in dissociation constants and reaction equilibrium. But it doesn't make your answer wrong, at least IMHO.
     
  7. Aug 28, 2011 #6
    ooh okay :smile:

    so then in the case of HCO^-3, to show it is a base, it would be HCO3^-1 + H3O+ -> H2CO3 + H2O?
     
  8. Aug 28, 2011 #7

    Borek

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    Staff: Mentor

    Yes.

    In this case, as H2CO3 is a weak acid, reaction with water:

    HCO3- + H2O <-> H2CO3 + OH-

    is quite common, so it will be as good as the one you wrote.
     
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