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Homework Help: Acid-base rxns

  1. Feb 22, 2005 #1
    i would like to see if my reasoning is correct (for aq. solutions):

    [tex]2HBr_(aq_) + 2NH_3_(aq_) -> 2NH_4_(aq_) + Br_2_(aq_) [/tex]
    HBR is an acid so it gives H to NH3, which leaves Br all alone. But Br can't exist by itself, so it becoems Br2?
    ionic:
    [tex]2H^+_(aq_) + 2Br^-_(aq_) + 2NH^-_3_(aq_) -> 2NH^+_4_(aq_) + 2Br^-_(aq_) [/tex]
    net ionic:
    [tex]2H^+_(aq_) + 2NH^-_3_(aq_) -> 2NH^+_4_(aq_) [/tex]

    Also, I have a question about water and its presence or lackthereof in net ionic equations (for aq. solutions).

    I have
    [tex]2HClO_4_(aq_) + Mg(OH)_2_(s_) -> Mg(ClO_4)_2 (s) + 2H_2O[/tex]
    ionic:
    (matter states are same as above)[tex]2H^+ + 2CLO^-_4 + Mg^+^2 + 2OH^- -> Mg(ClO_4)_2 + 2H^+ + 2OH^- [/tex]

    but that last part is really [tex]2H_2O[/tex]. But if i write it like that, then in my net equation i gotta keep [tex]H_2O[/tex] and the H and OH ... making my net equation the same as my ionic equation. That seems wrong to me. Is there a covention where I can write [tex]2H_2O[/tex] and still leave out OH and H as spectator ions in my net ionic equation? Thank you for your time.
     
    Last edited: Feb 22, 2005
  2. jcsd
  3. Feb 22, 2005 #2

    GCT

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    First check if [itex]Mg(ClO_4)_2[/itex] is a solid, otherwise it would remain hydrated and would not appear in the net ionic equation. And I believe that water will actually be in the net ionic equation, that is [itex]H^+_{(aq)} + OH^-_{(aq)} \xrightarrow{\leftarrow} H_2O_{(l)}[/itex], there's a equilibrium associated with this equation although not on a large scale.
     
  4. Feb 22, 2005 #3

    GCT

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    that is most of the ionized hydrogen cation and hydroxide will be converted to water.
     
  5. Feb 22, 2005 #4
    ERROR - yeah that mg clo4 is solid, it has (s) in subscript beside it.
    EDIT - whoops. That was my assumption. If HCLO is breaking it down, then it'd have to be a liquid, as this kind of reaction goes to completion.

    That makes the net equation for (2)
    [tex]2H^+ + 2CLO^-_4 + Mg^+^2 + 2OH^- -> Mg(ClO_4)_2_(aq_) + 2H_2O[/tex]

    I take it the reasoning in my first equation involving HBr and NH3 is correct?
     
    Last edited: Feb 22, 2005
  6. Feb 22, 2005 #5

    GCT

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    First, think about it.......how is bromine gas even formed? You've got two anions, the two cannot combine unless one of them is a cation. So bromine anion is the final product, usually the ammonium cation interacts ionically with bromine anion to a degree to form a salt adduct (l). However, I don't believe that you need to get this technical here. Thus your net ionic equation seems correct.
     
  7. Feb 22, 2005 #6
    thanks for your help.
     
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