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Homework Help: Acids and bases

  1. Nov 20, 2008 #1
    1. The problem statement, all variables and given/known data
    An acid and a base is supposed to form a salt and water during a neutralization reaction such as NaOH + HCl --> NaCl + H2O. However, based on Brønsted–Lowry acid-base theory, shouldn't NaOH become NaH2O or something like that after accepting an H+ ion from HCl since the base is a proton acceptor? Why does the OH- ion from NaOH get separated?

    My next question is....
    What is the result when the base, NH3 reacts with the acid HCl, for example? Is the result NH4+ and Cl-?
     
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  3. Nov 21, 2008 #2

    Borek

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    NaOH in water is fully dissociated. What is a proton acceptor in this solution?
     
  4. Nov 24, 2008 #3
    In Brownsted- Lowry Theory, bases are said to be proton acceptors and acids are said to be proton donors and since NaOH is a base in this solution, shouldn't it receive a H+ion from HCl, which is an acid? When NH3, a base and H2O react, NH4+ and OH- are produced. In this case, a H+ ion is transferred from H2O to NH3 whereas in the NaOH and HCl reaction, a H+ ion is not tranferred from the acid to the base. Why is this so? Does this have something to do with water having a unique property?
     
  5. Nov 25, 2008 #4

    Borek

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    You are still ignoring fact that NaOH is fully dissociated. There is no such thing as NaOH in solution, so it can't react.
     
  6. Nov 26, 2008 #5
    Oh ya, I completely forgot about that. Thank you. However I do have another question on how if NH3 and HCl reacted? Would the result be NH4+ and OH-? But if that is so, then it would mean that when an acid reacts with a base, a salt and water is not always the result. If so, when does this rule apply and when does a reaction between a base and an acid result in another acid and base?
     
  7. Nov 26, 2008 #6

    Borek

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    Why OH-?
     
  8. Dec 1, 2008 #7
    OH- because NH4+ gains a H+ ion since it is the base resulting in OH-. So are NH4+ and OH_ the products for such a reaction?
     
  9. Dec 2, 2008 #8

    Borek

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    NH3 has a lone pair that can accomodate H+, thats right. However, where is the source of OH-? Please remember you are reacting NH3 with strong acid.
     
  10. Dec 4, 2008 #9
    Are you referring to H2O as the strong acid since it is left to react with NH3?
     
  11. Dec 5, 2008 #10

    Borek

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    No, please take a look again at the original question you have posted.

    You listed two substances, one of them is a strong, mineral acid. One of the most common strong acids.
     
    Last edited: Dec 5, 2008
  12. Dec 17, 2008 #11
    Sorry about that. I forgot about my original question in mind. What I mean is, I do have question on how if NH3 and HCl reacted? Would the result be NH4+ and Cl-? But if that is so, then it would mean that when an acid reacts with a base, a salt and water is not always the result. If so, when does this rule apply and when does a reaction between a base and an acid result in another acid and base?
     
  13. Dec 17, 2008 #12

    Borek

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    Yes.

    Look for extended definitions of acids and bases. Classic one (Arrhenius) treats everything in terms of OH- and H+ - it fails for ammonia and HCl reaction. This one can be easily understood in terms of Brønsted-Lowry's acids and bases.
     
  14. Dec 21, 2008 #13
    Actually, I'm very new to the topics od Acids and Bases so I don't think I was able to understand everything on that webpage very clearly. Still, this is what I think it meant and how it applies to my question; Ammonia and HCl for NH4+ an Cl-. NH4+ is a conjugate acid and Cl- is a conjugate base. Am I right?
     
  15. Dec 22, 2008 #14

    Borek

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    No, conjugate acid and conjuage base differ just by one proton.
     
  16. Jan 1, 2009 #15
    So let me try to get what u are saying and from the hyperlink you attached; The reaction I stated above results in NH4+ and Cl- and both aren't a conjugate acid and base pair. That is because the base, NH3 does not contain OH-. Is that true?
     
  17. Jan 1, 2009 #16

    Borek

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    My English fails me here and I am not sure what you mean (word 'both' make the statement unclear for me). IF you mean that NH4+ is not conjugate acid of Cl- and vice versa - then you are right.

    It can be put this way, but it doesn't have to. You are all the time trying to use Arrhenius definition of base to ammonia - that's why you still have problems understanding the case.
     
  18. Jan 2, 2009 #17
    What is that other way or definition that my explanation can be put? Is it that a base is defined as any substance that can accept a hydrogen ion?
     
    Last edited: Jan 2, 2009
  19. Jan 2, 2009 #18

    Borek

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  20. Jan 4, 2009 #19
    Ok thank you very much for your help so far.
     
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