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Questions about polyatomic ion compounds

  1. Sep 24, 2008 #1

    Ser

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    I'm extremely confused in regards to many aspects concerning polyatomic ions. For one, am I correct in thinking that covalent polyatomic ions gain charges by having components dissociated? Ex. A and B are sharing an electron, A has a significantly greater hold, B is dissociated by something leaving A with the electron and the compound now having a - charge?

    If my understanding of that is correct, here are some questions about polyatomic ion compounds:
    1) Acids are covalent, right? Well how do they form, then? The polyatomic ion dissociates H+(s) from other compounds?
    2) Well how does, say, Hydrobromic acid form? How can, in a covalent bond, an H+ and Br- come together? That seems ionic...they both dissociated from something else and happened to come together covalently but the electronegativity is even enough that the partial charge isn't significant so the + and - parts balance the charge?
    3) Finally, how do ionic compounds with polyatomic ions work? If, say, Nitrate pulls off an electron from a group 1 element that element is now +1 and Nitratre is -2...how does that balance?

    So confused..thanks in advance for bearing with my questions. I'm undoubtedly missing something here.
     
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  3. Sep 25, 2008 #2

    Borek

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    Staff: Mentor

    This is more or less correct, although nomenclature you use - "dissociated by something" - doesn't make sense to me. Just dissociated.

    No idea what you are asking, this is again nomenclature problem. However, you may assume that some bonds are ionic while others are covalent in the same molecule.

    HX (where X is any halogen) are a little bit tricky. The molecule - in tha gas phase - is covalet. However, when it gets dissolved, it dissociates because both cation and anion are solvated by water molecules, and they are much more stable in water as solvated ions,. than as covalent molecule.

    Nitrate doesn't pull electron. If anything, reaction goes like

    2Na + 2H+ -> 2Na+ + H2

    and nitrate doesn't change in the reaction. Reality is a little bit more complicated, as sodium is very reactive and it will not "wait" for H+, it will react directly with water, but that's the general idea.
     
  4. Sep 25, 2008 #3

    Ser

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    What I meant is that since the polyatomic ions have negative charges, how do they covalently bond with hydrogen to become neutral? The hydrogens would have to be already be cations when they bond?

    With the Nitrate question I was referring to the Nitrate polyatomic ion NO3-1. When it forms an ionic bond it's pulling off electrons, right? Well I can understand ionic bonds with elements perfectly but here it looks like Nitrate pulls off an electron, giving it an additional negative charge, and the other thing yields its electron, giving that a positive charge. The negative and positive cross out but the original -1 charge seems like it's still there, regardless of how many it pulls? With, say, potassium and fluoride the fluoride pulls one, K is +, F is -, even. But with, for example, Nitrate if it goes into an ionic bond and pulls electrons from something like K (just as a random example) the charge will stay "evened out" like with K and F but the -1 will still be there. How is that removed?

    Also, with your explanation of Hydrobromic acid, does that mean that the hydrogen and bromide are neutral until they get dissociated?

    Thanks :)
     
    Last edited: Sep 25, 2008
  5. Sep 25, 2008 #4

    Borek

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    Staff: Mentor

    Could be thats thing that you are missing here. Both covalent and ionic compounds are neutral when seen from the outside. HCl molecule is neutral, HNO3 is neutral as well. However, charge INSIDE of the molecule doesn't have to be distributed evenly - one of electrons is moved from H to NO3, so they are already charged, although only partially. When making acid you synthesize NEUTRAL molecule in which charge is not distributed evenly, and when it dissociates, charge gets separated - H becomes H+, NO3 becomes NO3-. If - for any reason - they recombine - negatively charged anions reacts with positively charged cation, giving neutral molecule.
     
  6. Sep 25, 2008 #5

    Ser

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    I still seem to be missing something...My thought process is that Nitrate has a net -1 charge by itself. When you say they detach into H+ NO3- I guess that means the hydrogen was initially a cation before the nitric acid was formed? If so, I understand it now and my main confusion then would be polyatomic ionic compounds.

    NO3- + Na, how would that work? Obviously you need a + to balance the overall molecule but how does that happen? If I'm correct, the sodium gives up an electron to the nitrate, which gives sodium a +. But that makes Nitrate -2, so it isn't balanced?
     
  7. Sep 25, 2008 #6

    Borek

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    Staff: Mentor

    Nitrate anion, not nitric acid nor - for example - sodium nitrate, which are neutral.

    Depends on what do you mean by "before the nitric acid was formed". If we assume you make nitric acid dissolving dinitrogen pentoxide in water:

    N2O5 + H2O -> 2HNO3

    you start with neutral molecules and you end with neutral molecules. Charges appear during dissociation:

    HNO3 -> H+ + NO3-

    No, it is not NO3- + Na, it is already NO3- + Na+. Why do you think you start with atomic sodium and NO3-? To synthesize sodium nitrate you have to either start with nitric acid and metallic sodium (in which case sodium will react with H+ from acid dissociation, NO3- is untouched) or with - for example - sodium hydroxide - in which case sodium is already in the cationic form (and NO3- is untouched again).
     
  8. Sep 25, 2008 #7

    Ser

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    Aha! I knew it was something dumb I was thinking. Now I understand that, too. Thanks for the help, much calmer in my head now that I get it.
     
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