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Weak Bases... More Stable?

Discussion in 'MCAT Study Question Q&A' started by coolchix321, 08.16.09.

  1. coolchix321

    coolchix321

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    Could someone please explain to me why weak bases are generally unreactive because they are stable? I am having trouble understanding this concept...
    Is it the smaller the anion, the weaker the base? Why?
    I need some clarification on qualitative weak/strong acid/base chemistry....
    Thanks
  2. Rolling

    Rolling

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    This would be the best way I can explain it.

    Look at the Halogen groups, and the monoprotic acids you can form with those. Ignore H-F momentarily.

    If you look at the pKa of those monoprotic acids, you'll see

    H-Br
    H-I
    H-Cl

    H-Br is stronger than H-I which is stronger than H-Cl

    EDIT VERY VERY SORRY H-I IS STRONGER THAN H-BR. I had not looked at the periodic table at this point in making the post and forget that Iodine has a bigger radius. My sincerest apologies.

    Now the reason for this is because, the reaction I will layout.

    H-A + H2O ---> <---- (my best attempt to show reversibility) A- + H3O+

    So anyhow, we can see here that the Anion forms. All of these acids are strong. As we can expect, their conjugate bases will be weak, and the reason they are unreactive, is because they are stable.

    out of all 3 atoms, I can BEST stabilize that negative charge when it is an anion, and the reason is because it is the largest atom, and so can "spread out" the negative charge better. Unlike Br-/Cl- which are smaller, and thus would be slightly more basic (but are still considered weak bases).

    I hope that helps. If that does NOT help, I can try to clarify more. Let me know.
    Last edited: 08.18.09
  3. Baylor2012

    Baylor2012

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    Wow, that makes alot of sense. Props for you ;)
  4. coolchix321

    coolchix321

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    I am still confused
    i need more clarification please
    also, i thought HCl was a stonger acid than HI and HBr...
  5. ThirdEye

    ThirdEye

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    Unstable molecules are reactive. Strong acids and bases are less stable than weak acids and bases because they are more reactive.

    HCl is strong but HI>HBr>HCl and HF isn't even strong. The bigger the size, the easier it is to give up the H, and the more unstable it is.

    A weak base is resistive to being protonated because it is stable. A strong base is going to accept a proton easily because it's less stable.
  6. inaccensa

    inaccensa

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    To understand the strength of acids and bases or their conjugates, you need to really follow 2 things - Inductive effect and polarizability.btw the strength of acids increases from top to bottom here, HI being the most acidic.

    HF
    HCl
    HBr
    HI

    if you look at the periodic table, the electronegativity decreases from top to bottom and size increases from top to bottom. I used to mix the concepts of electronegativity with strength. But it is not really how it works

    Now Fluorine is the most electronegative. When HF dissociates, you get H+ and F- . Fluoride ion is not stable because it is small in size and it cannot spread the negative charge over a larger space. This creates the higher affinity of fluoride ion for an electrophile or a proton. If you compare HF with HCl, which anion will be more stable? Which will be able to spread the charge more? Another way to think of this is in terms of dissociation, strong acids dissociate completely, HF cannot because the fluoride ion will quickly reform HF, it won't stay as F-.
  7. minutemen11

    minutemen11

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    H-I is the strongest not H-Br...

    another way to look at this is in terms of nucleophillicity for SN1/SN2 rxns
    if you have a protic solvent you'd want largest anion (I^-) because it can shed the solvation easier than a small anion like (F^-) this is also why F is pretty much unreactive in most rxns and is only a weak acid.

    or you could even look at in terms of a leaving group, I is the best in the halogens because it is the largest and will therefor be the most stable. F is the least stable as a leaving group because its so small and so electronegative that it will not be as stable as I. Pretty much i think a general rule is that as you go down a group in the table, the acidity increases, if you remember from substiutent effects for a benzene ring the more acidic it is the more stable the ring is.
  8. coolchix321

    coolchix321

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    Thanks
    so why doesnt HF want to stay as F- exactly? Because it's small? How does size play a role exactly? How does spreading an anion over a larger space make something more stable??
  9. ThirdEye

    ThirdEye

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    Because of the difference in effective nuclear charge
  10. ThirdEye

    ThirdEye

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    .

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