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General Question Regarding Ions and Reactivity

Discussion in 'MCAT Study Question Q&A' started by SuperSneaky24, 05.12.14.

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  1. SuperSneaky24

    SuperSneaky24

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    So say I have a N 3- ion. Why is that reactive as a base? Shouldn't it not be reactive as it fills up its valence shell into a noble gas configuration?
     
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  3. Teleologist

    Teleologist 2+ Year Member

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    Are you saying the azide ion is reactive as a Bronsted base or as a Lewis base?

    Oops, you're talking about the triply negative nitrogen anion. That is very reactive as a base because that's just ammonia (weak base) ionized three times (all three hydrogen protons removed).

    With each successive ionization we

    1) Increase the magnitude of the negative formal charge on the nitrogen atom. We also are increasing the magnitude of the negative partial charge on the nitrogen atom.

    2) decrease the size of the molecule (eventually the nitrogen atom). This increases negative charge density.

    The above two factors make the N^3- anion very nucleophilic. It wants to grab onto a positively charged hydrogen proton. This is simply Coulomb's law - opposites attract.

    Also remember conjugate theory. The conjugate base of a weak acid is strong. Ammonia is a very weak acid. Heck, we almost never think about ammonia as an acid because that's how weak it is as an acid. So it's conjugate base must be strong.

    The amide ion (H2N^-) is an even weaker acid. (Why? Think about its negative charge density compared to ammonia.) So it's conjugate base must be even stronger.

    And so on until we get to HN^2- and N^3-.
     
    Last edited: 05.12.14
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  4. SuperSneaky24

    SuperSneaky24

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    Thanks for responding. And right I understand why it's so reactive, but I'm just wondering why it is reactive in the first place. In theory, an atom wants to have an electron configuration of a noble gas so it can be most stable right? But doesn't a noble gas electron configuration make it inert? So I don't get why the N 3- ion, which has the noble gas configuration of neon, would be reactive in the first place.
     
  5. Teleologist

    Teleologist 2+ Year Member

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    N^3- has a high negative charge density. Note that noble gases are neutral - i.e. no formal charge.

    Also formation of a sigma bond creates a bonding orbital with a low energy.
     
  6. SuperSneaky24

    SuperSneaky24

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    Ah gotcha, so it's because of the large negative charge that makes it so reactive. Thanks man! Lol I feel so stupid because I know the really complex stuff pretty well, but the simple conceptual things just bother me.
     
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