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Thread: Electronegativity

  1. #1 Electronegativity 
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    I am fairly familiar with this concept but I don't understand how people use it to predict a reaction. Say for example you didn't KNOW that Mg(NO3)2 reacted with NaOH to make Mg(OH)2 and NaNO3, how using the different elements electronegativity do you work out that this reaction will occur and that there will be no reaction therefore between NaNO3 and Mg(OH)2??

    (Bonus question) How do scientists work out the electronegative values for each element empirically?

    Thanks very much
    09jnewington


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  3. #2  
    Bullshit Intolerant PhDemon's Avatar
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    I've never heard of it being used to predict a reaction (I would use Gibbs Free Energy). All electronegativity would tell you is about a tendency to form ions or polarity of bonds.

    It cannot be directly measured and is calculated, usually from bond dissociation energies or ionization energies. (Electronegativity - Wikipedia, the free encyclopedia)


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  4. #3  
    exchemist
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    Quote Originally Posted by 09jnewington View Post
    I am fairly familiar with this concept but I don't understand how people use it to predict a reaction. Say for example you didn't KNOW that Mg(NO3)2 reacted with NaOH to make Mg(OH)2 and NaNO3, how using the different elements electronegativity do you work out that this reaction will occur and that there will be no reaction therefore between NaNO3 and Mg(OH)2??

    (Bonus question) How do scientists work out the electronegative values for each element empirically?

    Thanks very much
    09jnewington
    There may be some misunderstanding here. Electronegativity is not used to predict the outcome of this reaction. These are all fully ionised substances, so when you mix them in aqueous solution the ions will in principle just float around independently. You have Mg++(aq), NO3 -- (aq), Na+(aq) and OH-(aq), all independent. BUT of course Mg (OH)2 is poorly soluble, so it precipitates out, leaving behind Na+ and NO3 --.

    Now you may ask why is it that Mg(OH)2 is insoluble. Well it must be that Mg and OH can get to a much lower energy state in the solid than in the aqueous phase. This will be to do with the efficiency of packing the ions in the crystal (size of the ions is important) and the resulting strength of the ionic attractive forces. This is about electrostatics, if you like, but not about electronegativity.

    Very much as PhDemon says, electronegativity, on the other hand, is the tendency to gain electrons. It is what predicts that an Mg atom will easily lose electrons to form a cation and that O and N atoms will tend to gain electrons, forming anions. Electronegativity also gives you an idea of the direction of dipoles (partial charge separation) in covalent molecules, which can be important, for example in organic chemistry.
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  5. #4  
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    at 2:30 he mentions chlorine reacting because it is more electronegative, this is what I was referring to specifically.
    Iodine (new and improved) (element #56) - YouTube
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  6. #5  
    Bullshit Intolerant PhDemon's Avatar
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    He seems to be using electronegativity in a hand waving way to explain the underlying energetics outlined by exchemist in post #3. I suppose it does give an "explanation" but it is firmly in the "lies to children" school of chemistry (like the octet rule and similar) and not strictly rigourous to my mind, it depends what level you are at but I would always rather go to the more fundamental thermodynamics for explanations.
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