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Hydrogen bonding between water and other molecules.

  1. Sep 23, 2012 #1
    1. The problem statement, all variables and given/known data

    I have attached a solution to one of my homework questions that states "Draw hydrogen bonds that might be formed between a water molecule and the following compounds: a) H3PO4; b) PO43-; c) aniline; d) pyridine; e) ethylamine; f) ethyl ammonium ion; g) acetic acid."

    My question is specifically for H3P04 and Acetic acid. Why is it that in H3PO4, the H on the OH makes the hydrogen bond with the O of a water molecule, but in acetic acid, the lone electron pair on the OH makes the hydrogen bond with the H of a water molecule.

    Does it make a difference if I instead drew a hydrogen bond from the lone electron pair of the OH on H3PO4 and connected this to the H of water? Is it the same thing or is there a reason these are like this?


    2. Relevant equations



    3. The attempt at a solution
     

    Attached Files:

  2. jcsd
  3. Sep 23, 2012 #2

    AGNuke

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    Gold Member

    A Polarized Hydrogen atom makes Hydrogen bond with Polarized F, O, N and to some extent, Cl.

    Water molecule will form Hydrogen Bonds from both Oxygen and Hydrogen atoms as the species you mentioned, Phosphoric Acid and Acetic Acid, have both polarized O and H atom.

    But I feel that, in Acetic Acid, Water must form H-Bond from Doubly Bonded Oxygen atom and Acidic Hydrogen, as the O atom in C=O is more electronegative than O atom in C-O-H, hence having more electron density and tendency to attract partially positive H atom of water molecule.

    If there are polarized H and F/O/N atom, you can draw H-Bond with both of them, which you should.
     
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