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Can anyone explain WHY acids dissociate?

  1. Feb 2, 2004 #1

    jimmy p

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    Can anyone explain WHY acids dissociate?
     
  2. jcsd
  3. Feb 2, 2004 #2

    ShawnD

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    Stronger acids consist of hydrogen(s) having ionic bonds with non-metals or polyatomic ions (which are mostly non-metal). Ionic bonding means that the non-metal somewhat takes the electrons off the hydrogen and the hydrogen is attracted to the non-metal because the hydrogen becomes positive and the non-metal becomes negative. Ions follow the same rules as every other charges in the universe - more positive and more negative mean more attraction.
    When an acid is dissolved in water, the electron on the hydrogen has already been pulled off. Now as for why it doesn't just stay bonded with the non-metal; look at how electronegative the non metals are.
    http://members.shaw.ca/mkorney/c214pdf04/pertab.PDF (scroll down)

    As you can see, Oxygen is the most electronegative with the exception of fluorine (which makes weird acids). The hydrogen will move from the non-metal it was joined with and go to the oxygen on the water making H3O+.
     
  4. Feb 2, 2004 #3

    GCT

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    The dissociation of an arrehnius acid commonly pertains to the acid in water. There are two basic factors in the degree of dissociation of a proton-the bond dissociation energy and the degree of polarity. As you can imagine, the greater polarity the bond associated with hydrogen the more "proton-like" the hydrogen becomes. For instance, chlorine bonded to hydrogen...the hydrogen has a fairly large positive charge. Water can surround this charge, and since the work of developing the charge has already been done, water can effectively dissociate the proton. Other factors include entropical issues which I will not go into here.
     
  5. Feb 2, 2004 #4

    jimmy p

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    ok so dissociation occurs because when an acid is in solution, the oxygen in the water is more electronegative so attracts hydrogens from the acids. Is the amount of dissociation balanced, eg there are the same amounts of H3O+ and X- ions?

    Also what happens in the case of fluoride acids?
     
  6. Feb 2, 2004 #5

    GCT

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    Flourine is always an exception in its group, the first element of each column usually is an exception.

    Yes, HF has a large dipole moment (polarity), yet it has a very high bond dissociation energy, this offsets the polarity factor.

    Just keep in mind the two factors which I mentioned on the former post. Yes, water helps in the dissociation of a proton and formation of other ions but the oxygen being more electronegative is not a factor. It stabilizes charges that develop for instance the negatively charged conjugate base of the acid. So in a sense, it helps with the dissociation by stabilizing the developing charges.

    Yes.
     
  7. Feb 3, 2004 #6

    jimmy p

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    So how do you distinguish between weak and strong acids?
     
  8. Feb 3, 2004 #7
    Strong acids are hydrogens attached to the highly electronegative halogens, in addition to:

    H2SO4 -> sulphuric acid
    HNO3 -> nitric acid
    H3PO4 -> phosphoric acid

    And some others I'm sure.
    Weak acids are pretty much everything else, mainly organic acids like acetic acid, citric acid, etc. Basically acids with carbon and oxygen are weak.
     
  9. Feb 3, 2004 #8

    GCT

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    Again, there are two main ways we can judge the acidity of a particular proton from its location in a molecular structure. You should review my first post. I don't think that you will need to worry about this too much until you get to organic chemistry, judging the acidity from molecular structure can be a bit complicating.

    Strong acids (referring to arrehnius acids) dissociate completely in water. Weak acids do not dissociate completely and thus an equilibrium constant is applicable.
     
  10. Feb 3, 2004 #9

    jimmy p

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    i kinda get that much because i have studied that, what i mean is in terms of dissociation how can you tell the difference? Do strong acids dissociate more or something?
     
  11. Feb 3, 2004 #10

    GCT

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    Strong acids dissociate completely meaning that all of the acid in the sample dissociates in water. For weak acids, only a part of the sample dissociates and equilibrium constant (which is constant for a certain weak acid at certain temperature) is

    [H3O][A-]/[HA]

    Hope this answers your question.
     
  12. Feb 4, 2004 #11

    jimmy p

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    Thanks for your help both of you, that makes it a lot easier. I like it when people OTHER than my teachers explain things. Different perspective i suppose.
     
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