Dehydrating agents

  1. Why is conc H2SO4 , KOH and phosphorus pentoxide dehydrating agents?
    I dont understand how they work. Do they adsorb water , react with water to from new compounds or anything else ??? How do they dehydrate?
  2. jcsd
  3. Borek

    Staff: Mentor

    Both ways possible. Although this is a thin ice - the difference between physical and chemical change is blurry, so it is not always easy say what is happening.

    Classic approach is that first two of those you listed absorb (not adsorb) water without a chemical reaction, third reacts with water.
  4. The SO--4 anion has a very high negative charge density and so
    will very strongly attract the Hydrogen's end of the polar H2O molecule.
    If you pour conc. H2SO4 on paper the water is removed with
    only Carbon left.

    Note: Im not sure if you were aware that SO4 is very hydrophilic for the above reasons . If so ,then Borek answered your question.
    Last edited: Mar 27, 2012
  5. Borek

    Staff: Mentor

    That's not the way it works. Sulfuric acid is very strong and it protonates water converting it to ionic H3O+. H3O+ is no longer volatile.

    Edit: changed H3+ to H3O+, obvious mistake by me.
    Last edited: Mar 29, 2012
  6. Well Sulfuric acid being a strong acid and protonating water does not exclude the
    following : The Sulfate ion forms a hydration shell consisting of a symmetrical
    arrangement of 16 Hydrogen bonded water molecules -
    SO4-- (H2O)16

    So are you saying that it is
    If so then it looks like we are about back where we started
  7. Borek

    Staff: Mentor

    I guess it must be a function of how much water is added, but if you start with the concentrated acid I am sure first stage is protonation. Note that the 16 water molecules arrangement means 16 moles of water per mole of sulfuric acid - that's 3M solution, so diluted its drying properties are negligible (compare
  8. I think we are talking about different stages of a process:
    Sulfuric acid as a dehydrating acid, followed by a diprotic acid. Sucrose plus conc. Sulfuric acid.
    C12H22O11 --> conc.--> 12C + 11H2O
    + mixture: H2SO4 + H2O --> H3O+ + HSO4-
    HSO4- + H2O > H3O++ SO4--

    Then it is this Sulfate ion that is hydrated by hydrogen bonded water molecules

    Last edited: Mar 29, 2012
  9. Borek

    Staff: Mentor

    Yes and no - we are talking about different stages of dilution, but after we are past the first stage sulfuric acid is hardly a dehydrating agent. It is an effective dehydrating agent only when it is concentrated.

    Can you elaborate, I am not sure what you mean. Sounds to me like you are separating its dehydrating properties from its acidic properties. Water protonation is already an acid base reaction, so dehydrating properties are the effect of the sulfuric acid acidity.
  10. This is getting complicated. Is it correct to just say that the three above processes:
    dehydration , diprotonation and hydration can occur together.And in this order:dehydration first ,that is when the Sulfuric acid is most concentrated .
    Followed by diprotonation, followed by hydration. Just the same sequence in the above post.
    If it is possible to analyze the exact sequence here in terms of rates and changing concentrations ( the water produced from sucrose )
    then I am going to let you do it.
    Last edited: Mar 29, 2012
  11. Borek

    Staff: Mentor

    I don't think you can treat dehydration and protonation as separate processes. Protonation is the driving force behind dehydration, so they occur simultaneously. My bet is that in the case of sugars "free" proton from the sulfuric acid protonates -OH group, making it possible to remove water molecule from the sugar molecule (leaving a carbocation behind). Then this water molecule is protonated again.
  12. Thanks - I was wondering what the mechanism was on that.
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