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General Chem—Telluric Acid

  1. Sep 4, 2014 #1

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    According to Wikipedia, the proper chemical formula for Telluric Acid is [itex]Te(OH)_6[/itex].

    From what I've learned in chemistry thus far, acids made from polyatomic ions within the same group follow a similar pattern. For example, Selenic Acid [itex]H_2SeO_4[/itex] is analogous to Sulphuric acid [itex]H_2SO_4[/itex]. If this is the case, why is it that Telluric Acid differs from this pattern? I would expect its chemical formula to be [itex]H_2TeO_4[/itex], but apparently this represents Metatelluric acid which is the analogue of sulfuric acid and "is unknown." (Wikipedia)

    Why is this the case?

    Thank you!
     
  2. jcsd
  3. Sep 4, 2014 #2

    jim mcnamara

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    from http://tellurium.atomistry.com/telluric_acids.html
    H2TeO4.2H2O can be written as Te(OH)6 making the water of hydration look like hydroxyl groups.
     
  4. Sep 5, 2014 #3

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    Thanks, Jim.

    So would ##H_2TeO_4\cdot2H_2O## then be considered a hydrate due to the presence of water molecules within it?

    In addition, how are we supposed to know that telluric acid is a hydrate? (i.e., that it has a ##nH_2O##)
     
    Last edited: Sep 5, 2014
  5. Sep 5, 2014 #4

    Borek

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    I feel like you are trying to describe reality with simplified formulas, which won't work.

    Hydrates often have well defined water molecules in the crystal lattice (and I believe that's the way you are trying to picture the telluric acid). But that's not the case here. We call observed substances "hydrates" because they differ by a water molecule, but they are actually different (although related) compounds. This is quite common - similar thing happens for example with sodium bisulfite and sodium metabisulfite.
     
  6. Sep 7, 2014 #5
    In Group IV, carbonic acid is [itex]H_2CO_3[/itex], but silicic acid is [itex]H_4SiO_4[/itex] - metasilicic acid [itex]H_2SiO_3[/itex] is polymeric.
    In Group V, nitric acid is [itex]HNO_3[/itex], but phosphoric acid is [itex]H_3PO_4[/itex] - metaphosphoric acid [itex]HPO_3[/itex] is polymeric.
    The reason both cases being that the bigger central atom prefers bonding to more oxygens.

    Same reasoning applies to tellurium and iodine - [itex]Te(OH)_6[/itex] has Te bonded to 6 equal oxygens, whereas metatelluric acids are polymeric structures where some oxygens are bonded to two tellurium atoms.
     
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