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Chem Lab ID of Unknowns

  1. Oct 6, 2006 #1
    Chem Lab "ID of Unknowns"

    I had to find 5 unknown solutions in a recent chem lab and now I am doing the equations, but im not sure about two of them:

    Deep blue percipitate
    NH3 (aq) + CuSO4 (aq) -------> Cu(NH3)4 SO4 (s)

    brown heavy percipitate
    4NaI (aq) + 2CuSO4 (aq) -------> 2CuI (s) + I2 (s) + 2Na2SO4 (aq)

    Are these reactions correct? I got percipitates for both of them.
  2. jcsd
  3. Oct 6, 2006 #2
    Copper(II)Sulphate is a blue color, crystalises and often used to grow crystals. As is Copper(II)Nitrate.

    looks good :)

    You should show the oxidation states of the elements ( the metals ) because it is the key to the color. Metals in different oxidation states are often different colors. This is why they are used extensivley for titrations as dyes & indicators.
    Last edited: Oct 6, 2006
  4. Oct 6, 2006 #3
    Deep blue percipitate
    NH3 (aq) + CuSO4 (aq) -------> Cu(NH3)4SO4 (s)

    Should there be a + between the Cu(NH3)4 and SO4?

    brown heavy percipitate
    4NaI (aq) + 2CuSO4 (aq) -------> 2CuI (s) + I2 (s) + 2Na2SO4 (aq)

    Is the I2 solid or aqueous? What gives the brown color?
  5. Oct 6, 2006 #4
    No, i dont think so. Its a crystalline salt of copper, and its solid. The ions are packed in a lattice.

    Iodine is brown (and i think aqueous? I think you have mis-balenced that equation, not sure), the color comes from the metals electronic structure, absorption of photons of certain energies and not others means that it appears brown, it acts like a color filter.

    Changing the oxidation state of the metal or the electronic configuration through complex formation changes the frequency of photons that it absorbs, and so too its color.
  6. Oct 7, 2006 #5
    Iodine is brown, actually a very deep brown liquid. You are correct
  7. Oct 7, 2006 #6
    isn't iodine some kinda indicator like phenothayleine (mind the spelling) ?
  8. Oct 8, 2006 #7
    For the first reaction: you should not be expecting any precipitation - tetra-amino cuprate is indeed deep blue in colour but water soluble. The blue precipitate observed was a side reaction of Cu2+ with OH- as the solution become more and more basic on the addition of NH3:

    Cu2+(aq) + OH-(aq) -----> Cu(OH)2(s)

    Which is a also blue in colour.

    For the latter reaction:

    Something's werid... You should not expect a brown colour because none of the species in the reaction is brown. Iodine is purple, not brown (well, bromine is brown...). There should have been nothing more than a clear solution with white precipitate of CuI(s).
  9. Oct 10, 2006 #8
    Where did you get the OH- from? So the neither one of these reactions should be a percipitate?
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