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Homework Help: Selective Reduction

  1. Nov 23, 2007 #1
    1. The problem statement, all variables and given/known data

    Given: Sn4+ + 2e- --->Sn2+ Estandard=+0.15V
    For a galvanic cell under standard condition, which of the following anodic half reaction would produce at the cathode a spontaneous reduction of Sn4+ to Sn2+ but NOT Sn2+ to Sn.

    Pb --> Pb2+ + 2e-
    Sn2+ --> Sn4+ + 2e-
    Pb + SO42- --> PbSO4 + 2e-
    Sn --> Sn2+ + 2e-
    Fe --> Fe3+ + 3e-
    Ni --> Ni2+ + 2e-

    3. The attempt at a solution

    I found the oxidation potential for all of the above reactions:

    +0.126
    -0.139
    +0.355
    +0.141
    +0.036
    +0.236, respectively

    For a spontaneous reaction, the Ecell has to be positive. I calculated the Ecell for each of those reactions with Sn4+ + 2e- --->Sn2+ Estandard=+0.15V and with Sn2+ + 2e---->Sn

    I found that Pb --> Pb2+ + 2e-, Sn2+ --> Sn4+ + 2e-, and Fe --> Fe3+ + 3e- are the only three that produce a Ecell that is positive for Sn4+ to Sn2+ and NOT Sn2+ to Sn

    but for Sn --> Sn2+ + 2e- reacting with Sn2+ + 2e---->Sn, the Ecell is obviously equation to zero, would that considered to be spontaneous??

    can someone check if I am doing this right?

    Thanks a lot =)
     
  2. jcsd
  3. Nov 26, 2007 #2
    am I on the right track here?
     
  4. Nov 26, 2007 #3
    do u go to the UoW? :)
     
  5. Nov 26, 2007 #4
    what is "UoW"?
     
  6. Nov 26, 2007 #5
    lol i guess not


    by the way in ur questions do u have one like this..

    Selective Oxidation (not reduction)

    --------------------------------------------------------------------------------

    1. The standard reduction potential for the half-reaction

    Sn4+ + 2e- Sn2+
    is +0.15 V.
    Consider data from the table of standard reduction potentials for common half-reactions, in your text.

    For a galvanic cell under standard conditions, which of the following cathodic half reactions would produce, at the anode, a spontaneous oxidation of Sn to Sn2+ but not Sn2+ to Sn4+.

    Sn2+ + 2e- Sn
    Pb2+ + 2e- Pb
    2H2O + 2e- H2 + 2OH-
    PbSO4 + 2e- Pb + SO42-
    Fe2+ + 2e- Fe
    Fe3+ + 3e- Fe
     
  7. Nov 27, 2007 #6
    yeah, something similar to that.
     
  8. Nov 27, 2007 #7
    can someone please check what i'm doing wrong here... i'm still not getting the correct answer =(
     
  9. Nov 28, 2007 #8
    if the Ecell = 0 (in equilibrium) is that still considered to be spontaneous?
     
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