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Homework Help: Nernst Equation Problem

  1. Mar 26, 2010 #1
    This problem is homework from a lab I did yesterday. Normally I would just ask my TA, but since it's homework I can't. I could email him, but I feel like I'll probably get a faster response if I post this here

    1. The problem statement, all variables and given/known data
    Use the Nernst equation to determine the cell potential for the cell involving Ag+. This is from a voltaic cell I made using a copper anode and CuSO4 solution and a silver cathode and AgNO3, with a KNO3 solution as a salt bridge. I used a multimeter to calculate the cell potentials for other cells I built (using other metal cathodes and relevant solutions) because their concentrations were 1M, standard conditions. The AgNO3 was 0.1M so I need to use the Nernst equation to calculate cell potential (assuming all other factors, ex. temperature were standard).

    Measured potential for the cell: -0.390V
    Given oxidation potential for the copper half reaction: +0.34V


    2. Relevant equations
    The Nernst Equation:

    E[tex]_{cell}[/tex] = E[tex]^{o}[/tex][tex]_{cell}[/tex] + 0.0592V(mol)/n * log[tex]_{10}[/tex]Q

    Half reaction at the cathode:

    Ag+(aq) + e- -> Ag(s), since [Ag+] = 0.1M, Q and n will both be 0.1

    3. The attempt at a solution
    Honestly, this comes down to me not understanding what the terms mean. I just started electrochemistry and I haven't been able to find a decent explanation of the terms online.

    I'm not sure whether I need to solve for E[tex]^{o}[/tex][tex]_{cell}[/tex] (I did and got -0.982V) or E[tex]_{cell}[/tex] (I got +0.202V). Solving for E[tex]^{o}[/tex][tex]_{cell}[/tex] makes more sense, given my answer, since I know that silver is a poor reducing agent. It should give a negative answer of fairly high magnitude in relation to the other cells, which it did.

    Basically what I'm asking is, E[tex]^{o}[/tex][tex]_{cell}[/tex] is the quantity I'm looking for...right? Is E[tex]_{cell}[/tex] the measured potential for a cell under non-standard conditions? Also, is my answer for E[tex]^{o}[/tex][tex]_{cell}[/tex] of a reasonable magnitude? Thanks.

    EDIT: Looks like my subscripts were messed up somehow. I'm pretty sure you'll be able to figure out what I meant though.
     
  2. jcsd
  3. Mar 27, 2010 #2

    Borek

    User Avatar

    Staff: Mentor

    No, you are looking for Ecell of a silver half cell. E0cell you can take from standard potential tables.

    Q is 0.1, that's OK, but n is not.

    Once you have Ecell of silver half cell, you can calculate E for whole cell.

    --
    methods
     
  4. Mar 28, 2010 #3
    Okay, so for the whole cell, n would be 1.1 mol then? My lab manual says that n is the total amount (in moles) of electrons transferred, so I just add the 0.1 to the 1 from the CuSO4, correct?
     
  5. Mar 28, 2010 #4

    Borek

    User Avatar

    Staff: Mentor

    No, n is amount of electrons transferred in the half cell reaction. It is always an integer.

    --
     
  6. Mar 28, 2010 #5
    Ah okay, looking back on it that makes a lot more sense than what I have. Thanks a lot for the help.
     
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