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Homework Help: Electrochemical cell question - strip on Zn in contact with zn2+/CN solution

  1. Mar 12, 2012 #1
    The potential at which a metal is deposited in the electrowinning process can significantly affect the quality of the deposit. The potential itself can be dramatically affected by the addition of complexing agents and by adjusting the pH of the solution as the following calculations demonstrate. (assume 298 K)

    a) Calculate the potential of an electrode consisting of a strip of Zn in contact with a solution containing 0.01 mol Zn2+ and 3.0 mol CN‐ per litre. The ε° value for aquated Zn2+ is ‐0.76 V, and β = 1 x 10^9 for the Zn2+ / CN‐ equilibrium.

    b) ε = ε° ‐ (RT/nF) ln Q
    where Q is the reaction quotient. Also useful is that β= [Zn(CN)]/[CN]^2[Zn]

    n is the number of electrons transferred to transform Zn ion to Zn solid (2)

    c) I tried to do a RICE table using β = 10^9 = x / (0.01-x) (3-2x)^2. assuming that on the bottom, the x values are negligible. However, this isn't correct.

    I am given that ε = -0.76, RT/nF = 0.0128. The main problem in this computation is that I have no idea what the reaction quotient should be. I know it is usually based on the half cell equation, which would be ln (1/Zn2+), which I thought I could figure out using the RICE table...but given that the x is negligible.........arhghweoufwe!

  2. jcsd
  3. Mar 13, 2012 #2


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    Staff: Mentor

    Your approach looks OK to me. However, you should calculate not concentration of a complex, but concentration of a free ion - and put this free ion concentration the into Nernst equation.
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