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Homework Help: Electrolysis, coating

  1. Apr 26, 2008 #1
    [SOLVED] Electrolysis, coating

    2. Relevant equation
    Why electrolysis of aqueous zinc sulfate is not used for coating with zinc metal?

    3. The attempt at a solution
    The following reaction has more positive electrode potential: (SO4)2- + 4H+ + 2e => H2SO3 + H2O than the reaction 2H+ + 2e- => H2(g); so H2SO3 would form at cathode. However, the right answer is that H+(aq) discharged preferentially because cations in solution =H+(aq),Zn2+(aq).
    Do you know why?

    Thank you in advance!
    Last edited: Apr 26, 2008
  2. jcsd
  3. Apr 26, 2008 #2


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    Cations assist in transfer of the proton throughout the solution. It's similar to the notion that water conducts electricity better when it is more "ionic".
  4. Apr 27, 2008 #3
    Do you mean that I cannot look straight away at the substance of greatest electrode potential?
  5. Apr 27, 2008 #4


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    This question seems to be rather simple - when you have cations in solution it attracts the negatively charged sulfate

    In addition this latter reaction requires 4 molar equivalence of protons compared to only 2 for the other reaction.

    What are the respective values of the electrode potentials for both reactions and also please inform us on the exact answer for this problem. It seems to me that the conversion of the proton to hydrogen gas would be more favored as the formation of sulfuric acid from sulfate is not favored in many circumstances.
  6. Apr 27, 2008 #5
    Standard Electrode Potentials:

    (SO4)2- + 4H+ + 2e => H2SO3 + H2O, E = +0.17V

    2H+ + 2e- => H2(g), E = +0.00V

    Exact answer:
    Cations in the solution are H+(aq) and Zn2+(aq).
    => H+(aq) discharged preferentially (I do not understand this in the light of above potentials.)
    Last edited: Apr 27, 2008
  7. Apr 27, 2008 #6


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    What the answer is referring to is comparing the zinc reactions with that of the proton - in particular note the standard reduction potential of reducing zinc 2+ .

    Remember , this isn't a galvanic pathway it is an electrolytic pathway.
  8. Apr 27, 2008 #7
    Thanks! Now, I understand. The key is the aqueous solution and electrolytic cell.
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