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Homework Help: Galvanic Cells and Cl2 Gas

  1. Jun 7, 2010 #1
    When I was reading back over my book to re-write my notes, I saw the words, "The Cl2 is the electrode (cathode). It must be in a solution with its ions i.e Cl- has KCL."

    My question is, why does the Chlorine gas have to be in a solution with its ions? If anyone could answer this, it would be greatly appreciated.
     
  2. jcsd
  3. Jun 7, 2010 #2

    Borek

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    I am not sure if "must" is the best wording.

    What is the electrode reaction?
     
  4. Jun 7, 2010 #3
    It was between Iron and Chloride gas (Platinum was the electrode.) The iron electrode was in iron nitrate solution and the Chloride gas/ platinum was in Potassium Chloride.

    i.e

    Fe | Fe2+ || Cl2 | Cl-, Pt
     
  5. Jun 7, 2010 #4

    Borek

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    Even if at the beginning there are no chlorides, but just chlorine - what happens when the reaction starts?

    --
     
  6. Jun 7, 2010 #5
    It produces chloride gas? Would it be aqueous since it would be dissolved in a solution?
     
  7. Jun 8, 2010 #6

    Borek

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    No such thing as chloride gas. There is such a thing as chlorine gas - and that was present from the very beginning, wasn't it?
     
  8. Jun 8, 2010 #7
    Yes, so the chlorine gas reduces to form chloride.
     
  9. Jun 8, 2010 #8

    Borek

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    And now you have both chlorine and chlorides...

    Not that it means you need something that will reduce the chlorine. Without a reducing agent there should be just gaseous chlorine - although it is not as simple, as chlorine reacts with water, producing some amounts of hydrochloric and hypochlorous acid. So chlorides will be present always, but not because they "must" be present to allow reaction, rather they "will" be present as a product.

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  10. Jun 8, 2010 #9
    Ok thankyou.

    However, while I am still on the topic of chlorine; if it is in an aqueous solution (in a compound), it will never reduce in electrolysis since it is too dilute. That's what my notes say, and I just wanted to verify whether that is correct.

    The other exceptions to electrolysis are also that Group 1 ions can never oxidise as they are too stable and neither can polyatomic ions.
     
  11. Jun 8, 2010 #10

    Borek

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    Please elaborate, not sure what you mean. Chlorine in compounds is usually alreay reduced, and no idea why it is "diluted".

    Correct.

    Well, almost correct. That works OK as a first approximation. Counter examples do exist, but they are rare and they usually require rather exotic conditions.

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    methods
     
  12. Jun 8, 2010 #11
    Ok, I shall explain in more detail:

    Say, NaCl (aq) was being electrolysed. I am just going to focus on the Cl here. The Cl ion will not oxidise (apologies as I accidentally said reduced before) as it is not concentrated (?) enough so the water is oxidised instead forming an acidic aqueous solution and oxygen gas.
     
  13. Jun 8, 2010 #12

    Borek

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    Quite the opposite. If you will electrolyse solution of NaCl you will observe Cl2 evolving.

    Actually this is a little bit more complicated. If you look at tables of standard potentials, water should decompose first, so you should observe bubbles of oxygen. However, dfference between oxidation potentials is not that large, and oxygen evolution is notoriously slow, that means to speed up the reaction we need to apply higher potential than the one calculated from the standard potential and Nernst equation - this is called "overpotential". Standard (or formal) potential plus overpotential means we get into the area where chlorine gets oxidized - and chlorine evolves much easier. Net effect is that whenever you electrolyze brines instead of getting just oxygen you will get it mixed with chlorine.

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    methods
     
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