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Homework Help: Electrolysis of sodium sulphate

  1. Aug 12, 2018 #1
    1. The problem statement, all variables and given/known data
    I want to know what happens at the anode and why it happens during the electrolysis of sodium sulphate.
    2. The attempt at a solution
    Na+ and H+ move towards cathode, H+ is discharged due to Electrode potential values. What happens to the SO42- ions and how is O2 produced at anode?
     
  2. jcsd
  3. Aug 12, 2018 #2

    Borek

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    Hint: just like nothing happens to Na+, nothing happens to SO42-.

    What else is present in the solution?
     
  4. Aug 12, 2018 #3
    OH- is present, but how does OH- convert to O2?
     
  5. Aug 12, 2018 #4

    Borek

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    Try to guess the reaction, there are no many possibilities.
     
  6. Aug 12, 2018 #5
    I'm guessing OH- breaks into O2- and H+ and then O2- loses an electron
     
  7. Aug 12, 2018 #6

    Borek

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    Don't go for a mechanism, just write and balance the half reaction.
     
  8. Aug 12, 2018 #7
    O2- + e gives O2
     
  9. Aug 12, 2018 #8
  10. Aug 12, 2018 #9

    Borek

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    No, you have to start with what is present in the solution.
     
  11. Aug 13, 2018 #10
    3OH- + 3e gives O2 + H2O ??
    Its seems counter-intuitive to write either SO42- and OH- that is present directly as O2
     
  12. Aug 13, 2018 #11

    Borek

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    Can you balance it? If not, it can't be a right reaction equation.

    But in general you are on the right track - start with OH-, put O2 on the right side and see what you can do to balance the equation. You can use any ions and substances that are already present in the solution.
     
  13. Aug 23, 2018 #12
    2H2O gives 4H+ + O2 + 4e- ?
     
  14. Aug 23, 2018 #13
    How do I guess nothing happens to SO42- in an exam? or predict O2 evolves from H2O?
     
  15. Aug 23, 2018 #14

    Borek

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    Will do, although commonly we rather assume it is OH- that is being oxidized (it nicely cancels out with H+ reduced on the other electrode).

    These are things you just have to remember. SO42- is quite stable in the solution and doesn't easily get involved in the redox reactions, electrolysis in water very often ends up with some oxygen and hydrogen being produced.
     
  16. Aug 23, 2018 #15
    Oh, alright. Thank you very much for your help :D
    P.S-I couldn't help but notice that PF has a great physics and mathematics community but a relatively inactive one in chemistry. E.g. you end up resolving most of my problems and some related to organic chemistry aren't resolved at all. Is there something like chemistryforums.com where I can look for help in case you aren't present here for a while?
    Thank you
     
  17. Aug 23, 2018 #16

    Borek

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    Make it chemical*
     
  18. Aug 23, 2018 #17
    Alright, Thank you :D
     
  19. Aug 25, 2018 #18

    epenguin

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    Pity if you have never seen it demonstrated experimentally, if you had you would remember. I can remember it was one of the very first demonstrations in chemistry lessons in my school a long long time ago, supposed to demonstrate that water is made out of a combination of hydrogen and oxygen. School teaching laboratories used to have a special apparatus for the purpose of showing this. If you are learning chemistry without a school laboratory, which I sense is increasingly common, this gross lack is these days compensated by YouTube, where you can actually see quite a lot of chemistry. For example here:



    A nice thing pointed out is how the volume of hydrogen is twice that of the oxygen - a very significant fact as you might know and might be worth following up immediately looking up the Gay-Lussac law and the kinetic theory of gases.I don't remember Mr Bayford pointing this out the first time I saw it, but that might be my fault. I think he did show the nature of the gases, collecting them in test tube and the one causing a glowing splint to light up brightly, the other making a pop when combusted.

    Many YouTube sites illustrate standard visible chemistry, a number on electrolysis of water – I just took a quick look
    Here is a nice one.


    You'll notice in all cases although they say they are electrolysing water they throw other stuff in. Like Mr Bayford those years ago poured some sulphuric acid in explaining that water was such a bad conductor that this was needed to make the current flow.I thought at the time that deprived the experiment of rigour, and he was just electrolysing sulphuric acid. But at least I remember it! :oldbiggrin:
     
  20. Aug 25, 2018 #19
    The trouble is quite the opposite, we have so many experiments here (typical Indian high-school) that it's hard to remember any of the experiments you do in the lab. I'll certainly look into it though, thanks :D
     
  21. Aug 25, 2018 #20

    epenguin

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    I'm relieved to hear it, and if there is a happy medium, at least your course is better than learning just from books. Many things you can learn later in life, but if you miss the sights and sounds of chemistry at school, it is almost never made up for later. I'm not saying Borek wrong by the way, But these demonstrations and revisiting your lab notes can give you a lot of hooks to latch memory on to.
     
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