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Electrochemistry question

  1. Feb 28, 2005 #1
    Hi people, I have the following question. Fe2+, Fe3+, Cr2O7(2-) and Cr3+ are contained in a solution in standard states with dilute sulphuric acid. Deduce what happens. I am wondering here what effect the sulphuric acid has, is it simply a catalyst for the reaction in which the Cr2O72- is reduced to Cr3+ and the Fe2+ is oxidised to Fe3+? Could someone check this for me, thx. Joe
  2. jcsd
  3. Feb 28, 2005 #2
    [tex]Fe^{2+} _{(aq)} + Fe^{3+} _{(aq)} + Cr_2 O_7 _{(aq)}^{2-} + Cr^{3+} _{(aq)} + H_2 SO_4 _{(aq)}[/tex]

    By the looks of it, I would say the sulphuric acid is a catalyst because all the other substances are ions and the sulphuric acid is not so is not really in a state to be used. If this is the case then I can see nothing wrong with your redox statements.

    However if it was not used as a catalyst, I think the sulpate ion from the sulphuric acid would bond with the [tex]Fe^{2+}[/tex] I haven't got time to see if this will change the oxidation states because I have to be out of the house in -3 minutes.

    Hope that might have helped. Gurus, enlighten us :smile:

    The Bob (2004 ©)
  4. Feb 28, 2005 #3


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    Staff: Mentor

    H2SO4 in aqueous solution is dissociated as well. So you have made a mistake at the beginning of your reasoning...

    Here goes the enlightenement:

    Sulphuric acid server two purposes.

    First, it is necessary in the solution containing Fe3+ to ensure pH is low enough to disallow precipitation of Fe(OH)3.

    Second, in order for the Cr2O72- ions to work as oxidizer H+ ions are necessary (so they are not catalysing reaction, they are a substrate), as the half reaction is

    [tex]Cr_2O_7^{2-} + 14H^+ +6e^- \rightarrow 2Cr^{3+} + 7H_2O[/tex]

    Fe2+ ions are oxidized to Fe3+.
  5. Feb 28, 2005 #4
    I realised what I had implied when I turned my PC off but I didn't have time to turn it on, change it and get to college on time.

    I do apologise.

    The Bob (2004 ©)
  6. Feb 28, 2005 #5


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    Simply balance the redox reaction equation, your gen chem text should teach you how to do this, it's very simple.
  7. Feb 28, 2005 #6


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    Overall, it may be that the addition of the acid is necessary to increase the conductivity of water; this is a redox reaction after all. This is my guess, though I'm not certain.
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