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Thread: EMF involving H+ ions?

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    EMF involving H+ ions?

    I can do basic questions asking to calculate EMFs such as

    Sn + Mg2+ → Sn2+ + Mg

    where you'd just write the 2 half equations and then use the table of standard potentials but the ones involving water make no sense to me at all. For example,

    MnO2 + 2I- → Mn2+ + I2 + 2H2O

    I can get as far as writing the 2 half equations as

    2I- → I2 + 2e- and
    MnO2 + 4H+ + 2e- → Mn2+ + 2H2O

    but then I'm unsure as to how you'd get a value for the second half equation involving H+ ions. Help is greatly appreciated ^^

    Edit: Oh the answers state the final answer is +0.68V btw

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    Junior Member trecex1's Avatar
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    Re: EMF involving H+ ions?

    That second equation is on the standard reduction potentials. MnO2 is reduced, I- is oxidised so just calculate it as usual.

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    Re: EMF involving H+ ions?

    Quote Originally Posted by trecex1 View Post
    That second equation is on the standard reduction potentials. MnO2 is reduced, I- is oxidised so just calculate it as usual.
    So I disregard the H2O part??

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    Junior Member trecex1's Avatar
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    Re: EMF involving H+ ions?

    Quote Originally Posted by Treedom View Post
    So I disregard the H2O part??
    What do you mean disregard? That's part of the half reaction.
    The reduction potential of MnO2 + 4H+ + 2e- → Mn2+ + 2H2O is 1.22 V
    The potential for 2I- → I2 + 2e- is -0.54 V (assuming it's solid otherwise you won't get your answer)
    If you add these you get 0.68 V

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    Re: EMF involving H+ ions?

    Quote Originally Posted by trecex1 View Post
    What do you mean disregard? That's part of the half reaction.
    The reduction potential of MnO2 + 4H+ + 2e- → Mn2+ + 2H2O is 1.22 V
    The potential for 2I- → I2 + 2e- is -0.54 V (assuming it's solid otherwise you won't get your answer)
    If you add these you get 0.68 V
    How did you get 1.22V for MnO2 + 4H+ + 2e- → Mn2+ + 2H2O?? This equation isn't present on the standard potentials data sheet?

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    Re: EMF involving H+ ions?

    Trecex1, out of curiosity did u do industrial chem?
    Want to try some discovery essay questions:http://community.boredofstudies.org/...ml#post7194501
    Quote Originally Posted by anomalousdecay View Post
    Discover your own stuff. Be proactive. Find your own path. We can't hold your hand. Cha Cha real smooth.

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    Junior Member trecex1's Avatar
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    Re: EMF involving H+ ions?

    Quote Originally Posted by Treedom View Post
    How did you get 1.22V for MnO2 + 4H+ + 2e- → Mn2+ + 2H2O?? This equation isn't present on the standard potentials data sheet?
    It's not but you can look it up - http://www.ed.gov.nl.ca/edu/k12/eval...potentials.pdf

    Quote Originally Posted by eyeseeyou View Post
    Trecex1, out of curiosity did u do industrial chem?
    yes

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