G-chem, EMF?

Dencology

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    Hg2+ + 2e- --> Hg Eo = 0.85

    Zn2+ + 2e----> Zn Eo= -0.76

    115. Given the cell potentials shown above, the equilibrium constant at 298 K for the reaction
    Zn + Hg2+ --> Zn2+ + Hg

    Is closest to which of the following?


    • 2.5 × 1054
    • 54
    • 1.6
    • 1.8×102-

    My answer would be C. but the answer is A. how?



    X+ + e- ---> X(s) Eo= -2.174

    11. For the half rxn above, which of the following is a correct statement?


    • X+ is readily reduced
    • X+ is a good oxidizing agent
    • X is a poor reducing agent
    • X is a good oxidizing agent
    • X is readily oxidized

    Here we see that X+ is being reduced, yet the Eo shows an oxidizing rxn (because it is negative value), would this be the reason.

    My answer is A but the correct ans. is E. how come?
     
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    peyman2002

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      For 115 I think choice C is correct.

      For #11 choice E is correct b/c the E0 for the rxn shown is a big negative number and as u know the more positive an E0 is the better its ability to oxidize hence a better oxidizing agent, so in our case the E0=-2.174 thus its a good reducing agent which again means that it readily oxidizes. you can think of X(s) as an element like Li(s) which really wants to get rid of one electron and become oxidized.
       

      zuma35

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        1.6 would be the Cell Voltage. The question asked for the equilibrium constant. That would be found using the Nerst equation, which I believe is not necessary for the DAT. Google it if you wish.

        For 2. the negative E0 means that it is nonspontaneous. This is the reduction and therefore X+ is poorly reduced. However, any nonspontaneous reaction becomes spontaneous when you flip it. So flipping the reaction we see that X is oxidized spontaneously.
         

        Dencology

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          For 115 I think choice C is correct.

          For #11 choice E is correct b/c the E0 for the rxn shown is a big negative number and as u know the more positive an E0 is the better its ability to oxidize hence a better oxidizing agent, so in our case the E0=-2.174 thus its a good reducing agent which again means that it readily oxidizes. you can think of X(s) as an element like Li(s) which really wants to get rid of one electron and become oxidized.


          so i was right when i wrote that the reason the answer is E is because of the Eo, it is negative so it means it is being oxidized.
           
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          zuma35

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            so i was right when i wrote that the reason the answer is E is because of the Eo, it is negative so it means it is being oxidized.

            A negative Eo does not mean that it is being oxidized. There can be a negative Eo when a reduction is spontaneous also. Here, since the the reduction half reaction happens to have a negative Eo, we know that the oxidation would have a positive, and would hence be spontaneous.
             
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