chemistry

if you're given a chart of half cell potentials (reduction potentials), how can you figure out which metals produce the highest voltage?

For ex.
Cadmium (Cd) with copper (Cu)
Silver (Ag) with zinc (Zn)
Lead (Pb) with zinc (Zn)

would you say that pb and zn produce the highest voltage because they result in the biggest difference? even though pb and zn have the largest negative voltage.

what about for:
Silver (Ag) with copper (Cu)
Copper (Cu) with lead (Pb)
Nickel (Ni) with cadmium (Cd)

i think it would be ag and cu for the same reason as above.

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  1. similarly, from the chart, how can you determine which combinations of metals (if used to create an electrochemical cell) produce the largest voltage?

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  2. do i have to see which of the two metals are bring reduced and oxidized to determine the voltage?

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  3. This is usually a trying subject for students and I'll answer the last post and ignore the others. I think that will be the best for all of these questions since all of them are related. So, "do i have to see which of the two metals are bring reduced and oxidized to determine the voltage?".

    Yes. As an example, do this. Suppose you have a cell consisting of Cu and Zn, how do you know which is the anode and which the cathode?
    Reduction potential for Cu is + 0.337
    Reduction potential for Zn is -0.763 or
    Cu^2+ + 2e ==> Cu Eo = +0.337
    Zn^2+ + 2e ==> Zn Eo = -0.763
    I think the texts say to choose the one with the largest negative voltage and reverse that equation. That would be
    Zn ==> Zn^2+ + 2e Eox = +0.763 and combine with Cu reduction
    Cu^2+ + 2e ==> Cu Ered = 0.337 then add the Eox + Ered
    ----------------------------------------------------
    Zn + Cu^2+ + 2e ==> Zn^2+ + Cu + 2e Etotal = 1.100 volts. Of course in the final equation the 2e on each side cancel. How do you know if this cell is possible? Because you get a POSITIVE voltage. A positive voltage tells you any reaction you've set up will react as written. A negative value will tell you the reaction will not go as written but will go in the opposite reaction.
    Which electrode is the anode and which the cathode? The definition of anode is "oxidation occurs at the anode". Which of the two are oxidized. Since oxidation is the loss of electrons the Zn metal is oxidized so Zn is the anode anode and Cu is the cathode.
    In practice I have my own method but it's no better than what the texts teach. I KNOW that the metal with the largest oxidation potential will be the anode so I mentally reverse those numbers. That tells me Zn will be the anode and Cu the cathode so I KNOW to write Zn as an oxidation equation and to write Cu as the reduction equation, then I add the two. Hope this helps. Another way is just go with luck; i.e., choose EITHER one, reverse it, leave the other equation alone, then add the two voltages (remember to change the sign of the one you reverse). If the cell is negative you reversed the wrong one so do the opposite. If the cell is + you made a lucky choice.

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    DrBob222

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