Given these standard reduction potentials at 25oC:

Cr3+ + e- -> Cr2+ (E1^o = -0.407V)
Cr2+ + 2e- -> Cr(s) (E2^o = -0.913V)

Determine the standard reduction potential at 25oC for the half-reaction equation:
Cr3+ + 3e- -> Cr(s)

This seems like a really simple problem (just simply add E1 and E2 together), but conversely the value seen in tables for that final reaction -0.74V. Which way is the right one to go?

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  1. Add E1 to 2*E2 and divide by 3.

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  2. Thanks for the help! But I'm curious - why multiply E2 by 2, and then divide the answer by 3?

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  3. First convert both E^o to dG values then you can add them and reconvert the dG value back to E^o
    use n=1 for E1
    n=2 for E2
    and n=3 for Etotal
    (n=number of electrons transfered)

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  4. - convert E for each half reaction into Delta G
    - add the Delta G values
    - convert back to E
    - answer is -0.744V

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