Given these standard reduction potentials at 25oC:

Question

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?

Answer 1

Add E1 to 2*E2 and divide by 3.

Answer 2

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

Answer 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)

Answer 4

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