Question

Assuming that this half cell I2 + 2e- + =>2I- as standard electrode and assumed its potential that is 0.00 V :
a) what is standard potential of this half cell : Br2 + 2e- + => 2Br-

b) what is standard potential of this half cell : Al^+3 + 3e- => Al

Answer 1

Look up the standard reduction potentials for I2, Br2, and Al^3+. Then set them up on a sheet of paper something like this, along with H2.
..........................Eo vs SHE
Br2 + 2e --> 2Br^-..........1.07

I2 + 2e --> 2I^- ...........0.54

2H^+ + 2e --> H2............0.00

Al^3+ + 3e --> Al..........-1.66

So I2 is 0.54 above H2, Br2 is 1.07 above H2 and Al^3+ is 1.66 below H2.

If you call I2 0.00, then Br2 is just 1.07-0.54 = 0.53 above I2 and that would be the Eo value vs I2 as 0.00. Then Al^3+ is 1.66 to get to H2 and another 0.54 to get to I2; therefore, on this scale Al^3+ will be 1.66 + 0.54 = 2.20 below I2 and it's Eo value will be -2.20v.