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
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.
To determine the standard potential of a half cell, you can use the standard reduction potential table. The standard reduction potential of a half cell is the measure of the tendency of a species to be reduced, relative to the standard hydrogen electrode (SHE), with a potential of 0.00 V.
a) To find the standard potential of the half cell reaction: Br2 + 2e- => 2Br-
Step 1: Identify the species involved in the half reaction.
In this case, the species involved are Br2 (bromine) and Br- (bromide ion).
Step 2: Look up the reduction potentials for each species.
Using the standard reduction potential table, you can find the reduction potential for the reduction of Br2 to Br-. The half-cell potential for Br2 + 2e- => 2Br- is +1.09 V.
b) To find the standard potential of the half cell reaction: Al^+3 + 3e- => Al
Step 1: Identify the species involved in the half reaction.
In this case, the species involved are Al^+3 (aluminum ion) and Al (aluminum metal).
Step 2: Look up the reduction potentials for each species.
Using the standard reduction potential table, you'll see that the reduction potential for the reduction of Al^+3 to Al is -1.66 V.
So, the standard potential for the half cell reaction: Al^+3 + 3e- => Al is -1.66 V.