Question

why is O2 listed twice in the standard Electrode Potentials ?

I found both O2 + H3O+ + 4e <--> 6H20 and O2 + 2H20 + 4e- <--> 4OH- help please

Answer 1

it can be reduced to two from one case it can be reduced to H2O in acid media and in case of neutral media it can be reduced to OH- hence it has two different electrode potential
not only this you can find several examples here i have given for Tin :)
Sn4+ + 2 e−> Sn2+ +0.15
Sn4+ + 2 e− Sn2+ +0.15
Sn2+ + 2 e− Sn(s) −0.13
it doesn't mean that each element should have only one electrode potential but it also depends upon the oxidation state it is changed and the product it is formed in equilibrium
hope it helps you :)

Answer 2

thanks for answering, I understand now. I appreciate the help!!

Answer 3

you are welcome :)