Find stndrd potential of 0.5 rcn using stndrd delta G values

[Rachael_1H]

When calculating the standard potential for a half reaction, such as U4+ + 4e- ---> U, why do you have to convert the standard potentials of the half reactions U4+ + e- ---> U3+ and U3+ + 3e- ---> U to their standard delta G values, and then add their standard delta G values to find the standard potential of the desired half reaction? Why cant you just add the two different half reactions' standard potentials?

[Chem_Mod]

cell potential is not a state function so we need to convert that to gibbs free energy(which is a state function) to calculate the overall cell potential.