Equilibrium Constant from Electrode Potentials

[zorro]

Homework Statement

Evaluate the equilibrium constant for the formation of triodide ion.

I₂ + I^- ------> I₃^-

a 298K, if E_I2|I-^o = 0.6197V and E_I3-|I-^o=0.5355V

I don't understand one thing - At equilibrium, ΔG=0 so that E^o_cell=0 (from the equation ΔG=-nfE) but E^o_cell is not zero for the above reaction. Where is my mistake?

 

[vaibhavsharma]

I don't understand one thing - At equilibrium, ΔG=0 so that E^o_cell=0 (from the equation ΔG=-nfE) but E^o_cell is not zero for the above reaction. Where is my mistake?

$$\Delta$$G^o = -nFE^o and $$\Delta$$G = -nFE are two different equations.You are combining them.At equilibrium $$\Delta$$G=0 doesn't
mean that E^o=0 but it means E=0 i.e. electric potential at that time is equal to zero.E^o is constant quantity for any reaction and if it becomes zero the reaction can't proceed.For calculation of equilibrium constant just use the above result in nerst equation ie at equilibrium

E = E^o - (0.059*logQ)/n = 0 [Q= equilibrium constant]

 

[zorro]

Thank you very much.