Using the standard electrode potential, find out the pair between which redox reaction is not feasible.
E°Values: Fe3+/ fe2+ = +0.77; I2/I- = +0.54;
Cu2+/ Cu = -0.34; Ag+ /Ag = + 0.80 V
Feasibility of a redox reaction is decided by the electric potential of the cell made out of two elements. So the net E°cell for the cell should be positive. The relative position of Ag and Fe3+ in electrochemical series and the reduction potential of cell should be determined for all cases.
(i) 
The 
Since the value is positive the reaction proceeds spontaneously and is feasible.
∴ We can calculate for remaining three reduction potential similarly,
(ii) 
(iii) 
(iv) 
Since the value of potential of cell for Ag and Fe3+ is negative we can say that the reaction is non-spontaneous and is not feasible as the redox reaction resists undergoing changes under mentioned configuration of Ag which undergoes oxidation, as its emf is less as compared to Fe3+, it readily undergoes reduction. Thus reaction in Option (iv) is not feasible.