Question

Using the standard electrode potential, find out the pair between which redox reaction is not feasible.

E^Θ values: \[\ce{Fe^{3+} / Fe^{2+} = + 0.77; I2/I- = + 0.54}\];

\[\ce{Cu^{2+} / Cu = + 0.34; Ag+ / Ag = + 0.80 V}\]

Options

• \[\ce{Fe^{3+} and I-}\]

• \[\ce{Ag+ and Cu}\]

• \[\ce{Fe^{3+} and Cu}\]

• \[\ce{Ag and Fe^{3+}}\]

Answer 1

\[\ce{Ag and Fe^{3+}}\]

Explanation:

(i) \[\ce{2Fe^{3+} + 2e- -> 2Fe^{2+}; E^Θ = + 0.77 V}\]
                      \[\ce{2I- -> I2 + 2e- ; E^Θ = - 0.54 V (sign of E^Θ is revised)}\]
                                                                                           
     \[\ce{2Fe^{3+} + 2I- -> 2Fe^{2+} + I2; E^Θ_{cell} = + 0.23 V}\]

This reaction is feasible since \[\ce{E^Θ_{cell}}\] = + ve.

(ii)                \[\ce{Cu -> Cu^{2+} + 2e-; E^Θ = - 0.34 V (sign of E^Θ is revised)}\]
      \[\ce{2Ag+ 2e- -> Ag; E^Θ = + 0.80 V}\]
                                                                                               
     \[\ce{Cu + 2Ag+ -> Cu^{2+} + 2Ag; E^Θ = + 0.46 V}\]

This reaction is feasible since \[\ce{E^Θ_{cell}}\] = + ve.

(iii)  \[\ce{2Fe^{3+} + 2e- -> 2Fe^{2+}; E^Θ = + 0.77 V}\]
                           \[\ce{Cu -> Cu^{2+} + 2e-; E^Θ = - 0.34 V (sign of E^Θ is revised)}\]
                                                                                                    
          \[\ce{2Fe^{3+} + Cu -> 2Fe^{2+} + Cu^{2+}; E^Θ_{cell} = + 0.43 V}\]

This reaction is feasible since \[\ce{E^Θ_{cell}}\] = + ve.

(iv)                 \[\ce{Ag -> Ag^{+} + e-; E^Θ = - 0.80 V (sign of E^Θ is revised)}\]
     \[\ce{Fe^{3+} + 2e- -> Fe^{2+}; E^Θ = - 0.03 V}\]
                                                                                               
      \[\ce{Cu + 2Ag+ -> Cu^{2+} + 2Ag; E^Θ = + 0.46 V}\]

This reaction is not feasible since \[\ce{E^Θ_{cell}}\] = – ve.