Ncert Exercises & Solutions

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

$E^{⊝} values : {Fe}^{3 +} / {Fe}^{2 +} = + 0.77; I_{2} / I^{-} = + 0.54;$
${Cu}^{2 +} / Cu = + 0.34; {Ag}^{+} / Ag = + 0.80 V$

1. Fe³⁺ and I^-

2. Ag⁺ and Cu

3. Fe³⁺ and Cu

4. Ag and Fe³⁺

Hint:

E_{cell}^{o}

value must be positive

Calculate the value of

E_{cell}^{o}

for every options as follows:

(1)

2 {Fe}^{3 +} + 2 e^{-} \to 2 {Fe}^{2 +}; E ° = + 0.77 V

2 I^{-} \to I_{2} + 2 e^{-}; E ° = - 0.54 V (sign of E ° is reversed)

2 {Fe}^{3 +} + 2 I^{-} \to 2 {Fe}^{2 +} + I_{2}; E °_{cell} + 0.23 V

This reaction is feasible since

E °_{cell}

is positive.

(2)

Cu \to {Cu}^{2 +} + 2 e^{-}; E ° = - 0.34 V (sign of E ° has been reversed)

2 {Ag}^{+} + 2 e^{-} \to 2 Ag; E ° = + 0.80 V

Cu + 2 {Ag}^{+} \to 2 {Cu}^{2} + 2 Ag; E ° = + 0.46 V

This reaction is feasible since

E °_{cell}

is positive.

(3)

2 {Fe}^{3 +} + 2 e^{-} \to 2 {Fe}^{2 +}; E ° = + 0.77 V

Cu \to {Cu}^{2 +} + 2 e^{-}; E ° = - 0.34 V 2 {Fe}^{3 +} + Cu \to 2 {Fe}^{2 +} + {Cu}^{2 +}; E ° = + 0.43 V

(sign of E ° is reversed)

This reaction is feasible since

E °_{cell}

is positive.

(4)

Ag \to {Ag}^{+} + e^{-}; E ° = - 0.80 V (sign of E ° is reversed)

{Fe}^{3 +} + e^{-} \to {Fe}^{2 +}; E ° = + 0.77 V

Ag + {Fe}^{3 +} \to {Ag}^{+} + {Fe}^{2 +}; E ° = - 0.03 V

This reaction is not feasible since

E °_{cell}

is negative