Question

If E° Fe²⁺ /Fe = -0.441V and E° Fe³⁺ / Fe²⁺ = +771V,
the standard EMF of the reaction
Fe + 2Fe³⁺ ® 3Fe²⁺ will be
(a) 1.653 V (b) 1.212 V
(c) 0.111 V (d) 0.330 V

Answer 1

Answer (d) 0.330 V

Explanation:

According to me, the value of $E^{o}$ in question for equation (ii) should be +0.771 V.

$E^{o}_{cell}=E^{o}_({cathode})-E^{o}_({anode})$

Write the half cell and standard potential value ($E^{o}$) given in question

$Fe^{+2}+2e^{-}\rightarrow Fe$          $E_{1}^{o}=-441V$       ......(i)

$Fe^{+3}+e^{-}\rightarrow Fe^{+2}$        $E_{2}^{o}=+0.771V$    ......(ii)

Calculate $E_{3}^{o}$ for overall reaction,

$Fe+2Fe^{+3}\rightarrow 3Fe^{+2}$    $E_{3}^{o}$=?       .......(iii)

To obtain equation (iii), multiply equation (ii) by 2 and reverse the equation (i)  .On multiplying any factor in half cell reaction, there is no change in $E^{o}$ value.

$Fe\rightarrow Fe^{+2}+2e^{-}$             $E_{1}^{o}=+441V$         At Anode

$2Fe^{+3}+2e^{-}\rightarrow 2Fe^{+2}$    $E_{2}^{o}=+0.771V$        At Cathode

$E^{o}_{cell}=E^{o}_({cathode})-E^{o}_({anode})$

$E_{3}^{o}=E_{2}^{o}-E_{1}^{o}$

$E_{3}^{o}=0.771V-0.441V$

$E_{3}^{o}=+0.330V$