First slide

Standard EMF of a cell

Question

Given below are the half-cell reactions,
$\begin{array}{l} {Mn}^{2 +} + 2 e^{-} ⟶ Mn; E^{\circ} = - 1 .18 eV \\ 2 ({Mn}^{3 +} + e^{-} ⟶ {Mn}^{2 +}); E^{\circ} = + 1 .51 eV \end{array}$
The $E^{\circ} for 3 {Mn}^{2 +} ⟶ Mn + 2 {Mn}^{3 +}$ will be

Difficult

A

-2.69 V, the reaction will not occur
B

-2.69 V, the reaction will occur
C

-0.33 V, the reaction will not occur
D

-0.33 V, the reaction will occur

Solution

If $E_{cell}^{o}$ is positive, reaction is spontaneous, otherwise it is non-spontaneous.
$\begin{array}{l} {Mn}^{2 +} + 2 e^{-} ⟶ Mn; E^{\circ} = - 1 .18 V \\ 2 {Mn}^{2 +} ⟶ 2 {Mn}^{3 +} + 2 e^{-}; E^{\circ} = - 1 .51 V \\ 3 {Mn}^{2 +} ⟶ Mn + 2 {Mn}^{+ 3} \\ E^{\circ} = E_{oxi}^{\circ} + E_{red}^{\circ} = - 1 .51 + (- 1 .18) = - 2 .69 V \end{array}$
Negative $E_{cell}^{o}$ of the reaction indicates that reaction is non-spontaneous.

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