First slide

Standard EMF of a cell

Question

Given, $E_{{Ni}^{2 +} / Ni = 0 .25 V}^{\circ}, E_{{Cu}^{2 +} / Cu = 0 .34 V}^{\circ} E_{{Ag}^{+} / Ag = 0 .80 V}^{\circ}$ and $E_{{Zn}^{2 +} / Zn = - 0 .76 V}^{\circ}$
Which of the following reactions under standard condition will not take place in the specified direction?

Easy

A

${Ni}^{2 +} (aq) + Cu (s) ⟶ Ni (s) + {Cu}^{2 +} (aq)$
B

$Cu (s) + 2 {Ag}^{+} (aq) ⟶ {Cu}^{2 +} (aq) + 2 Ag (s)$
C

$Cu (s) + 2 H^{+} (aq) ⟶ {Cu}^{2 +} (aq) + H_{2} (g)$
D

$Zn (s) + 2 H^{+} (aq) ⟶ {Zn}^{2 +} (aq) + 3 H_{2} (g)$

Solution

Because $E_{cell}^{o}$ is not positive only for these reaction.
$\begin{array}{l} E_{cell}^{\circ} = {E_{{Ni}^{2 +} / Ni}^{\circ}}^{0}^{\circ} {Cu}^{2 +} / Cu \\ = 0 .25 - 0 .34 = - 0 .09 V \end{array}$
Similarly, $\begin{array}{l} E_{cell}^{\circ} = E_{H^{+} / H_{2}}^{\circ} - E_{{Cu}^{2 +} / Cu}^{\circ} \\ = - 0 .34 V \end{array}$

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