Question

The standard electrode potentials of

$Z n^{2 +} \, \left|\right. \, Zn \, , \, C u^{2 +} \, \left|\right. \, Cu \, and \, A g^{+} \, \left|\right. \, Ag$ are respectively - 0.76, 0.34 and 0.8 V. The following cells were constructed

I - $Zn \, \left|\right. \, Z n^{2 +} \, \left|\right.\left|\right. \, \, C u^{2 +} \, \left|\right. \, Cu$

II - $Zn \, \left|\right. \, Z n^{2 +} \, \left|\right.\left|\right. \, \, A g^{+} \, \left|\right. \, Ag$

III - $Cu \, \left|\right. \, C u^{2 +} \, \left|\right.\left|\right. \, A g^{+} \, \left|\right. \, Ag$

What is the correct order of $\text{E}_{c e l l}^{^\circ }$ of these cells?

• A. II > III > I
• B. II > I > III
• C. I > II > III
• D. III > I > II

Answer 1

Answer: (B)

Given,

$\text{Zn}^{2 +} \, \text{+} \, \, \, \text{2e}^{-}\text{ } \rightarrow \, \text{Zn} \, , \, \text{E}^{^\circ }=-0.76 \, \text{V}$

$\text{Cu}^{2 +}\text{ +} \, \, \text{2e}^{-} \, \rightarrow \, \text{Cu} \, , \, \, \, \text{E}^{^\circ } \, = \, 0.34 \, \text{V}$

$\text{Ag}^{+}\text{ +} \, \, \text{e}^{-} \rightarrow \, \text{Ag }, \, \, \text{E}^{^\circ } \, = \, 0.8 \, \text{V}$

Cell reaction of (I) is

$Zn \, + \, C u^{2 +} \, \rightarrow \, Z n^{2 +} \, + \, Cu$

$E_{cell}^{o}=E_{red}^{o}\left(\right.cathode\left.\right)-E_{red}^{o}\left(\right.anode\left.\right)$

$= 0.34 - \left(- 0.76\right) \\ = 1.10 \, \text{V}$

Cell reaction of (II) is

$\text{Zn } + \text{ 2Ag}^{+} \, \rightarrow \text{ Zn}^{2 +} \, + \text{ 2Ag}$

$\text{E}_{\text{Cell}}^{o} = + 0.80 - \left(- 0.76\right) = + 1.56 \, \text{V}$

Cell reaction of (III) is

$\text{Cu } + \text{ 2Ag}^{+} \, \rightarrow \text{ Cu}^{2 +} \, + \text{ 2Ag}$

$\text{E}_{\text{Cell}}^{o} = + 0.80 - \left(0.34\right) = + 0.46 \, \text{V}$

So, the correct order of $\text{E}_{c e l l}^{^\circ }$ of these cell is

II > I > III.