Question

The standard electrode potentials of

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

I - $Zn|Z{n}^{2+}||C{u}^{2+}|Cu$

II - $Zn|Z{n}^{2+}||A{g}^{+}|Ag$

III - $Cu|C{u}^{2+}||A{g}^{+}|Ag$

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

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

Answer 1

Answer: (B)

Given,

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

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

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

Cell reaction of (I) is

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

$E_{cell}^o=E_{red}^o(cathode)-E_{red}^o(anode)$

$$\begin{gathered} =0.34-\left(-0.76\right) \\ =1.10\text{V} \end{gathered}$$

Cell reaction of (II) is

$\text{Zn }+{\text{ 2Ag}}^{+}\to {\text{ Zn}}^{2+}+\text{ 2Ag}$

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

Cell reaction of (III) is

$\text{Cu }+{\text{ 2Ag}}^{+}\to {\text{ Cu}}^{2+}+\text{ 2Ag}$

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

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

II > I > III.