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The standard reduction potentials of $\mathrm{Zn}^{2+}\left|\mathrm{Zn}, \mathrm{Cu}^{2+}\right| \mathrm{Cu}$ and $\mathrm{Ag}^{+} \mid \mathrm{Cu}$ and $\mathrm{Ag}^{+} \mid \mathrm{Ag}$ are respectively $-0.76,0.34$ and $0.8 \mathrm{~V}$. The following cells were constructed :
(1) $\mathrm{Zn}\left|\mathrm{Zn}^{2+}\right|\left|\mathrm{Cu}^{2+}\right| \mathrm{Cu}$
(2) $\mathrm{Zn}\left|\mathrm{Zn}^{2+}\right|\left|\mathrm{Ag}^{+}\right| \mathrm{Ag}$
(3) $\mathrm{Cu}\left|\mathrm{Cu}^{2+} \| \mathrm{Ag}^{+}\right| \mathrm{Ag}$
What is the correct order of $E_{\text {cell }}^{\circ}$ of these cells?
Options:
(1) $\mathrm{Zn}\left|\mathrm{Zn}^{2+}\right|\left|\mathrm{Cu}^{2+}\right| \mathrm{Cu}$
(2) $\mathrm{Zn}\left|\mathrm{Zn}^{2+}\right|\left|\mathrm{Ag}^{+}\right| \mathrm{Ag}$
(3) $\mathrm{Cu}\left|\mathrm{Cu}^{2+} \| \mathrm{Ag}^{+}\right| \mathrm{Ag}$
What is the correct order of $E_{\text {cell }}^{\circ}$ of these cells?
Solution:
1150 Upvotes
Verified Answer
The correct answer is:
$2>1>3$
Given
$\mathrm{Zn}^{2+} \longrightarrow \mathrm{Zn} \quad E^{\circ}=-0.78 \mathrm{~V}$
$\mathrm{Cu}^{2+} \longrightarrow \mathrm{Cu} \quad E^{\circ}=0.34 \mathrm{~V}$
$\mathrm{Ag}^{+} \longrightarrow \mathrm{Ag} \quad E^{\circ}=0.8 \mathrm{~V}$
Cell reaction of (1) is
$\mathrm{Zn}+\mathrm{Cu}^{2+} \longrightarrow \mathrm{Zn}^{2+}+\mathrm{Cu}$
$E_{\text {cell }}^{\circ}=E_{\text {oxidation }}+E_{\text {reduction }}$
$=+0.78+0.34$
$=+1.12 \mathrm{~V}$
Cell reaction of (2) is
$\mathrm{Zn}+\mathrm{Ag}^{+} \longrightarrow \mathrm{Zn}^{2+}+\mathrm{Ag}$
$E_{\text {cell }}^{\circ}=0.78+0.8$
$=+1.58 \mathrm{~V}$
Cell reaction of (3) is :
$\mathrm{Cu}+\mathrm{Ag}^{+} \longrightarrow \mathrm{Cu}^{2+}+\mathrm{Ag}$
$E_{\text {cell }}^{\circ}=-0.34+0.8$
$=+0.46$
So, the correct order of $E_{\text {cell }}^{\circ}$ of these cell is -
$\mathrm{b}>\mathrm{a}>\mathrm{c} .$
$\mathrm{Zn}^{2+} \longrightarrow \mathrm{Zn} \quad E^{\circ}=-0.78 \mathrm{~V}$
$\mathrm{Cu}^{2+} \longrightarrow \mathrm{Cu} \quad E^{\circ}=0.34 \mathrm{~V}$
$\mathrm{Ag}^{+} \longrightarrow \mathrm{Ag} \quad E^{\circ}=0.8 \mathrm{~V}$
Cell reaction of (1) is
$\mathrm{Zn}+\mathrm{Cu}^{2+} \longrightarrow \mathrm{Zn}^{2+}+\mathrm{Cu}$
$E_{\text {cell }}^{\circ}=E_{\text {oxidation }}+E_{\text {reduction }}$
$=+0.78+0.34$
$=+1.12 \mathrm{~V}$
Cell reaction of (2) is
$\mathrm{Zn}+\mathrm{Ag}^{+} \longrightarrow \mathrm{Zn}^{2+}+\mathrm{Ag}$
$E_{\text {cell }}^{\circ}=0.78+0.8$
$=+1.58 \mathrm{~V}$
Cell reaction of (3) is :
$\mathrm{Cu}+\mathrm{Ag}^{+} \longrightarrow \mathrm{Cu}^{2+}+\mathrm{Ag}$
$E_{\text {cell }}^{\circ}=-0.34+0.8$
$=+0.46$
So, the correct order of $E_{\text {cell }}^{\circ}$ of these cell is -
$\mathrm{b}>\mathrm{a}>\mathrm{c} .$
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