NEET UG 2022 — Chemistry Physical Chemistry

2022mcqeasy

At $298K$ , the standard electrode potentials of ${\mathrm{Cu}}^{2+}/\mathrm{Cu},{\mathrm{Zn}}^{2+}/\mathrm{Zn},{\mathrm{Fe}}^{2+}/\mathrm{Fe}$ and ${\mathrm{Ag}}^{+}/\mathrm{Ag}$ are $0.34V$ , $-0.76V$ , $-0.44V$ and $0.80V$ , respectively.

On the basis of standard electrode potential, predict which of the following reaction cannot occur?

Official previous-year question

Held on 30 Apr 2022 · Verified 9 Jul 2026.

Options

A
${\mathrm{CuSO}}_{4}(\mathrm{aq})+\mathrm{Fe}(s)\rightarrow {\mathrm{FeSO}}_{4}(\mathrm{aq})+\mathrm{Cu}(s)$
B
${\mathrm{FeSO}}_{4}(\mathrm{aq})+\mathrm{Zn}(s)\rightarrow {\mathrm{ZnSO}}_{4}(\mathrm{aq})+\mathrm{Fe}(s)$
C
$2{\mathrm{CuSO}}_{4}(\mathrm{aq})+2\mathrm{Ag}(s)\rightarrow 2\mathrm{Cu}(s)+{\mathrm{Ag}}_{2}{\mathrm{SO}}_{4}(\mathrm{aq})$
D
${\mathrm{CuSO}}_{4}(\mathrm{aq})+\mathrm{Zn}(s)\rightarrow {\mathrm{ZnSO}}_{4}(\mathrm{aq})+\mathrm{Cu}(s)$

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Solution

The given standard reduction potentials of $\mathrm{Zn},\mathrm{Cu},\mathrm{Ag}$ and $\mathrm{Fe}$ are;

$E{^{\circ}}_{\frac{{\mathrm{Zn}}^{2+}}{\mathrm{Zn}}}=-0.76V,E{^{\circ}}_{\frac{{\mathrm{Cu}}^{2+}}{\mathrm{cu}}}=0.34V E{^{\circ}}_{\frac{{\mathrm{Ag}}^{+}}{\mathrm{Ag}}}=0.80V,\mathrm{and}E{^{\circ}}_{\frac{{\mathrm{Fe}}^{2+}}{\mathrm{Fe}}}=-0.44V$

The condition for reaction to occur, $E{^{\circ}}_{\mathrm{Cell}}={(E{^{\circ}}_{\mathrm{SRP}})}_{\mathrm{reduction}\mathrm{reaction}}-{(E{^{\circ}}_{\mathrm{SRP}})}_{\mathrm{oxidation}\mathrm{reaction}}$ must have the positive value.

$(A){\mathrm{Cu}}^{2+}(\mathrm{aq})+\mathrm{Fe}\rightarrow \mathrm{Cu}+{\mathrm{Fe}}^{2+}(\mathrm{aq})$

$E{^{\circ}}_{\mathrm{Cell}}=0.34+0.44 E{^{\circ}}_{\mathrm{Cell}}=0.78V$

$(B)\mathrm{Zn}+{\mathrm{Fe}}^{2+}(\mathrm{aq})\rightarrow {\mathrm{Zn}}^{2+}(\mathrm{aq})+\mathrm{Fe}$

$E{^{\circ}}_{\mathrm{Cell}}=-0.44+0.76 E{^{\circ}}_{\mathrm{Cell}}=+0.32V$

$(C){\mathrm{Cu}}^{2+}(\mathrm{aq})+2\mathrm{Ag}\rightarrow 2{\mathrm{Ag}}^{+}(\mathrm{aq})+\mathrm{Cu}$

$E{^{\circ}}_{\mathrm{Cell}}=0.34-0.80 E{^{\circ}}_{\mathrm{Cell}}=-0.46V$

$(D)\mathrm{Zn}+{\mathrm{Cu}}^{2+}(\mathrm{aq})\rightarrow {\mathrm{Zn}}^{2+}(\mathrm{aq})+\mathrm{Cu}$

$E{^{\circ}}_{\mathrm{Cell}}=0.34+0.76 E{^{\circ}}_{\mathrm{Cell}}=+1.10V$

Hence, the reaction given in $C$ option is not possible.

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Options

${\mathrm{CuSO}}_{4}(\mathrm{aq})+\mathrm{Fe}(s)\rightarrow {\mathrm{FeSO}}_{4}(\mathrm{aq})+\mathrm{Cu}(s)$

${\mathrm{FeSO}}_{4}(\mathrm{aq})+\mathrm{Zn}(s)\rightarrow {\mathrm{ZnSO}}_{4}(\mathrm{aq})+\mathrm{Fe}(s)$

$2{\mathrm{CuSO}}_{4}(\mathrm{aq})+2\mathrm{Ag}(s)\rightarrow 2\mathrm{Cu}(s)+{\mathrm{Ag}}_{2}{\mathrm{SO}}_{4}(\mathrm{aq})$

${\mathrm{CuSO}}_{4}(\mathrm{aq})+\mathrm{Zn}(s)\rightarrow {\mathrm{ZnSO}}_{4}(\mathrm{aq})+\mathrm{Cu}(s)$