Question

The standard reduction potential data at $25^{\circ }C$ is given below.
$E^{\circ }(F{e}^{3+},F{e}^{2+})=+0.77V;E^{\circ }(F{e}^{2+},Fe)=-0.44V$
$E^{\circ }(C{u}^{2+},Cu)=+0.34V;E^{\circ }(C{u}^{+},Cu)=+0.52V$
$E^{\circ }[O_{2(g)}+4H^{+}+4e^{-}\to 2H_{2}O]=+1.23V;$
$E^{\circ }[O_{2(g)}+2H_{2}O+4e^{-}\to 4O{H}^{-}]=+0.40V$
$E^{\circ }(C{r}^{3+},Cr)=-0.74V;E^{\circ }(C{r}^{2+},Cr)=-0.91V$
Match $E^{\circ }$ of the redox pair in List I with the values given in List II and select the correct answer using the code given below the lists:

List I	List II
P. $E^{\circ }(F{e}^{3+},Fe)$	1. $-0.18V$
Q. $E^{\circ }(4H_{2}O\to 4H^{+}+4O{H}^{-})$	2. $-0.4V$
R. $E^{\circ }(C{u}^{2+}+Cu\to 2C{u}^{+})$	3. $-0.04V$
S. $E^{\circ }(C{r}^{3+},C{r}^{2+})$	4. $-0.83V$

• A. $(P)-(4),(Q)-(1),(R)-(2),(S)-(3)$
• B. $(P)-(2),(Q)-(3),(R)-(4),(S)-(1)$
• C. $(P)-(1),(Q)-(2),(R)-(3),(S)-(4)$
• D. $(P)-(3),(Q)-(4),(R)-(1),(S)-(2)$

Answer 1

Answer: (D)

$F{e}^{3+}+e^{-}\to F{e}^{2+};\Delta G_1^{\circ }=-1F\times 0.77$
$F{e}^{2+}+2e^{-}\to Fe;{\Delta }^{\circ }{G}_2=+2F\times 0.44$
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$F{e}^{3+}+3e^{-}\to Fe;\Delta G_3^{\circ }=-3F\times E_{F{e}^{3+}/Fe}^{\circ }$
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$\Delta G_3^{\circ }=\Delta G_1^{\circ }+\Delta G_2^{\circ }$
$-3F\times E_{F{e}^{3+}/Fe}^{\circ }=-0.77F+0.88F$
$-3E_{F{e}^{3+}/Fe}^{\circ }=0.11V$
$Q:2H_{2}O\to O_2+4H^{+}+4e^{-};E^{\circ }=-1.23V$
$O_2+2H_{2}O+4e^{-}\to 4O{H}^{-};E^{\circ }=+0.40V$
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$4H_{2}O\to 4H^{+}+4O{H}^{-};E^{\circ }=-0.83V$
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$R:C{u}^{2+}+2e^{-}\to Cu;E^{\circ }=+0.34V$
$2C{u}^{2+}\to 2C{u}^{+}+2e^{-};E^{\circ }=-0.52V$
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$C{u}^{2+}+Cu\to 2C{u}^{+};E^{\circ }=-0.18V$
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$S:C{r}^{3+}+3e^{-}\to Cr;\Delta G_1^{\circ }=+3F\times 0.74$
$Cr\to C{r}^{2+}+2e^{-};\Delta G_2^{\circ }=-2F\times 0.91$
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$C{r}^{3+}+e^{-}\to C{r}^{2+};\Delta G_3^{\circ }=-1F\times E_{C{r}^{3+}/C{r}^{2+}}^{\circ }$
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$\Delta G_3^{\circ }=\Delta G_1^{\circ }+\Delta G_2^{\circ }$
$-1F\times E_{C{r}^{3+}/C{r}^{2+}}^{\circ }=2.22F-1.82F=0.4F$
$E_{C{r}^{3+}/C{r}^{2+}}^{\circ }=-0.4V$