1

### JEE Main 2016 (Online) 10th April Morning Slot

Identify the correct statement :
A
Iron corrodes in oxygen-free water.
B
Iron corrodes more rapidly in salt water because its electrochemical potential is higher.
C
Corrosion of iron can be minimized by forming a contact with another metal with a higher reduction potential.
D
Corrosion of iron can be minimized by forming an impermeable barrier at its surface.
2

### JEE Main 2017 (Offline)

Given
$E_{C{l_2}/C{l^ - }}^o$ = 1.36 V, $E_{C{r^{3 + }}/Cr}^o$ = - 0.74 V
$E_{C{r_2}{O_7}^{2 - }/C{r^{3 + }}}^o$ = 1.33 V, $E_{Mn{O_4}^ - /Mn ^{2+}}^o$ = 1.51 V
Among the following, the strongest reducing agent is
A
Mn2+
B
Cr3+
C
Cl
D
Cr

## Explanation

$E_{C{l_2}/C{l^ - }}^o$ = 1.36 V, $E_{C{r^{3 + }}/Cr}^o$ = - 0.74 V
$E_{C{r_2}{O_7}^{2 - }/C{r^{3 + }}}^o$ = 1.33 V, $E_{Mn{O_4}^ - /Mn ^{2+}}^o$ = 1.51 V

More negative the E° value of the species, more stronger is the reducing agent. Since Cr3+ is having least reducing potential, so Cr would be strongest reducing agent.
3

### JEE Main 2017 (Online) 8th April Morning Slot

What is the standard reduction potential (Eo) for Fe3+ $\to$ Fe ?
Given that :
Fe2+ + 2e$-$ $\to$ Fe; $E_{F{e^{2 + }}/Fe}^o$ = $-$0.47 V
Fe3+ + e$-$ $\to$ Fe2+; $E_{F{e^{3 + }}/F{e^{2 + }}}^o$ = +0.77 V
A
$-$ 0.057 V
B
+ 0.057 V
C
+ 0.30 V
D
$-$ 0.30 V

## Explanation

For the given reaction :

Fe3+ + e$-$ $\to$ Fe2+; $E_{F{e^{3 + }}/F{e^{2 + }}}^o$ = +0.77 V

$\Delta$Go = -nFEo

$\Rightarrow$ $\Delta G_1^o = - \left( 1 \right)F\left( {0.77} \right)$ = -0.77F

For the following reaction :

Fe2+ + 2e$-$ $\to$ Fe; $E_{F{e^{2 + }}/Fe}^o$ = $-$0.47 V

$\Delta G_2^o = - \left( 2 \right)F\left( {-0.47} \right)$ = 0.47$\times$2F

Overall reaction :

Fe3+ + 3e$-$ $\to$ Fe;

$\Delta G_3^o = - 3FE_3^o$

$\Rightarrow$ $\Delta G_1^o$ + $\Delta G_2^o$ = $3FE_3^o$

$\Rightarrow$ -0.77F + 0.47$\times$2F = $3FE_3^o$

$\Rightarrow$ $E_3^o = - {{0.17} \over 3}$ = -0.057 V
4

### JEE Main 2017 (Online) 8th April Morning Slot

Consider the following standard electrode potentials (Eo in volts) in aqueous solution :

Element M3+ /M M+ /M
A1 -1.66 + 0.55
T1 +1.26 - 0.34

Based on these data, which of the following statements is correct ?
A
T1+ is more stable than A13+
B
A1+ is more stable than A13+
C
T1 + is more stable than A1+
D
T13+ is more stable than A13+