1

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

Oxidation of succinate ion produces ethylene and carbon dioxide gases. On passing 0.2 Faraday electricity through an aqueous solution of potassium succinate, the total volume of gases (at both cathode and anode) at STP (1 atm and 273 K) is :
A
2.24 L
B
4.48 L
C
6.72 L
D
8.96 L
2

### 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.
3

### 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.
4

### 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