Ncert Exercises & Solutions

$E_{Cell}^{°}$ of some half cell reactions are given below.

I. $H^{+} (a q) + e^{-} ⟶ \frac{1}{2} H_{2} (g); {E^{°}}_{cell} = 0.00 V$

II. $2 H_{2} O (l) ⟶ O_{2} (g) + 4 H^{+} (a q) + 4 e^{-}; E_{cell}^{⊖} = 1.23 V$

III. $2 {SO}_{4}^{2 -} (a q) ⟶ S_{2} O_{8}^{2 -} (a q) + 2 e^{-}; E_{cell}^{⊖} = 1.96 V$

The correct choice among the given is -

a.	In dilute sulphuric acid solution, hydrogen will be reduced at cathode
b.	In concentrated sulphuric acid solution, water will be oxidised at anode
c.	In dilute sulphuric acid solution, water will be oxidised at anode
d.	In dilute sulphuric acid solution, SO₄^2- ion will be oxidised to tetrathionate ion at anode

Choose the correct option
1. (a, b)
2. (b, c)
3. (c, d)
4. (a, c)

Hint: Higher the reduction potential, the higher will be oxidising power

During the electrolysis of dilute sulphuric acid above given three reactions occur each of which represents a particular reaction either oxidation half cell reaction or reduction half cell reaction.

Oxidation half cell reactions that occur at the anode are as follows

2 {SO}_{4}^{2 -} (aq) ⟶ S_{2} O_{8}^{2 -} + 2 e^{-} E_{cell}^{°} = 1.96 V

2 H_{2} O^{+} (l) ⟶ O_{2} (g) + 4 H^{+} (aq) + 4 e^{-}; E_{cell}^{°} = 1.23 V

Reaction having a lower value of

E_{Cell}^{°}

will undergo faster oxidation.

Hence, oxidation of water occur preferentially reduction half cell reaction occurs at cathode

H^{+} (aq) + e^{-} ⟶ \underset{At Cathode}{\frac{1}{2} H_{2} (g)}

Hence, options (a) and (c) are correct

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