CBSE Class 12 · 2011 · All India · Redox Reactions and ElectrochemistryLong Answer

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a) What type of battery is the lead storage battery? Write the anode and cathode reactions and the overall cell reaction occurring in the operation of a lead storage battery.
b) Calculate the potential for the half-cell containing 0.10 M K

_{2}

_{2}

_{7}

(aq), 0.20 M Cr

^{3 +}

(aq) and

1.0 \times 1 0^{- 4}

M H

^{+}

(aq). The half-reaction is

C r_{2} O_{7}^{2 -} (aq) + 14 H^{+} (aq) + 6 e^{-} ⟶ 2C r^{3 +} (aq) + 7 H_{2} O (l)

and the standard electrode potential is E° = 1.33 V.

Answer: (a) Secondary cell; reactions as standard. (b) E ≈ 0.78 V

Solution

(a) It is a secondary (rechargeable) cell.
Anode:

Pb + S O_{4}^{2 -} \to PbS O_{4} + 2 e^{-}

Cathode:

Pb O_{2} + S O_{4}^{2 -} + 4 H^{+} + 2 e^{-} \to PbS O_{4} + 2 H_{2} O

Overall:

Pb + Pb O_{2} + 2 H_{2} S O_{4} \to 2PbS O_{4} + 2 H_{2} O

(b) Nernst equation (

n = 6

E = 1.33 - \frac{0.059}{6} lo g \frac{[ C r ^{3 +} ] ^{2}}{[ C r _{2} O _{7}^{2 -} ] [ H ^{+} ] ^{14}}

\frac{( 0.20 ) ^{2}}{0.10 \times ( 1 0 ^{- 4} ) ^{14}} = \frac{0.04}{1 0 ^{- 57}} = 4 \times 1 0^{55}

E = 1.33 - \frac{0.059}{6} \times 55.60 = 1.33 - 0.547

E = 0.78 V

CBSE Class 12 Board · 2011 · All India · Subjective

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CBSE Class 12 · 2011 · All India · Redox Reactions and ElectrochemistryLong Answer

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