i) Account for the following: (a) An alkaline medium inhibits the rusting of iron. (b) Iron does not rust even if the zinc coating is broken in a galvanised iron pipe. ii) Given: $\mathrm{Cu^{2+}+2e^-\rightarrow Cu}$, E° = +0.34 V; $\mathrm{Ag^++e^-\rightarrow Ag}$, E° = +0.80 V. (a) Construct a galvanic cell using the above data. (b) For what concentration of Ag$^+$ ions will the emf of the cell be zero at 25°C, if the concentration of Cu$^{2+}$ is 0.01 M? (log 3.919 = 0.593)

(i)(a) OH⁻ removes the H⁺ needed for the cathodic step. (b) Zn, more reactive, keeps acting as sacrificial anode. (ii)(a) Cu|Cu²⁺||Ag⁺|Ag, E° = 0.46 V. (b) [Ag⁺] ≈ 1.6 × 10⁻⁹ M (*)

Redox Reactions and Electrochemistry — CBSE Class 12 · 2006 · Delhi Board Question with Solution

i) Account for the following:
(a) An alkaline medium inhibits the rusting of iron.
(b) Iron does not rust even if the zinc coating is broken in a galvanised iron pipe.
ii) Given:

C u^{2 +} + 2 e^{-} \to Cu

, E° = +0.34 V;

A g^{+} + e^{-} \to Ag

, E° = +0.80 V.
(a) Construct a galvanic cell using the above data.
(b) For what concentration of Ag

^{+}

ions will the emf of the cell be zero at 25°C, if the concentration of Cu

^{2 +}

is 0.01 M? (log 3.919 = 0.593)

Solution

(i)(a) Rusting needs H

^{+}

for the cathodic reaction O

_{2}

+ 4H

^{+}

+ 4e

^{-} \to

_{2}

O; an alkaline medium neutralises H

^{+}

, so rusting is inhibited.
(b) Zinc is more reactive than iron; even when the coating breaks, zinc (not iron) is oxidised in the local cell — sacrificial protection continues.
(ii)(a) Silver, with the higher E°, is the cathode:

Cu (s) ∣ C u^{2 +} (aq) ∣∣ A g^{+} (aq) ∣ Ag (s), E_{ce l l}^{\circ} = 0.80 - 0.34 = 0.46 V

(b) Setting

E_{ce l l} = 0

(

n = 2

0 = 0.46 - \frac{0.059}{2} lo g \frac{[ C u ^{2 +} ]}{[ A g ^{+} ] ^{2}}

lo g \frac{0.01}{[ A g ^{+} ] ^{2}} = \frac{0.46 \times 2}{0.059} = 15.59

[A g^{+}]^{2} = 1 0^{- 17.59} \Rightarrow [A g^{+}] = 1 0^{- 8.80}

[A g^{+}] \approx 1.6 \times 1 0^{- 9} M

(*) Solved with the exact Nernst relation; the paper's printed log-hint (log 3.919 = 0.593) points to the same order of magnitude.

CBSE Class 12 Board · 2006 · Delhi · Subjective

CBSE Class 12 · 2006 · Delhi · Redox Reactions and ElectrochemistryLong Answer

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