i) Explain with an example how weak and strong electrolytes can be distinguished. ii) In the button cell used in watches the following reaction occurs: $$\mathrm{Zn(s)+Ag_2O(s)+H_2O(l)\longrightarrow Zn^{2+}(aq)+2Ag(s)+2OH^-(aq)}$$ Determine E° for the cell and $\Delta_rG^{\circ}$ for the reaction. Given: $\mathrm{Zn^{2+}+2e^-\rightarrow Zn}$, E° = −0.76 V; $\mathrm{Ag_2O+H_2O+2e^-\rightarrow 2Ag+2OH^-}$, E° = +0.80 V; 1 F = 96500 C mol$^{-1}$.

(i) Strong electrolytes: Λ_m falls slowly/linearly with √c (e.g. KCl); weak: Λ_m small but rises steeply on dilution (e.g. CH₃COOH). (ii) E° = 1.56 V; ΔG° ≈ −301 kJ mol⁻¹.

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

WhatsApp Email

i) Explain with an example how weak and strong electrolytes can be distinguished.
ii) In the button cell used in watches the following reaction occurs:

Zn (s) + A g_{2} O (s) + H_{2} O (l) ⟶ Z n^{2 +} (aq) + 2Ag (s) + 2O H^{-} (aq)

Determine E° for the cell and

Δ_{r} G^{\circ}

for the reaction.
Given:

Z n^{2 +} + 2 e^{-} \to Zn

, E° = −0.76 V;

A g_{2} O + H_{2} O + 2 e^{-} \to 2Ag + 2O H^{-}

, E° = +0.80 V; 1 F = 96500 C mol

^{- 1}

Solution

(i) Plot molar conductivity against

c

. A strong electrolyte (e.g. KCl) shows a small, linear decrease of

Λ_{m}

with increasing

c

, and

Λ_{m}^{\circ}

is found by extrapolation. A weak electrolyte (e.g. CH

_{3}

COOH) has much lower

Λ_{m}

which rises very steeply at high dilution (dissociation increases), so its

Λ_{m}^{\circ}

cannot be found by extrapolation (Kohlrausch's law is used instead).
(ii) Zn is oxidised, Ag

_{2}

O reduced,

n = 2

E_{ce l l}^{\circ} = 0.80 - (- 0.76)

E_{ce l l}^{\circ} = 1.56 V

Δ_{r} G^{\circ} = - n F E^{\circ} = - 2 \times 96500 \times 1.56

Δ_{r} G^{\circ} = - 3.01 \times 1 0^{5} J = - 301 kJ mo l^{- 1}

CBSE Class 12 Board · 2006 · Delhi Compartment · Subjective

Practise more: all CBSE board PYQs on Redox Reactions and Electrochemistry →

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

Solution

More board questions on Redox Reactions and Electrochemistry