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Question #44789

Difficulty: 8/10 | 🟢 Free

The Question

Consider the following electrochemical cell with transference operating at 298 K298\text{ K}:

Pt∣H2(1 bar)∣Sol L ⋮ Sol R∣H2(1 bar)∣PtPt | H_2 (1\text{ bar}) | \text{Sol L} \,\vdots\, \text{Sol R} | H_2 (1\text{ bar}) | Pt

Sol L\text{Sol L} is an ideal dilute aqueous buffer containing 0.1 M RNH20.1\text{ M } RNH_2 and 0.1 M RNH3+Cl−0.1\text{ M } RNH_3^+Cl^-. Assume the activity coefficients for all species in Sol L\text{Sol L} are unity (γ=1\gamma = 1).

Sol R\text{Sol R} is a concentrated aqueous buffer containing 1.0 M RNH21.0\text{ M } RNH_2 and 1.0 M RNH3+Cl−1.0\text{ M } RNH_3^+Cl^-. Due to the high concentration of the organic amine, the dielectric constant of the medium is significantly lowered, altering the solvation dynamics. The activity coefficients in Sol R\text{Sol R} are determined to be: γRNH3+=0.5\gamma_{RNH_3^+} = 0.5, γCl−=0.5\gamma_{Cl^-} = 0.5, and γRNH2=2.0\gamma_{RNH_2} = 2.0 (due to the salting-out effect).

The thermodynamic acid dissociation constant of RNH3+RNH_3^+ is Ka=10−9K_a = 10^{-9}. The transport numbers of the ions across the liquid junction are tCl−=0.75t_{Cl^-} = 0.75 and tRNH3+=0.25t_{RNH_3^+} = 0.25.

Given: 2.303RTF=0.06 V\frac{2.303 RT}{F} = 0.06\text{ V}, log⁡2=0.30\log 2 = 0.30, and log⁡5=0.70\log 5 = 0.70.

Calculate the total electromotive force (EcellE_{cell}) of the cell.

Options

  • A.

    -0.015 V

  • B.

    -0.036 V

  • C.

    -0.006 V

  • D.

    +0.030 V

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Correct Answer

Step-by-Step Solution

Explanation

How the Examiner Thinks

Exception_Exposure

Learn to identify edge cases and boundary exceptions that JEE examiners hide in math problems.

Concepts Behind This Question

Liquid Junction Potential|Electrode Potential Dependencies|Basicity-Solvation Competition

Why This Question Is Tricky

Common Mistakes

This problem tests the 'Basicity-Solvation Competition' and the breakdown of the ideal Nernst equation.

Students typically assume that in a buffer where [Base]=[ConjugateAcid][Base] = [Conjugate Acid], the pH=pKapH = pK_a.

However, at high concentrations, especially with organic amines, the solvent's dielectric constant drops, drastically altering activity coefficients (γ≠1\gamma \neq 1).

The trap is twofold: (1) The Nernst equation strictly relies on thermodynamic activity, not concentration, shifting the electrode potential. (2) The Liquid Junction Potential (LJP) must also be calculated using mean ionic activities (a±a_{\pm}), not molarities.

Failing to account for γ\gamma in either the buffer equilibrium (Option B) or the LJP equation (Option C) leads to incorrect potentials.

Quick Facts About This Question

Question Information

SubjectChemistry
ChapterElectrochemistry - Class 12
ConceptLiquid Junction Potential
Difficulty8
Cognitive Load7
Target Solve Time160 sec
AccessFree