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JEE Advanced 2020 Paper 2 · Chemistry · Electrochemistry · medium

A Daniell-type cell Zn(s) | Zn^2+(aq) \| Cu^2+(aq) | Cu(s) has E^°cell=1.10 V at 298 K. Take (2.303RT)/(F)=0.059 V.

One or more options correct

A Daniell-type cell Zn(s) ∣ Zn2+(aq) ∥ Cu2+(aq) ∣ Cu(s)\mathrm{Zn(s)\,|\,Zn^{2+}(aq)\,\|\,Cu^{2+}(aq)\,|\,Cu(s)} has Ecell∘=1.10 VE^\circ_{cell}=1.10\ \mathrm{V} at 298 K298\ \mathrm{K}. Take 2.303RTF=0.059 V\dfrac{2.303RT}{F}=0.059\ \mathrm{V}. Which of the following statements is/are correct?
  1. A. With [Zn2+]=1.0 M[\mathrm{Zn^{2+}}]=1.0\ \mathrm{M} and [Cu2+]=0.010 M[\mathrm{Cu^{2+}}]=0.010\ \mathrm{M} the cell emf is about 1.04 V1.04\ \mathrm{V}.
  2. B. With [Zn2+]=0.10 M[\mathrm{Zn^{2+}}]=0.10\ \mathrm{M} and [Cu2+]=1.0 M[\mathrm{Cu^{2+}}]=1.0\ \mathrm{M} the cell emf is lower than 1.10 V1.10\ \mathrm{V}.
  3. C. The equilibrium constant of the cell reaction at 298 K298\ \mathrm{K} lies between 103010^{30} and 103510^{35}.
  4. D. Doubling the surface area of both electrodes at fixed concentrations doubles the emf of the cell.
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Answer: A

The cell reaction is Zn+Cu2+→Zn2++Cu\mathrm{Zn+Cu^{2+}\to Zn^{2+}+Cu} with n=2n=2, so E=E∘−0.0592log⁡[Zn2+][Cu2+]E=E^\circ-\dfrac{0.059}{2}\log\dfrac{[\mathrm{Zn^{2+}}]}{[\mathrm{Cu^{2+}}]}. Option A: log⁡(1/0.01)=2\log(1/0.01)=2, so E=1.10−0.059=1.041 VE=1.10-0.059=1.041\ \mathrm{V}, correct. Option B: log⁡(0.1/1)=−1\log(0.1/1)=-1, so E=1.10+0.0295=1.1295 VE=1.10+0.0295=1.1295\ \mathrm{V}, which is higher than 1.10 V1.10\ \mathrm{V}, so B is wrong. Option C: log⁡K=nE∘0.059=2×1.100.059=37.3\log K=\dfrac{nE^\circ}{0.059}=\dfrac{2\times1.10}{0.059}=37.3, so K≈2×1037K\approx2\times10^{37}, which is outside the stated range, so C is wrong. Option D: emf is an intensive property fixed by E∘E^\circ and the concentrations, and electrode area changes only the current capacity, so D is wrong. Only A is correct. As a further check on the structure of the problem, remember that the Nernst correction depends only on the reaction quotient Q=[Zn2+]/[Cu2+]Q=[\mathrm{Zn^{2+}}]/[\mathrm{Cu^{2+}}], so any change that lowers QQ raises the emf, and the emf falls to zero only when QQ equals KK, that is, at equilibrium.
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