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Standard reduction potentials at 298 K are given: Pb2+/Pb = -0.13 V Ni2+/Ni = -0.24 V Cd2+/Cd = -0.40 V Fe2+/Fe = -0.44 V A metal rod X and a rod Y are dipped into a solution containing 0.001 M X2+ and 0.1 M Y2+ at 298 K and connected by a conducting wire. Metal X is observed to dissolve. Which of the following could be the correct pair (X, Y)?

1. Cd and Ni
2. Cd and Fe
3. Ni and Pb
4. Ni and Fe

Correct answer: Cd and Ni

Solution

X dissolves (acts as anode), so we need E_cell > 0. Using Nernst equation with n=2: E_cell = [E°(Y) - E°(X)] + 0.059 V (the Nernst term adds 0.059 V because [X2+] < [Y2+]). Checking each pair: (A) Cd/Ni: 0.16 + 0.059 = +0.22 V > 0. Valid. (B) Cd/Fe: -0.04 + 0.059 = +0.019 V > 0. Valid. (C) Ni/Pb: 0.11 + 0.059 = +0.17 V > 0. Valid. (D) Ni/Fe: -0.20 + 0.059 = -0.14 V < 0. X would not dissolve. Options A, B, C are all correct; D is incorrect. The first listed correct answer is Cd and Ni.

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