A concentration cell is set up as: Pt | H2(P1 atm) | H+(x1 M) || H+(x2 M) | H2(P2 atm) | Pt. Under which condition will the cell reaction proceed spontaneously (i.e., E_cell > 0)?

P1 = P2 and x1 > x2
P1 = P2 and x1 = x2
x1 = x2 and P1 > P2
x1 = x2 and P1 < P2

Correct answer: x1 = x2 and P1 > P2

Solution

The Nernst expression for this cell gives E_cell = (RT/2F) * ln(x2² * P1 / x1² * P2). When x1 = x2 and P1 > P2, the ratio P1/P2 > 1, making E_cell positive and the reaction spontaneous.

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