concentration cell

Meaning: A concentration cell is a galvanic cell with the same electrode material and the same ion in both half-cells, at different concentrations. Because the half-reactions are identical, E°cell = 0, and the voltage comes from the concentration difference. The dilute half-cell is the anode (oxidation adds ions to it) and the concentrated half-cell is the cathode (reduction removes ions from it). The two concentrations move toward each other until Q = 1 and E = 0.

Equation: Ecell = (RT/nF) ln([concentrated]/[dilute]), where Ecell is in V, R = 8.314 J/(mol·K), T is in K, n is the moles of electrons transferred per reaction (mol), and F = 96,485 C/mol. At 25 °C this becomes Ecell = (0.0592/n) log([conc]/[dilute]).

On the MCAT: A passage shows two identical Cu electrodes in 0.01 M and 1.0 M Cu²⁺ solutions. You name the anode, give the direction of electron flow, predict which concentration rises, and calculate the voltage. A tenfold ratio with n = 1 gives about 0.059 V. You may also be asked how the voltage changes as the cell runs: it falls toward zero.

Common confusion: E° = 0 does not mean zero voltage. The standard potential is zero, but Q ≠ 1, so the actual potential is not. To place the anode, remember that the spontaneous process evens out the concentrations, so oxidation occurs on the dilute side.