In a concentration cell with identical redox couples on both sides, what determines the sign of Ecell?

• A. The ratio of activities (concentrations) on the two electrodes
• B. The solvent viscosity
• C. The applied external pressure
• D. The electrode surface area

Answer: (A)

In a concentration cell the driving force is the difference in concentrations of the same redox couple on the two sides. The Nernst equation shows that each half-cell’s potential depends on the ratio of activities (essentially concentrations) of its oxidized and reduced forms. When you have identical couples on both sides, the overall cell potential reduces to a term that depends only on the ratio of those activities between sides. The sign comes from which side has the higher activity of the oxidized species: the electrode with the larger oxidized-form activity tends to oxidize (becomes the anode), and the other side tends to reduce (becomes the cathode), giving a positive Ecell in the defined direction if the cathode side has the higher oxidized-form activity. The ratio of activities on the two electrodes is therefore what sets the sign.

Other factors like solvent viscosity, external pressure, or electrode surface area do not determine the EMF sign in this ideal description; they can influence kinetics or practical performance but not the thermodynamic driving force.