9.10 Cell Potential Under Nonstandard Conditions

Explain the relationship between deviations from standard cell conditions and changes in the cell potential. Thermodynamics and Electrochemistry UNIT 9

• In a real system under nonstandard conditions, the cell potential will vary depending on the concentrations of the active species. The cell potential is a driving force toward equilibrium; the farther the reaction is from equilibrium, the greater the magnitude of the cell potential.
• Equilibrium arguments such as Le Châtelier’s principle do not apply to electrochemical systems, because the systems are not in equilibrium.
• The standard cell potential Eo corresponds to the standard conditions of Q = 1. As the system approaches equilibrium, the magnitude (i.e., absolute value) of the cell potential decreases, reaching zero at equilibrium (when Q = K). Deviations from standard conditions that take the cell further from equilibrium than Q = 1 will increase the magnitude of the c relative to oE . Deviations from standard ell potential conditions that take the cell closer to equilibrium than Q = 1 will decrease the magnitude of the cell potential relative toEo. In concentration cells, the direction of spontaneous electron flow can be determined by considering the direction needed to reach equilibrium.
• Algorithmic calculations using the Nernst equation are insufficient to demonstrate an understanding of electrochemical cells under nonstandard conditions. However, students should qualitatively understand the effects of concentration on cell potential and use conceptual reasoning, including the qualitative use of the Nernst equation: EQN: E = Eo − (RT/nF) ln Q to solve problems.