Explain potential changes with the reaction quotient

Composition carries a free-energy contribution

At fixed temperature, ΔrG=ΔrG°+RT ln Q. Combine this with ΔrG=−nFE to obtain E=E°−(RT/nF)ln Q. Q is a dimensionless reaction quotient, not electrical charge. The formula concerns the written direction and a compatible activity convention.

The main task is conceptual: products accumulating relative to reactants raises Q and decreases signed E for the written reaction. When Q=K, ΔrG=0 and E=0. Beyond that point the favorable direction reverses. It is incorrect to say that increasing Q must always decrease the absolute magnitude of the voltage.

Compare four prepared states

The numerical Nernst predictions below are an optional model extension, not a substitute for the AP conceptual target: explain how reaction quotient changes affect cell potential and favorable direction. Memorizing this numerical procedure is not a completion requirement.

For the original n=2, E°=0.050 V cell at 298.15 K, each decade increase in Q subtracts approximately 0.02958 V. Thus Q=0.01 gives E≈0.10916 V; Q=1 gives 0.05000 V; Q=100 gives approximately −0.00916 V. The zero occurs at log10K≈1.690, between the last two preparations. These are illustrative ideal predictions, not measurements.

Before displaying each state, explain the likely electron direction relative to the named reaction. A positive E supports forward reaction if both electronic and ionic paths are available. A negative E supports the reversed chemistry, not a negative amount of electron transfer in a physical wire.

Concentration-cell reasoning

Identical electrode couples have E°cell=0 when compared in compatible standard states. Unequal activities can still produce a nonzero cell potential. In a metal/metal-ion concentration cell, the dilute side tends to produce metal ions through oxidation and the concentrated side tends to consume them through reduction. That direction reduces the concentration difference under the stated simple model.

This is a drive toward equilibrium, not an assertion that a working cell is already at equilibrium. Avoid applying an equilibrium-shift slogan while ignoring electron transfer or the quotient. Calculate or reason from the actual reaction and held conditions.

Boundaries and changed case

Real activity coefficients, junction potentials, overpotentials and internal resistance can matter. Our model excludes them and does not predict current or power. Temperature is fixed; changing it can alter both the logarithmic factor and E°.

For E°=0 and Q=0.1, predict positive E for the chosen direction; for Q=10 predict its negative with equal magnitude. Explain why the equality of magnitudes follows here from reciprocal quotients, not from a universal rule about arbitrary paired concentrations.

Source scope and continuation

Original teaching explanation, calculations and practice. Framework alignment: AP Chemistry topic 9.10 in the College Board course framework, effective Fall 2024, checked 30 October 2026. These pages do not reproduce restricted examination questions or establish AP authorization. The investigation is a bounded computational model, not a conducted laboratory.

Return to the preceding Unit 9 lesson when its reasoning is unfamiliar.

Continue to Infer deposited material from charge and stoichiometry.