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How does the cell potential change when the concentrations are not standard, and what happens as a cell discharges?

Topic 9.10 Cell Potential Under Nonstandard Conditions: predict how the cell potential changes with concentration using the Nernst relationship qualitatively, and explain why a cell potential falls to zero at equilibrium.

A focused answer to AP Chemistry Topic 9.10, covering how the cell potential changes with concentration, the qualitative use of the Nernst relationship and the reaction quotient Q, concentration cells, and why a cell reaches zero potential at equilibrium, with full worked examples.

Generated by Claude Opus 4.810 min answerUpdated 2026-06-04

Reviewed by: AI editorial process; not yet individually human-reviewed

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Quick answer

The cell potential depends on the concentrations of the species, not just the standard values. Qualitatively, the cell potential is related to the reaction quotient Q: increasing the reactant concentrations lowers Q and raises the cell potential above standard, while increasing the product concentrations raises Q and lowers the cell potential below standard. This is the qualitative content of the Nernst relationship, which says the cell potential decreases as Q increases. As a galvanic cell discharges, reactants are consumed and products build up, so Q rises toward K and the cell potential falls; when Q equals K the cell is at equilibrium and the cell potential is zero (a dead battery). A concentration cell has identical electrodes and ions but at different concentrations, so its standard potential is zero, yet it produces a positive potential by equalising the two concentrations, until they match and the potential reaches zero.

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What this topic is asking
Concentration and the cell potential
Discharge and equilibrium
Concentration cells
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What this topic is asking

The College Board (Topic 9.10) wants you to predict how the cell potential changes with concentration using the Nernst relationship qualitatively (through the reaction quotient $Q$ ), and to explain why a cell potential falls to zero at equilibrium. This applies the Q-versus-K thinking of Unit 7 to electrochemical cells.

Concentration and the cell potential

The driving force of a cell depends on how far it is from equilibrium. A cell with plenty of reactant and little product (small $Q$ ) is far from equilibrium and has a high potential; as product accumulates ( $Q$ rises), the cell is closer to equilibrium and the potential falls. This mirrors the $\Delta G = \Delta G^\circ + RT\ln Q$ relationship, since $\Delta G$ and the cell potential are linked by $\Delta G = -nFE$ .

Discharge and equilibrium

So a battery dies not because it runs out of material entirely but because it reaches chemical equilibrium, where $Q = K$ and the potential is zero. This is the electrochemical version of a reaction reaching equilibrium: the free energy change and the cell potential both go to zero together.

Concentration cells

A concentration cell has the same electrode and ion in both half-cells but at different concentrations. Because the half-reactions are identical, the standard cell potential is zero, yet the cell still produces a positive potential: the system can do work by transferring ions from the more concentrated to the more dilute side, equalising the concentrations. The potential is driven entirely by the concentration difference (the $Q$ term), and it falls to zero once the two concentrations become equal. Concentration cells are a vivid demonstration that the cell potential depends on concentration, not just on standard potentials.

Predicting the effect of concentration

A galvanic cell with $E^\circ_\text{cell} = +1.10$ V has its reactant metal-ion concentration increased well above standard. Predict the effect on the cell potential.

step 1 Identify the effect on Q

Increasing a reactant concentration decreases the reaction quotient $Q$ (more reactant in the denominator-like position of the cell reaction).

step 2 Apply the Nernst relationship qualitatively

A smaller $Q$ means the cell is further from equilibrium, so the cell potential rises above the standard value.

step 3 State the direction

The cell potential is now greater than $+1.10$ V.

step 4 Note the limiting behavior

If instead the product concentration were raised (larger $Q$ ), the potential would fall; at $Q = K$ the potential would reach zero.

Try this

Q1. A cell has its product ion concentration increased. State whether the cell potential rises or falls, and explain. [2 points]

Cue. It falls; more product raises $Q$ , bringing the cell closer to equilibrium and reducing the driving force.

Q2. Explain why a concentration cell has a positive potential even though its standard cell potential is zero. [2 points]

Cue. The two half-cells differ only in concentration; the system can do work by equalising the concentrations, giving a positive potential that falls to zero when the concentrations become equal.

Exam-style practice questions

Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AP 2023 (style)4 marksSection II (long FRQ, part). A galvanic cell has a positive standard cell potential. (a) Using the relationship between cell potential and the reaction quotient

Q

, predict whether the cell potential is higher or lower than standard when the reactant ion concentration is increased. (b) Predict the effect on the cell potential of increasing the product ion concentration. (c) Explain why the cell potential falls to zero as the cell discharges. (d) Explain what a concentration cell is and why it has a positive potential despite identical electrodes.

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A 4-point conceptual FRQ on nonstandard cell potential.

(a) More reactant (1 point): increasing the reactant concentration lowers $Q$ ; a smaller $Q$ makes the cell potential higher than standard (the cell is further from equilibrium, more driving force).
(b) More product (1 point): increasing the product concentration raises $Q$ , which lowers the cell potential below standard.
(c) Discharge to zero (1 point): as the cell discharges, reactants are consumed and products form, so $Q$ rises toward $K$ ; when $Q = K$ the cell reaches equilibrium and the cell potential is zero (a dead battery).
(d) Concentration cell (1 point): a concentration cell has the same electrode and ion on both sides but at different concentrations; the difference in concentration gives a nonstandard potential (positive) because the system can do work by equalising the concentrations, even though $E^\circ_\text{cell} = 0$ .

Markers reward the higher potential for more reactant, the lower potential for more product, the equilibrium-discharge reasoning, and the concentration-cell explanation.

AP 2021 (style)1 marksSection I (multiple choice). As a galvanic cell operates and approaches equilibrium, its cell potential (A) increases (B) stays constant (C) decreases toward zero (D) becomes negative. Justify your choice.

Show worked answer →

A 1-point conceptual MCQ. The answer is (C).

As the cell runs, reactants are consumed and $Q$ rises toward $K$ , so the driving force (cell potential) decreases; at equilibrium ( $Q = K$ ) the potential is zero (a dead battery). The trap is (D): the potential reaches zero at equilibrium, it does not go negative.

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Sources & how we know this

AP Chemistry Course and Exam Description — College Board (2020)