After midterm · Session 6 of 6 · 180 minutes
30 September 2026
What determines the sign of a concentration-cell potential?
Write your prediction before opening the lab. By the end, support an answer with one controlled comparison and an independent check.
| Activity | Minutes |
|---|---|
| Opening question and essential concepts | 30 |
| One hand calculation | 20 |
| One lab demonstration | 20 |
| Break | 10 |
| Student pair investigation | 70 |
| Discussion and exit question | 30 |
The 70-minute investigation is student work with instructor support.
Enter both half-reactions as reductions. Reverse the left reaction and add the right after matching electron counts.
E_{cell}^\circ=E_{right}^\circ-E_{left}^\circ
Both potentials need the same reference electrode and temperature. Multiplying a half-reaction does not multiply its potential.
Combine ΔrG=ΔrG°+RTlnQ with ΔrG=−nFE: E=E^\circ-\frac{RT}{nF}\ln Q
At 298.15 K, E=E^\circ-\frac{0.05915935\ \mathrm V}{n}\log_{10}Q
Changing T requires valid E° data at the new T.
At 298.15 K, n=2, aL=0.01 and aR=1: Q=0.01,\qquad E=+0.05915935\ \mathrm V
E°=0, K=1, but E is nonzero. Reversing the concentrations changes the sign of E. Equal activities give E=0.
ΔrG≈−11.416 kJ per mol of the written reaction.
Use Eright−Eleft and deltaG=−nFE with the balanced electron count. Equal activities must give zero potential for the same reduction couple.
Load the Lab 13 concentration-cell preset. Predict the sign, exchange left and right activities, then set them equal. Keep T and reference conventions fixed.
Keep one saved baseline for the pair investigation.
In Lab 13, compare a concentration cell with the activities exchanged. Predict and explain the sign of E, then independently check deltaG=−nFE.
Use 20 minutes to compare results and discuss unexpected behavior.
What determines the sign of a concentration-cell potential?
Use the final 10 minutes to explain your answer individually and name one assumption that limits it.
In Lab 13, compare a concentration cell with the activities exchanged. Predict and explain the sign of E, then independently check deltaG=−nFE.
Assessment 6 and independent checkpoint
Retain one study JSON, one comparison and a short explanation. Continue from your classroom case. The hand checkpoint is part of this time budget.
Choose one extension: scale the balanced reaction and electron count, examine activity corrections within the supported model, or explain why equilibrium voltage differs from voltage under load.
Session appendix · Module reference
Learning path · Offline package
Optional work is additional. Complete the required investigation first.