AP Chemistry · Handsworth Secondary 2026–27
Unit 9 · Study Guide
Dr. Ras Mulinta
Thermodynamics & Electrochemistry
Exam-focused review
A one-page map of what the AP exam expects from Unit 9. We give this unit no separate test (only 5 blocks), so this guide is your safety net, it is fully fair game on the AP exam. This is a checklist, not a re-teach: if a line doesn't click, return to that section of the notes package. A periodic table, equation sheet (R, F, ΔG = −nFE, Nernst), and a table of standard reduction potentials are provided on the exam.
Must be able to do
- 9.1 Predict the sign of ΔS by inspection, gas moles, phase changes, dissolving, temperature (KMT).
- 9.2 Calculate ΔS° = ΣS°(prod) − ΣS°(react); mind J vs. kJ and coefficients.
- 9.3 Use ΔG° = ΔH° − TΔS° and the four-case sign table to judge favorability and find the crossover T = ΔH°/ΔS°.
- 9.4 Explain why a favored reaction can be slow (kinetic control, high Eₐ), and that no reaction ≠ equilibrium.
- 9.5 Relate ΔG° = −RT ln K and ΔG = ΔG° + RT ln Q; sign of ΔG° ↔ K vs. 1.
- 9.6–9.7 Reason qualitatively about dissolution; couple an unfavorable step to a favorable one (sum the ΔG°).
- 9.8 Identify anode (oxidation) / cathode (reduction), electron & ion flow; write cell notation; galvanic vs. electrolytic.
- 9.9 E°cell = E°cathode − E°anode (don't scale E° by coefficients); ΔG° = −nFE°.
- 9.10 Use the Nernst equation qualitatively: which way does E drift as Q changes / the cell discharges (E → 0 at equilibrium).
- 9.11 Faraday's law chain: q = I·t → mol e⁻ = q/F → mol substance → mass.
The big idea that ties it together
One question, three lenses. "Which way does a change go, and how far?" is answered by ΔG° = ΔH° − TΔS°, which links to equilibrium through ΔG° = −RT ln K and to voltage through ΔG° = −nFE°. Favored ⇔ ΔG° < 0 ⇔ K > 1 ⇔ E°cell > 0, three faces of the same coin. Kinetics (Eₐ) is a separate axis: it sets the speed, never the destination.
Don't waste time on (excluded by the CED)
Labeling electrodes "+/−" (not assessed, identify by oxidation/reduction) · plug-and-chug algorithmic Nernst calculations (qualitative reasoning only) · applying Le Châtelier's principle to a running electrochemical cell (it isn't at equilibrium). Know the directions and the relationships, skip the rote arithmetic on Nernst.
Quick self-check (answer in your head, then verify)
- Sign of ΔS for CaCO₃(s) → CaO(s) + CO₂(g)?
- ΔG° at 298 K if ΔH° = −571.6 kJ and ΔS° = −326.4 J·K⁻¹? Favored?
- ΔG° at 298 K if K = 1.0 × 10⁴? (R = 8.314 J·mol⁻¹·K⁻¹.)
- E°cell for Zn | Zn²⁺ ‖ Ag⁺ | Ag, given E°(Ag⁺/Ag) = +0.80 V, E°(Zn²⁺/Zn) = −0.76 V?
- Using that cell (n = 2), what is ΔG°? (F = 96485 C·mol⁻¹.)
- Mass of Cu deposited by 2.00 A for 965 s? (Cu²⁺ + 2e⁻ → Cu, M = 63.55 g/mol.)
- As a galvanic cell discharges, which way does its voltage drift, and what is E at equilibrium?
Check yourself: 1) ΔS > 0 (a gas is produced from a solid) 2) ΔG° = −571.6 − (298)(−0.3264) = −474.3 kJ → favored 3) ΔG° = −(8.314)(298)(ln 10⁴) = −(2477.6)(9.210) = −22.8 kJ 4) E°cell = 0.80 − (−0.76) = +1.56 V 5) ΔG° = −(2)(96485)(1.56) = −3.01 × 10⁵ J = −301 kJ 6) q = 2.00 × 965 = 1930 C → 1930/96485 = 0.0200 mol e⁻ → ÷2 = 0.0100 mol Cu → ×63.55 = 0.636 g 7) E falls toward zero; at equilibrium (Q = K) E = 0 (a dead battery).